[libc++][pstl] Implement tag dispatching

[rarutyun]

Summary

I believe the approach is agreed.

Tag dispatching implementation is complete with all planned changes to do.

Welcome again for review:)

Depends on: https://reviews.llvm.org/D114624

[rarutyun]

pstl/include/pstl/internal/parallel_backend_serial.h

57

Note that this overload supposed to go away when all the changes are made. Currently there is a code that depends on that.

[• Quuxplusone]

pstl/include/pstl/internal/algorithm_fwd.h

19

I believe you need to add pstl/include to
https://github.com/google/llvm-premerge-checks/blob/main/scripts/clang-tidy.ignore
(Although why this file is in a Google repo instead of the llvm-project repo, I don't know!)

255

Throughout: On function declarations, I think it's unhelpful to have parameter names like _Tag __tag or (below) _ExecutionPolicy&& __exec. I would remove those parameter names. The names __first, __last, __pred are arguably more helpful; but for consistency I'd probably remove them too. (Or rather, don't add them.)

pstl/include/pstl/internal/execution_impl.h

31–33

decltype(T{}) is just T.

39–42

Could this be:

template <typename... _IteratorTypes>
using __are_random_access_iterators = decltype(__internal::__is_iterator_of<
    std::random_access_iterator_tag, _IteratorTypes...>(0));

(Notice the ADL-proofing on __is_iterator_of.)

IIUC, the intent is that __are_random_access_iterators<int*, char*> is true_type but __are_random_access_iterators<int*, std::list<int>::iterator> is false_type, right?

Or if C++17 is permitted, then

template <typename... _IteratorTypes>
using __are_random_access_iterators = std::bool_constant<
    (std::is_base_of_v<std::random_access_iterator_tag,
                       typename std::iterator_traits<_IteratorTypes>::iterator_category> && ...)
>;

107

Rather than split the declaration across #ifs, it would be easier to read (for both humans and computers) if this were just

#if defined(_PSTL_PAR_BACKEND_TBB)
 using __par_backend_tag = __tbb_backend;
#elif _PSTAL_PAR_BACKEND_SERIAL
 using __par_backend_tag = __serial_backend;
#else
 #error "A parallel backend must be specified"
#endif

(also note the probably-accidental inconsistent use of defined versus non-zero, and the bogus semicolon at the end of line 126)

116–129

IIUC, __is_vector is always true_type or false_type?
Would it be simpler to define...

template<bool _IsParallel, bool _IsVector>
struct __bikeshed_tag {
    using __is_vector = std::bool_constant<_IsVector>;
    using __is_parallel = std::bool_constant<_IsParallel>;
    using __backend_tag = std::conditional_t<_IsParallel, __par_backend_tag, void>;
};

template<bool _IsVector>
using __parallel_tag = __bikeshed_tag<true, _IsVector>;

template<bool _IsVector>
using __serial_tag = __bikeshed_tag<false, _IsVector>;

And then if you ever found any need to pattern-match on __bikeshed_tag<X, true>, it wouldn't be so awkward.

[zoecarver]

pstl/include/pstl/internal/algorithm_fwd.h

19

Additionally, it looks like these files may be nested one level too deep. I think this should be pstl/include/algorithm_fwd.h not pstl/include/pstl/include/algorithm_fwd.h.

[• Quuxplusone]

pstl/include/pstl/internal/algorithm_fwd.h

19

@zoecarver: No, I bet the nesting is right. Notice that it's

    pstl/include/pstl/internal/algorithm_fwd.h
not                   ^^^^^^^^
    pstl/include/pstl/include/algorithm_fwd.h

The pattern of "foo/include/foo/" ends up being pretty common in codebases I've seen, because there'll be a "foo/src" and a "foo/include", and then in order to make #include <foo/bar.h> to work after -I foo/include, you need to put your actual header file in "foo/include/foo/bar.h".

[rodgert]

pstl/include/pstl/internal/execution_impl.h

39–42

At least as it relates to libstdc++'s consumption of pstl, C++17 is the baseline expectation.

[rarutyun]

pstl/include/pstl/internal/algorithm_fwd.h

255

100% agree. Done

pstl/include/pstl/internal/execution_impl.h

31–33

There reason of decltype was SFINAE. iterator_category is not always the part of iterator_traits. I thought it might be useful when we adopt take this approach from PSTL upstream to oneDPL.

On the other hand, we don't have much to do when the passed types by users are not iterators. So, yeah, let's keep it simple. Probably if we need additional logic in our code we can make it in other place.

39–42

Or if C++17 is permitted, then

Sure, C++17 is permitted. std::conjunction is already C++17 API.

IIUC, the intent is that __are_random_access_iterators<int*, char*> is true_type but __are_random_access_iterators<int*, std::list<int>::iterator> is false_type, right?

right.

I think I made it even simpler (at least from my point of view) and I would prefer to leave it as is if no strong objections. I also would prefer to have such a logic separation for __are_random_access_iterators and __are_iterators_of

116–129

It probably would be simpler from searching standpoint but I don't think it's more correct comparing with what we have in the patch. serial_tag should not know about parallelism and backend. I don't tend to think that adding stubs (like void) is a good idea.

Probably I misunderstood the idea but I cannot find the applicability for using __is_parallel = std::bool_constant<_IsParallel>; line. It seems unnecessary to me because everything that we already have what we need to build the dispatch (I mean that __parallel_tag and __serial_tag are different types in your suggestion). Currently there is no need to read __is_parallel from somewhere.

What I do believe is we need to find better names for tags.

For backend tags I would propose serial_backend_tag and tbb_backend_tag instead of serial_backend and tbb_backend correspondently
For top level tags I would tend to think about parallel_pattern_tag, serial_pattern_tag, but probably "serial" is a bad word and "pattern" word makes the name too verbose. I am open for ideas

[• Quuxplusone]

pstl/include/pstl/internal/execution_impl.h

31–33

IIUC, you were thinking that using the decltype syntax would somehow "enable" SFINAE. That's not the case. SFINAE is happening regardless of the syntax you use.

template<class T> void f() -> decltype(typename T::nested_type{}) {}
template<class T> void f() -> typename T::nested_type {}

Both versions of f SFINAE away correctly when T=int (because substituting int into T::nested_type produces a failure, and substitution failure is not an error). The only thing that is different is that in the former case, we're also SFINAE-checking that T::nested_type{} is well-formed (i.e. T::nested_type is default-constructible from empty braces) whereas in the latter case we are checking only that it exists. (But this distinction shouldn't matter for tag types, which are always default-constructible.)

[rarutyun]

Add tag_dispatching mechanism for OpenMP backend

[rarutyun]

pstl/include/pstl/internal/execution_impl.h

31–33

Yes, you are right. Anyway, we current implementation it is not required anymore.

[rarutyun]

pstl/include/pstl/internal/algorithm_fwd.h

19

I believe you need to add pstl/include to
https://github.com/google/llvm-premerge-checks/blob/main/scripts/clang-tidy.ignore
(Although why this file is in a Google repo instead of the llvm-project repo, I don't know!)

I believe we will do that (if necessary) when enable CI for Parallel STL (which is one of the next steps after merging this patch). Currently I propose to just merge this changes ignoring clang-tidy (as it was done for previous patches as well)

19

    pstl/include/pstl/internal/algorithm_fwd.h
not                   ^^^^^^^^
    pstl/include/pstl/include/algorithm_fwd.h

I agree with @Quuxplusone here

[MikeDvorskiy]

In spite of my minor comments it LGTM.

pstl/include/pstl/internal/algorithm_fwd.h

402

I guess we have wrong clang format changes here. (As far as I remember our profile of clang-format options keeps return type on the separate line).

406

See the comment below.

pstl/include/pstl/internal/algorithm_impl.h

308

As far as I can see, here and in the other same places we don't in use parameter name tag in code, excepting type deduction (via decltype) of type that we already have. So, I would propose to omit this parameter name and define backend_tag as:

using backend_tag = typename parallel_tag<_IsVector>::__backend_tag;

It is not showstopper because it is not public API. So, it is up to you.

[rarutyun]

pstl/include/pstl/internal/algorithm_fwd.h

402

It's already inconsistent within the file. I suggest to make separate commit if we believe it's important. Locally I've applied clang-format for diff and that's the output

406

I guess you mean "above" instead of "below". Anyway, the answer is above:)

pstl/include/pstl/internal/algorithm_impl.h

308

Actually, in some places we use __tag parameter but not everywhere. Although, we could do using backend_tag with duplicating the whole type from template parameter I would prefer to keep it as it is because it (from my point of view) makes the code simpler and `backend_tag` line is not required to be changed if function parameter type is modified. Everything would be correctly deduced automatically would be correctly deduced automaticallyю

[rarutyun]

CI is green, @MikeDvorskiy and @rodgert approved that previously. I believe we can finally merge it.

pstl/include/pstl/internal/algorithm_fwd.h

 #define _PSTL_ALGORITHM_FWD_H
 
 #include <iterator>
 #include <type_traits>
 #include <utility>
 
 #include "pstl_config.h"
 
 _PSTL_HIDE_FROM_ABI_PUSH
 
 namespace __pstl
 {
 namespace __internal
 {
 
 //------------------------------------------------------------------------
 // any_of
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Pred>
 bool
 __brick_any_of(const _ForwardIterator, const _ForwardIterator, _Pred,
                /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Pred>
 bool
 __brick_any_of(const _RandomAccessIterator, const _RandomAccessIterator, _Pred,
                /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Pred, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Pred>
 bool
-__pattern_any_of(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Pred, _IsVector,
-                 /*parallel=*/std::false_type) noexcept;
+__pattern_any_of(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Pred) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Pred, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Pred>
 bool
-__pattern_any_of(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Pred, _IsVector,
-                 /*parallel=*/std::true_type);
+__pattern_any_of(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Pred);
 
 //------------------------------------------------------------------------
 // walk1 (pseudo)
 //
 // walk1 evaluates f(x) for each dereferenced value x drawn from [first,last)
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Function>
 void __brick_walk1(_ForwardIterator, _ForwardIterator, _Function,
                    /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Function>
 void __brick_walk1(_RandomAccessIterator, _RandomAccessIterator, _Function,
                    /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Function>
 void
-__pattern_walk1(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Function, _IsVector,
-                /*parallel=*/std::false_type) noexcept;
+__pattern_walk1(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Function) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Function>
 void
-__pattern_walk1(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Function, _IsVector,
-                /*parallel=*/std::true_type);
+__pattern_walk1(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Function);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Brick>
 void
-__pattern_walk_brick(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Brick,
-                     /*parallel=*/std::false_type) noexcept;
+__pattern_walk_brick(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Brick) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Brick>
 void
-__pattern_walk_brick(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Brick,
-                     /*parallel=*/std::true_type);
+__pattern_walk_brick(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                     _Brick);
 
 //------------------------------------------------------------------------
 // walk1_n
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Size, class _Function>
 _ForwardIterator __brick_walk1_n(_ForwardIterator, _Size, _Function,
                                  /*_IsVectorTag=*/std::false_type);
 
 template <class _RandomAccessIterator, class _DifferenceType, class _Function>
 _RandomAccessIterator __brick_walk1_n(_RandomAccessIterator, _DifferenceType, _Function,
                                       /*vectorTag=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Function>
 _ForwardIterator
-__pattern_walk1_n(_ExecutionPolicy&&, _ForwardIterator, _Size, _Function, _IsVector,
-                  /*is_parallel=*/std::false_type) noexcept;
+__pattern_walk1_n(_Tag, _ExecutionPolicy&&, _ForwardIterator, _Size, _Function) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Function>
 _RandomAccessIterator
-__pattern_walk1_n(_ExecutionPolicy&&, _RandomAccessIterator, _Size, _Function, _IsVector,
-                  /*is_parallel=*/std::true_type);
+__pattern_walk1_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _Size, _Function);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Brick>
 _ForwardIterator
-__pattern_walk_brick_n(_ExecutionPolicy&&, _ForwardIterator, _Size, _Brick,
-                       /*is_parallel=*/std::false_type) noexcept;
+__pattern_walk_brick_n(_Tag, _ExecutionPolicy&&, _ForwardIterator, _Size, _Brick) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Brick>
 _RandomAccessIterator
-__pattern_walk_brick_n(_ExecutionPolicy&&, _RandomAccessIterator, _Size, _Brick,
-                       /*is_parallel=*/std::true_type);
+__pattern_walk_brick_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _Size, _Brick);
 
 //------------------------------------------------------------------------
 // walk2 (pseudo)
 //
 // walk2 evaluates f(x,y) for deferenced values (x,y) drawn from [first1,last1) and [first2,...)
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _Function>
 _ForwardIterator2 __brick_walk2(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Function,
                                 /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _Function>
 _RandomAccessIterator2 __brick_walk2(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _Function,
                                      /*vector=*/std::true_type) noexcept;
 
 template <class _ForwardIterator1, class _Size, class _ForwardIterator2, class _Function>
 _ForwardIterator2 __brick_walk2_n(_ForwardIterator1, _Size, _ForwardIterator2, _Function,
                                   /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2, class _Function>
 _RandomAccessIterator2 __brick_walk2_n(_RandomAccessIterator1, _Size, _RandomAccessIterator2, _Function,
                                        /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Function>
 _ForwardIterator2
-__pattern_walk2(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Function, _IsVector,
-                /*parallel=*/std::false_type) noexcept;
+__pattern_walk2(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Function) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Function,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Function>
 _RandomAccessIterator2
-__pattern_walk2(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _Function,
-                _IsVector,
-                /*parallel=*/std::true_type);
+__pattern_walk2(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                _RandomAccessIterator2, _Function);
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2, class _Function,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2,
+          class _Function>
 _ForwardIterator2
-__pattern_walk2_n(_ExecutionPolicy&&, _ForwardIterator1, _Size, _ForwardIterator2, _Function, _IsVector,
-                  /*parallel=*/std::false_type) noexcept;
+__pattern_walk2_n(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _Size, _ForwardIterator2, _Function) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2,
-          class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _Size,
+          class _RandomAccessIterator2, class _Function>
 _RandomAccessIterator2
-__pattern_walk2_n(_ExecutionPolicy&&, _RandomAccessIterator1, _Size, _RandomAccessIterator2, _Function, _IsVector,
-                  /*parallel=*/std::true_type);
+__pattern_walk2_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _Size, _RandomAccessIterator2,
+                  _Function);
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Brick>
 _ForwardIterator2
-__pattern_walk2_brick(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Brick,
-                      /*parallel=*/std::false_type) noexcept;
+__pattern_walk2_brick(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                      _Brick) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Brick>
 _RandomAccessIterator2
-__pattern_walk2_brick(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                      _Brick,
-                      /*parallel=*/std::true_type);
+__pattern_walk2_brick(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                      _RandomAccessIterator2, _Brick);
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2,
+          class _Brick>
 _ForwardIterator2
-__pattern_walk2_brick_n(_ExecutionPolicy&&, _ForwardIterator1, _Size, _ForwardIterator2, _Brick,
-                        /*parallel=*/std::false_type) noexcept;
+__pattern_walk2_brick_n(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _Size, _ForwardIterator2, _Brick) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _Size,
+          class _RandomAccessIterator2, class _Brick>
 _RandomAccessIterator2
-__pattern_walk2_brick_n(_ExecutionPolicy&&, _RandomAccessIterator1, _Size, _RandomAccessIterator2, _Brick,
-                        /*parallel=*/std::true_type);
+__pattern_walk2_brick_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _Size,
+                        _RandomAccessIterator2, _Brick);
 
 //------------------------------------------------------------------------
 // walk3 (pseudo)
 //
 // walk3 evaluates f(x,y,z) for (x,y,z) drawn from [first1,last1), [first2,...), [first3,...)
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3, class _Function>
 _ForwardIterator3 __brick_walk3(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator3, _Function,
                                 /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _RandomAccessIterator3, class _Function>
 _RandomAccessIterator3 __brick_walk3(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                      _RandomAccessIterator3, _Function,
                                      /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3,
-          class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3,
+          class _Function>
 _ForwardIterator3
-__pattern_walk3(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator3,
-                _Function, _IsVector,
-                /*parallel=*/std::false_type) noexcept;
+__pattern_walk3(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator3,
+                _Function) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Function>
 _RandomAccessIterator3
-__pattern_walk3(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                _RandomAccessIterator3, _Function, _IsVector, /*parallel=*/std::true_type);
+__pattern_walk3(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                _RandomAccessIterator2, _RandomAccessIterator3, _Function);
 
 //------------------------------------------------------------------------
 // equal
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool __brick_equal(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _BinaryPredicate,
                    /* is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 bool __brick_equal(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _BinaryPredicate,
                    /* is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _BinaryPredicate,
-                _IsVector, /* is_parallel = */ std::false_type) noexcept;
+__pattern_equal(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                _BinaryPredicate, _IsVector, /* is_parallel = */ std::true_type);
+__pattern_equal(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                _RandomAccessIterator2, _BinaryPredicate);
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool __brick_equal(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2, _BinaryPredicate,
                    /* is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 bool __brick_equal(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2,
                    _BinaryPredicate, /* is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                _BinaryPredicate, _IsVector, /* is_parallel = */ std::false_type) noexcept;
+__pattern_equal(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                _RandomAccessIterator2, _BinaryPredicate, _IsVector, /* is_parallel = */ std::true_type);
+__pattern_equal(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                _RandomAccessIterator2, _RandomAccessIterator2, _BinaryPredicate);
 
 //------------------------------------------------------------------------
 // find_if
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Predicate>
 _ForwardIterator __brick_find_if(_ForwardIterator, _ForwardIterator, _Predicate,
                                  /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Predicate>
 _RandomAccessIterator __brick_find_if(_RandomAccessIterator, _RandomAccessIterator, _Predicate,
                                       /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 _ForwardIterator
-__pattern_find_if(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Predicate, _IsVector,
-                  /*is_parallel=*/std::false_type) noexcept;
+__pattern_find_if(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Predicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate>
 _RandomAccessIterator
-__pattern_find_if(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Predicate, _IsVector,
-                  /*is_parallel=*/std::true_type);
+__pattern_find_if(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                  _Predicate);
 
 //------------------------------------------------------------------------
 // find_end
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1 __brick_find_end(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                    _BinaryPredicate,
                                    /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1 __brick_find_end(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                         _RandomAccessIterator2, _BinaryPredicate,
                                         /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_find_end(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                   _BinaryPredicate, _IsVector,
-                   /*is_parallel=*/std::false_type) noexcept;
+__pattern_find_end(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                   _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_find_end(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                   _RandomAccessIterator2, _BinaryPredicate, _IsVector,
-                   /*is_parallel=*/std::true_type) noexcept;
+__pattern_find_end(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                   _RandomAccessIterator2, _RandomAccessIterator2, _BinaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // find_first_of
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1 __brick_find_first_of(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                         _BinaryPredicate,
                                         /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1 __brick_find_first_of(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                              _RandomAccessIterator2, _BinaryPredicate,
                                              /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_find_first_of(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                        _BinaryPredicate, _IsVector, /*is_parallel=*/std::false_type) noexcept;
+__pattern_find_first_of(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                        _ForwardIterator2, _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_find_first_of(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                        _RandomAccessIterator2, _BinaryPredicate, _IsVector, /*is_parallel=*/std::true_type) noexcept;
+__pattern_find_first_of(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                        _RandomAccessIterator2, _RandomAccessIterator2, _BinaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // search
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1 __brick_search(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                  _BinaryPredicate,
                                  /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1 __brick_search(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                       _RandomAccessIterator2, _BinaryPredicate,
                                       /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_search(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                 _BinaryPredicate, _IsVector,
-                 /*is_parallel=*/std::false_type) noexcept;
+__pattern_search(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                 _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_search(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                 _RandomAccessIterator2, _BinaryPredicate, _IsVector,
-                 /*is_parallel=*/std::true_type) noexcept;
+__pattern_search(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                 _RandomAccessIterator2, _RandomAccessIterator2, _BinaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // search_n
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
 _ForwardIterator
 __brick_search_n(_ForwardIterator, _ForwardIterator, _Size, const _Tp&, _BinaryPredicate,
                  /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Size, class _Tp, class _BinaryPredicate>
 _RandomAccessIterator
 __brick_search_n(_RandomAccessIterator, _RandomAccessIterator, _Size, const _Tp&, _BinaryPredicate,
                  /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate,
-          class IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
 _ForwardIterator
-__pattern_search_n(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Size, const _Tp&, _BinaryPredicate,
-                   IsVector,
-                   /*is_parallel=*/std::false_type) noexcept;
+__pattern_search_n(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Size, const _Tp&,
+                   _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp, class _BinaryPredicate,
-          class IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp,
+          class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_search_n(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Size, const _Tp&,
-                   _BinaryPredicate, IsVector,
-                   /*is_parallel=*/std::true_type) noexcept;
+__pattern_search_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Size,
+                   const _Tp&, _BinaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // copy_n
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _Size, class _OutputIterator>
 _OutputIterator __brick_copy_n(_ForwardIterator, _Size, _OutputIterator,
                                /*vector=*/std::false_type) noexcept;

 template <class _RandomAccessIterator, class _OutputIterator>
 _OutputIterator __brick_move(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
                              /*vector=*/std::true_type) noexcept;
 
 //------------------------------------------------------------------------
 // swap_ranges
 //------------------------------------------------------------------------
 template <class _ForwardIterator, class _OutputIterator>
-_OutputIterator
-__brick_swap_ranges(_ForwardIterator __first, _ForwardIterator __last, _OutputIterator __result,
+_OutputIterator __brick_swap_ranges(_ForwardIterator, _ForwardIterator, _OutputIterator,
                                     /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator>
-_OutputIterator
-__brick_swap_ranges(_RandomAccessIterator __first, _RandomAccessIterator __last, _OutputIterator __result,
+_OutputIterator __brick_swap_ranges(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
                                     /*vector=*/std::true_type) noexcept;
 
 //------------------------------------------------------------------------
 // copy_if
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _OutputIterator, class _UnaryPredicate>
 _OutputIterator __brick_copy_if(_ForwardIterator, _ForwardIterator, _OutputIterator, _UnaryPredicate,
                                 /*vector=*/std::false_type) noexcept;

 __brick_partition_by_mask(_ForwardIterator, _ForwardIterator, _OutputIterator1, _OutputIterator2, bool*,
                           /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator1, class _OutputIterator2>
 void
 __brick_partition_by_mask(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator1, _OutputIterator2, bool*,
                           /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryPredicate>
 _OutputIterator
-__pattern_copy_if(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator, _UnaryPredicate, _IsVector,
-                  /*parallel=*/std::false_type) noexcept;
+__pattern_copy_if(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator,
+                  _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _UnaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _UnaryPredicate>
 _OutputIterator
-__pattern_copy_if(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator, _UnaryPredicate,
-                  _IsVector, /*parallel=*/std::true_type);
+__pattern_copy_if(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                  _OutputIterator, _UnaryPredicate);
 
 //------------------------------------------------------------------------
 // count
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _Predicate>
 typename std::iterator_traits<_RandomAccessIterator>::difference_type
     __brick_count(_RandomAccessIterator, _RandomAccessIterator, _Predicate,
                   /* is_vector = */ std::true_type) noexcept;
 
 template <class _ForwardIterator, class _Predicate>
 typename std::iterator_traits<_ForwardIterator>::difference_type
     __brick_count(_ForwardIterator, _ForwardIterator, _Predicate,
                   /* is_vector = */ std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 typename std::iterator_traits<_ForwardIterator>::difference_type
-__pattern_count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Predicate,
-                /* is_parallel */ std::false_type, _IsVector) noexcept;
+__pattern_count(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Predicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate>
 typename std::iterator_traits<_RandomAccessIterator>::difference_type
-__pattern_count(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Predicate,
-                /* is_parallel */ std::true_type, _IsVector);
+__pattern_count(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                _Predicate);
 
 //------------------------------------------------------------------------
 // unique
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator __brick_unique(_ForwardIterator, _ForwardIterator, _BinaryPredicate,
                                 /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator __brick_unique(_RandomAccessIterator, _RandomAccessIterator, _BinaryPredicate,
                                      /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
-__pattern_unique(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _BinaryPredicate, _IsVector,
-                 /*is_parallel=*/std::false_type) noexcept;
+__pattern_unique(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _BinaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_unique(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _BinaryPredicate, _IsVector,
-                 /*is_parallel=*/std::true_type) noexcept;
+__pattern_unique(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                 _BinaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // unique_copy
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class OutputIterator, class _BinaryPredicate>
 OutputIterator __brick_unique_copy(_ForwardIterator, _ForwardIterator, OutputIterator, _BinaryPredicate,
                                    /*vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator, class _BinaryPredicate>
 _OutputIterator __brick_unique_copy(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator, _BinaryPredicate,
                                     /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryPredicate>
 _OutputIterator
-__pattern_unique_copy(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator, _BinaryPredicate,
-                      _IsVector, /*parallel=*/std::false_type) noexcept;
+__pattern_unique_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator,
+                      _BinaryPredicate) noexcept;
 
 template <class _ExecutionPolicy, class _DifferenceType, class _RandomAccessIterator, class _BinaryPredicate>
 _DifferenceType
 __brick_calc_mask_2(_RandomAccessIterator, _RandomAccessIterator, bool* __restrict, _BinaryPredicate,
                     /*vector=*/std::false_type) noexcept;
 
 template <class _DifferenceType, class _RandomAccessIterator, class _BinaryPredicate>
 _DifferenceType
 __brick_calc_mask_2(_RandomAccessIterator, _RandomAccessIterator, bool* __restrict, _BinaryPredicate,
                     /*vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _BinaryPredicate>
 _OutputIterator
-__pattern_unique_copy(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
-                      _BinaryPredicate, _IsVector, /*parallel=*/std::true_type);
+__pattern_unique_copy(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                      _OutputIterator, _BinaryPredicate);
 
 //------------------------------------------------------------------------
 // reverse
 //------------------------------------------------------------------------
 
 template <class _BidirectionalIterator>
 void __brick_reverse(_BidirectionalIterator, _BidirectionalIterator,
                      /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator>
 void __brick_reverse(_RandomAccessIterator, _RandomAccessIterator,
                      /*__is_vector=*/std::true_type) noexcept;
 
 template <class _BidirectionalIterator>
 void __brick_reverse(_BidirectionalIterator, _BidirectionalIterator, _BidirectionalIterator,
                      /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator>
 void __brick_reverse(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
                      /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator>
 void
-__pattern_reverse(_ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator, _IsVector,
-                  /*is_parallel=*/std::false_type) noexcept;
+__pattern_reverse(_Tag, _ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator>
 void
-__pattern_reverse(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _IsVector,
-                  /*is_parallel=*/std::true_type);
+__pattern_reverse(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator);
 
 //------------------------------------------------------------------------
 // reverse_copy
 //------------------------------------------------------------------------
 
 template <class _BidirectionalIterator, class _OutputIterator>
 _OutputIterator __brick_reverse_copy(_BidirectionalIterator, _BidirectionalIterator, _OutputIterator,
                                      /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator>
 _OutputIterator __brick_reverse_copy(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
                                      /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _OutputIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _OutputIterator>
 _OutputIterator
-__pattern_reverse_copy(_ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator, _OutputIterator, _IsVector,
-                       /*is_parallel=*/std::false_type) noexcept;
+__pattern_reverse_copy(_Tag, _ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator,
+                       _OutputIterator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator>
 _OutputIterator
-__pattern_reverse_copy(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator, _IsVector,
-                       /*is_parallel=*/std::true_type);
+__pattern_reverse_copy(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                       _OutputIterator);
 
 //------------------------------------------------------------------------
 // rotate
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator>
 _ForwardIterator __brick_rotate(_ForwardIterator, _ForwardIterator, _ForwardIterator,
                                 /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator>
 _RandomAccessIterator __brick_rotate(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
                                      /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator>
 _ForwardIterator
-__pattern_rotate(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _ForwardIterator, _IsVector,
-                 /*is_parallel=*/std::false_type) noexcept;
+__pattern_rotate(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _ForwardIterator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator>
 _RandomAccessIterator
-__pattern_rotate(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _IsVector,
-                 /*is_parallel=*/std::true_type);
+__pattern_rotate(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                 _RandomAccessIterator);
 
 //------------------------------------------------------------------------
 // rotate_copy
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _OutputIterator>
 _OutputIterator __brick_rotate_copy(_ForwardIterator, _ForwardIterator, _ForwardIterator, _OutputIterator,
                                     /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator>
 _OutputIterator __brick_rotate_copy(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
                                     _OutputIterator,
                                     /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator>
 _OutputIterator
-__pattern_rotate_copy(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _ForwardIterator, _OutputIterator,
-                      _IsVector,
-                      /*is_parallel=*/std::false_type) noexcept;
+__pattern_rotate_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _ForwardIterator,
+                      _OutputIterator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator>
 _OutputIterator
-__pattern_rotate_copy(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
-                      _OutputIterator, _IsVector,
-                      /*is_parallel=*/std::true_type);
+__pattern_rotate_copy(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                      _RandomAccessIterator, _OutputIterator);
 
 //------------------------------------------------------------------------
 // is_partitioned
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _UnaryPredicate>
 bool __brick_is_partitioned(_ForwardIterator, _ForwardIterator, _UnaryPredicate,
                             /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _UnaryPredicate>
 bool __brick_is_partitioned(_RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate,
                             /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 bool
-__pattern_is_partitioned(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate, _IsVector,
-                         /*is_parallel=*/std::false_type) noexcept;
+__pattern_is_partitioned(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 bool
-__pattern_is_partitioned(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate, _IsVector,
-                         /*is_parallel=*/std::true_type);
+__pattern_is_partitioned(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                         _UnaryPredicate);
 
 //------------------------------------------------------------------------
 // partition
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator __brick_partition(_ForwardIterator, _ForwardIterator, _UnaryPredicate,
                                    /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator __brick_partition(_RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate,
                                         /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator
-__pattern_partition(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate, _IsVector,
-                    /*is_parallel=*/std::false_type) noexcept;
+__pattern_partition(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_partition(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate, _IsVector,
-                    /*is_parallel=*/std::true_type);
+__pattern_partition(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                    _UnaryPredicate);
 
 //------------------------------------------------------------------------
 // stable_partition
 //------------------------------------------------------------------------
 
 template <class _BidirectionalIterator, class _UnaryPredicate>
 _BidirectionalIterator __brick_stable_partition(_BidirectionalIterator, _BidirectionalIterator, _UnaryPredicate,
                                                 /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator __brick_stable_partition(_RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate,
                                                /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _UnaryPredicate>
 _BidirectionalIterator
-__pattern_stable_partition(_ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator, _UnaryPredicate,
-                           _IsVector,
-                           /*is_parallelization=*/std::false_type) noexcept;
+__pattern_stable_partition(_Tag, _ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator,
+                           _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_stable_partition(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate, _IsVector,
-                           /*is_parallelization=*/std::true_type) noexcept;
+__pattern_stable_partition(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                           _UnaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // partition_copy
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _OutputIterator1, class _OutputIterator2, class _UnaryPredicate>
 std::pair<_OutputIterator1, _OutputIterator2>
     __brick_partition_copy(_ForwardIterator, _ForwardIterator, _OutputIterator1, _OutputIterator2, _UnaryPredicate,
                            /*is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator1, class _OutputIterator2, class _UnaryPredicate>
 std::pair<_OutputIterator1, _OutputIterator2> __brick_partition_copy(_RandomAccessIterator, _RandomAccessIterator,
                                                                      _OutputIterator1, _OutputIterator2,
                                                                      _UnaryPredicate,
                                                                      /*is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator1, class _OutputIterator2,
-          class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator1, class _OutputIterator2,
+          class _UnaryPredicate>
 std::pair<_OutputIterator1, _OutputIterator2>
-__pattern_partition_copy(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator1, _OutputIterator2,
-                         _UnaryPredicate, _IsVector,
-                         /*is_parallelization=*/std::false_type) noexcept;
+__pattern_partition_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator1,
+                         _OutputIterator2, _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator1, class _OutputIterator2,
-          class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator1,
+          class _OutputIterator2, class _UnaryPredicate>
 std::pair<_OutputIterator1, _OutputIterator2>
-__pattern_partition_copy(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator1,
-                         _OutputIterator2, _UnaryPredicate, _IsVector,
-                         /*is_parallelization=*/std::true_type);
+__pattern_partition_copy(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                         _OutputIterator1, _OutputIterator2, _UnaryPredicate);
 
 //------------------------------------------------------------------------
 // sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector,
-          class _IsMoveConstructible>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsMoveConstructible>
 void
-__pattern_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector /*is_vector*/,
-               /*is_parallel=*/std::false_type, _IsMoveConstructible) noexcept;
+__pattern_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare,
+               _IsMoveConstructible) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector /*is_vector*/,
-               /*is_parallel=*/std::true_type,
+__pattern_sort(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare,
                /*is_move_constructible=*/std::true_type);
 
 //------------------------------------------------------------------------
 // stable_sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_stable_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare,
-                      _IsVector /*is_vector*/,
-                      /*is_parallel=*/std::false_type) noexcept;
+__pattern_stable_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_stable_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare,
-                      _IsVector /*is_vector*/,
-                      /*is_parallel=*/std::true_type);
+__pattern_stable_sort(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                      _Compare);
 
 //------------------------------------------------------------------------
 // partial_sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_partial_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
-                       _Compare, _IsVector,
-                       /*is_parallel=*/std::false_type) noexcept;
+__pattern_partial_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
+                       _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_partial_sort(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
-                       _Compare, _IsVector,
-                       /*is_parallel=*/std::true_type);
+__pattern_partial_sort(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                       _RandomAccessIterator, _Compare);
 
 //------------------------------------------------------------------------
 // partial_sort_copy
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 _RandomAccessIterator2
-__pattern_partial_sort_copy(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                            _RandomAccessIterator2, _Compare, _IsVector,
-                            /*is_parallel=*/std::false_type) noexcept;
+__pattern_partial_sort_copy(_Tag, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                            _RandomAccessIterator2, _RandomAccessIterator2, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 _RandomAccessIterator2
-__pattern_partial_sort_copy(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                            _RandomAccessIterator2, _Compare, _IsVector,
-                            /*is_parallel=*/std::true_type);
+__pattern_partial_sort_copy(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1,
+                            _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2, _Compare);
 
 //------------------------------------------------------------------------
 // adjacent_find
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator
 __brick_adjacent_find(_RandomAccessIterator, _RandomAccessIterator, _BinaryPredicate,
                       /* IsVector = */ std::true_type, bool) noexcept;
 
 template <class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
 __brick_adjacent_find(_ForwardIterator, _ForwardIterator, _BinaryPredicate,
                       /* IsVector = */ std::false_type, bool) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
-__pattern_adjacent_find(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _BinaryPredicate,
-                        /* is_parallel */ std::false_type, _IsVector, bool) noexcept;
+__pattern_adjacent_find(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _BinaryPredicate, bool) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_adjacent_find(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _BinaryPredicate,
-                        /* is_parallel */ std::true_type, _IsVector, bool);
+__pattern_adjacent_find(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                        _BinaryPredicate, bool);
 
 //------------------------------------------------------------------------
 // nth_element
 //------------------------------------------------------------------------
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_nth_element(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare,
-                      _IsVector,
-                      /*is_parallel=*/std::false_type) noexcept;
+__pattern_nth_element(_Tag, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
+                      _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_nth_element(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare,
-                      _IsVector,
-                      /*is_parallel=*/std::true_type) noexcept;
+__pattern_nth_element(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                      _RandomAccessIterator, _Compare) noexcept;
 
 //------------------------------------------------------------------------
 // fill, fill_n
 //------------------------------------------------------------------------
 template <class _RandomAccessIterator, class _Tp>
 void
 __brick_fill(_RandomAccessIterator, _RandomAccessIterator, const _Tp&,
              /* __is_vector = */ std::true_type) noexcept;
 
 template <class _ForwardIterator, class _Tp>
 void
 __brick_fill(_ForwardIterator, _ForwardIterator, const _Tp&,
              /* __is_vector = */ std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 void
-__pattern_fill(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&,
-               /*is_parallel=*/std::false_type, _IsVector) noexcept;
+__pattern_fill(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Tp>
 _RandomAccessIterator
-__pattern_fill(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, const _Tp&,
-               /*is_parallel=*/std::true_type, _IsVector);
+__pattern_fill(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, const _Tp&);
 
 template <class _RandomAccessIterator, class _Size, class _Tp>
 _RandomAccessIterator
 __brick_fill_n(_RandomAccessIterator, _Size, const _Tp&,
                /* __is_vector = */ std::true_type) noexcept;
 
 template <class _OutputIterator, class _Size, class _Tp>
 _OutputIterator
 __brick_fill_n(_OutputIterator, _Size, const _Tp&,
                /* __is_vector = */ std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _OutputIterator, class _Size, class _Tp, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _OutputIterator, class _Size, class _Tp>
 _OutputIterator
-__pattern_fill_n(_ExecutionPolicy&&, _OutputIterator, _Size, const _Tp&,
-                 /*is_parallel=*/std::false_type, _IsVector) noexcept;
+__pattern_fill_n(_Tag, _ExecutionPolicy&&, _OutputIterator, _Size, const _Tp&) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp>
 _RandomAccessIterator
-__pattern_fill_n(_ExecutionPolicy&&, _RandomAccessIterator, _Size, const _Tp&,
-                 /*is_parallel=*/std::true_type, _IsVector);
+__pattern_fill_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _Size, const _Tp&);
 
 //------------------------------------------------------------------------
 // generate, generate_n
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _Generator>
 void __brick_generate(_RandomAccessIterator, _RandomAccessIterator, _Generator,
                       /* is_vector = */ std::true_type) noexcept;
 
 template <class _ForwardIterator, class _Generator>
 void __brick_generate(_ForwardIterator, _ForwardIterator, _Generator,
                       /* is_vector = */ std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Generator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Generator>
 void
-__pattern_generate(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Generator,
-                   /*is_parallel=*/std::false_type, _IsVector) noexcept;
+__pattern_generate(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Generator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Generator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Generator>
 _RandomAccessIterator
-__pattern_generate(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Generator,
-                   /*is_parallel=*/std::true_type, _IsVector);
+__pattern_generate(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                   _Generator);
 
 template <class _RandomAccessIterator, class Size, class _Generator>
 _RandomAccessIterator __brick_generate_n(_RandomAccessIterator, Size, _Generator,
                                          /* is_vector = */ std::true_type) noexcept;
 
 template <class OutputIterator, class Size, class _Generator>
 OutputIterator __brick_generate_n(OutputIterator, Size, _Generator,
                                   /* is_vector = */ std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class OutputIterator, class Size, class _Generator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class OutputIterator, class Size, class _Generator>
 OutputIterator
-__pattern_generate_n(_ExecutionPolicy&&, OutputIterator, Size, _Generator,
-                     /*is_parallel=*/std::false_type, _IsVector) noexcept;
+__pattern_generate_n(_Tag, _ExecutionPolicy&&, OutputIterator, Size, _Generator) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class Size, class _Generator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class Size, class _Generator>
 _RandomAccessIterator
-__pattern_generate_n(_ExecutionPolicy&&, _RandomAccessIterator, Size, _Generator,
-                     /*is_parallel=*/std::true_type, _IsVector);
+__pattern_generate_n(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, Size, _Generator);
 
 //------------------------------------------------------------------------
 // remove
 //------------------------------------------------------------------------
 template <class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator __brick_remove_if(_ForwardIterator, _ForwardIterator, _UnaryPredicate,
                                    /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator __brick_remove_if(_RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate,
                                         /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator
-__pattern_remove_if(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate, _IsVector,
-                    /*is_parallel*/ std::false_type) noexcept;
+__pattern_remove_if(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _UnaryPredicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_remove_if(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _UnaryPredicate, _IsVector,
-                    /*is_parallel*/ std::true_type) noexcept;
+__pattern_remove_if(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                    _UnaryPredicate) noexcept;
 
 //------------------------------------------------------------------------
 // merge
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_merge(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                               _OutputIterator, _Compare,
                               /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_merge(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                               _RandomAccessIterator2, _OutputIterator, _Compare,
                               /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_merge(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                _OutputIterator, _Compare, _IsVector, /* is_parallel = */ std::false_type) noexcept;
+__pattern_merge(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                _OutputIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_merge(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                _RandomAccessIterator2, _OutputIterator, _Compare, _IsVector,
-                /* is_parallel = */ std::true_type);
+__pattern_merge(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                _RandomAccessIterator2, _RandomAccessIterator2, _OutputIterator, _Compare);
 
 //------------------------------------------------------------------------
 // inplace_merge
 //------------------------------------------------------------------------
 
 template <class _BidirectionalIterator, class _Compare>
 void __brick_inplace_merge(_BidirectionalIterator, _BidirectionalIterator, _BidirectionalIterator, _Compare,
                            /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Compare>
 void __brick_inplace_merge(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare,
                            /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _Compare>
 void
-__pattern_inplace_merge(_ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator, _BidirectionalIterator,
-                        _Compare, _IsVector,
-                        /* is_parallel = */ std::false_type) noexcept;
+__pattern_inplace_merge(_Tag, _ExecutionPolicy&&, _BidirectionalIterator, _BidirectionalIterator,
+                        _BidirectionalIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_inplace_merge(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator,
-                        _Compare, _IsVector,
-                        /*is_parallel=*/std::true_type);
+__pattern_inplace_merge(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                        _RandomAccessIterator, _Compare);
 
 //------------------------------------------------------------------------
 // includes
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 bool
-__pattern_includes(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                   _Compare, _IsVector,
-                   /*is_parallel=*/std::false_type) noexcept;
+__pattern_includes(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                   _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 bool
-__pattern_includes(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                   _RandomAccessIterator2, _Compare, _IsVector,
-                   /*is_parallel=*/std::true_type);
+__pattern_includes(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                   _RandomAccessIterator2, _RandomAccessIterator2, _Compare);
 
 //------------------------------------------------------------------------
 // set_union
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_union(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                   _OutputIterator, _Compare,
                                   /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_union(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                   _RandomAccessIterator2, _OutputIterator, _Compare,
                                   /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_union(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                    _OutputIterator, _Compare, _IsVector, /*is_parallel=*/std::false_type) noexcept;
+__pattern_set_union(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                    _ForwardIterator2, _OutputIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_set_union(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                    _RandomAccessIterator2, _OutputIterator, _Compare, _IsVector, /*is_parallel=*/std::true_type);
+__pattern_set_union(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                    _RandomAccessIterator2, _RandomAccessIterator2, _OutputIterator, _Compare);
 
 //------------------------------------------------------------------------
 // set_intersection
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_intersection(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                          _OutputIterator, _Compare,
                                          /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_intersection(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                          _RandomAccessIterator2, _OutputIterator, _Compare,
                                          /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_intersection(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
-                           _ForwardIterator2, _OutputIterator, _Compare, _IsVector,
-                           /*is_parallel=*/std::false_type) noexcept;
+__pattern_set_intersection(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                           _ForwardIterator2, _OutputIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_set_intersection(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                           _RandomAccessIterator2, _OutputIterator, _Compare, _IsVector,
-                           /*is_parallel=*/std::true_type);
+__pattern_set_intersection(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1,
+                           _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2, _OutputIterator,
+                           _Compare);
 
 //------------------------------------------------------------------------
 // set_difference
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_difference(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                        _OutputIterator, _Compare,
                                        /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_difference(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                        _RandomAccessIterator2, _OutputIterator, _Compare,
                                        /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_difference(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                         _OutputIterator, _Compare, _IsVector, /*is_parallel=*/std::false_type) noexcept;
+__pattern_set_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                         _ForwardIterator2, _OutputIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_set_difference(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                         _RandomAccessIterator2, _OutputIterator, _Compare, _IsVector, /*is_parallel=*/std::true_type);
+__pattern_set_difference(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                         _RandomAccessIterator2, _RandomAccessIterator2, _OutputIterator, _Compare);
 
 //------------------------------------------------------------------------
 // set_symmetric_difference
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_symmetric_difference(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
                                                  _ForwardIterator2, _OutputIterator, _Compare,
                                                  /*__is_vector=*/std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator, class _Compare>
 _OutputIterator __brick_set_symmetric_difference(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                                  _RandomAccessIterator2, _OutputIterator, _Compare,
                                                  /*__is_vector=*/std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_symmetric_difference(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
-                                   _ForwardIterator2, _OutputIterator, _Compare, _IsVector,
-                                   /*is_parallel=*/std::false_type) noexcept;
+__pattern_set_symmetric_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                                   _ForwardIterator2, _OutputIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_set_symmetric_difference(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
-                                   _RandomAccessIterator2, _RandomAccessIterator2, _OutputIterator, _Compare, _IsVector,
-                                   /*is_parallel=*/std::true_type);
+__pattern_set_symmetric_difference(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1,
+                                   _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2,
+                                   _OutputIterator, _Compare);
 
 //------------------------------------------------------------------------
 // is_heap_until
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator __brick_is_heap_until(_RandomAccessIterator, _RandomAccessIterator, _Compare,
                                             /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator __brick_is_heap_until(_RandomAccessIterator, _RandomAccessIterator, _Compare,
                                             /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
-__pattern_is_heap_until(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector,
-                        /* is_parallel = */ std::false_type) noexcept;
+__pattern_is_heap_until(_Tag, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
-__pattern_is_heap_until(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector,
-                        /* is_parallel = */ std::true_type) noexcept;
+__pattern_is_heap_until(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                        _Compare) noexcept;
 
 //------------------------------------------------------------------------
 // min_element
 //------------------------------------------------------------------------
 
 template <typename _ForwardIterator, typename _Compare>
 _ForwardIterator __brick_min_element(_ForwardIterator, _ForwardIterator, _Compare,
                                      /* __is_vector = */ std::false_type) noexcept;
 
 template <typename _RandomAccessIterator, typename _Compare>
 _RandomAccessIterator __brick_min_element(_RandomAccessIterator, _RandomAccessIterator, _Compare,
                                           /* __is_vector = */ std::true_type) noexcept;
 
-template <typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare, typename _IsVector>
+template <typename _Tag, typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare>
 _ForwardIterator
-__pattern_min_element(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Compare, _IsVector,
-                      /* is_parallel = */ std::false_type) noexcept;
+__pattern_min_element(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Compare) noexcept;
 
-template <typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _IsVector>
+template <typename _IsVector, typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare>
 _RandomAccessIterator
-__pattern_min_element(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector,
-                      /* is_parallel = */ std::true_type);
+__pattern_min_element(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                      _Compare);
 
 //------------------------------------------------------------------------
 // minmax_element
 //------------------------------------------------------------------------
 
 template <typename _ForwardIterator, typename _Compare>
 std::pair<_ForwardIterator, _ForwardIterator> __brick_minmax_element(_ForwardIterator, _ForwardIterator, _Compare,
                                                                      /* __is_vector = */ std::false_type) noexcept;
 
 template <typename _RandomAccessIterator, typename _Compare>
 std::pair<_RandomAccessIterator, _RandomAccessIterator>
     __brick_minmax_element(_RandomAccessIterator, _RandomAccessIterator, _Compare,
                            /* __is_vector = */ std::true_type) noexcept;
 
-template <typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare, typename _IsVector>
+template <typename _Tag, typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare>
 std::pair<_ForwardIterator, _ForwardIterator>
-__pattern_minmax_element(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Compare, _IsVector,
-                         /* is_parallel = */ std::false_type) noexcept;
+__pattern_minmax_element(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Compare) noexcept;
 
-template <typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _IsVector>
+template <typename _IsVector, typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare>
 std::pair<_RandomAccessIterator, _RandomAccessIterator>
-__pattern_minmax_element(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Compare, _IsVector,
-                         /* is_parallel = */ std::true_type);
+__pattern_minmax_element(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                         _Compare);
 
 //------------------------------------------------------------------------
 // mismatch
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
 std::pair<_ForwardIterator1, _ForwardIterator2> __brick_mismatch(_ForwardIterator1, _ForwardIterator1,
                                                                  _ForwardIterator2, _ForwardIterator2, _Predicate,
                                                                  /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _Predicate>
 std::pair<_RandomAccessIterator1, _RandomAccessIterator2>
     __brick_mismatch(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2,
                      _Predicate,
                      /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
 std::pair<_ForwardIterator1, _ForwardIterator2>
-__pattern_mismatch(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
-                   _Predicate, _IsVector,
-                   /* is_parallel = */ std::false_type) noexcept;
+__pattern_mismatch(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
+                   _Predicate) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Predicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Predicate>
 std::pair<_RandomAccessIterator1, _RandomAccessIterator2>
-__pattern_mismatch(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                   _RandomAccessIterator2, _Predicate, _IsVector, /* is_parallel = */ std::true_type) noexcept;
+__pattern_mismatch(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
+                   _RandomAccessIterator2, _RandomAccessIterator2, _Predicate) noexcept;
 
 //------------------------------------------------------------------------
 // lexicographical_compare
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 bool __brick_lexicographical_compare(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _ForwardIterator2,
                                      _Compare,
                                      /* __is_vector = */ std::false_type) noexcept;
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare>
 bool __brick_lexicographical_compare(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
                                      _RandomAccessIterator2, _Compare,
                                      /* __is_vector = */ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 bool
-__pattern_lexicographical_compare(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
-                                  _ForwardIterator2, _Compare, _IsVector, /* is_parallel = */ std::false_type) noexcept;
+__pattern_lexicographical_compare(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2,
+                                  _ForwardIterator2, _Compare) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 bool
-__pattern_lexicographical_compare(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1,
-                                  _RandomAccessIterator2, _RandomAccessIterator2, _Compare, _IsVector,
-                                  /* is_parallel = */ std::true_type) noexcept;
+__pattern_lexicographical_compare(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1,
+                                  _RandomAccessIterator1, _RandomAccessIterator2, _RandomAccessIterator2,
+                                  _Compare) noexcept;
 
 } // namespace __internal
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_ALGORITHM_FWD_H */

pstl/include/pstl/internal/algorithm_impl.h

 template <class _RandomAccessIterator, class _Pred>
 bool
 __brick_any_of(const _RandomAccessIterator __first, const _RandomAccessIterator __last, _Pred __pred,
                /*__is_vector=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_or(__first, __last - __first, __pred);
 };
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Pred, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Pred>
 bool
-__pattern_any_of(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Pred __pred,
-                 _IsVector __is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_any_of(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Pred __pred) noexcept
 {
-    return __internal::__brick_any_of(__first, __last, __pred, __is_vector);
+    return __internal::__brick_any_of(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Pred, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Pred>
 bool
-__pattern_any_of(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Pred __pred,
-                 _IsVector __is_vector, /*parallel=*/std::true_type)
+__pattern_any_of(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                 _RandomAccessIterator __last, _Pred __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
-        return __internal::__parallel_or(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                         [__pred, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                                             return __internal::__brick_any_of(__i, __j, __pred, __is_vector);
+        return __internal::__parallel_or(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                         [__pred](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                                             return __internal::__brick_any_of(__i, __j, __pred, _IsVector{});
                                          });
     });
 }
 
 // [alg.foreach]
 // for_each_n with no policy
 
 template <class _ForwardIterator, class _Size, class _Function>

 template <class _RandomAccessIterator, class _Function>
 void
 __brick_walk1(_RandomAccessIterator __first, _RandomAccessIterator __last, _Function __f,
               /*vector=*/std::true_type) noexcept
 {
     __unseq_backend::__simd_walk_1(__first, __last - __first, __f);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Function>
 void
-__pattern_walk1(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Function __f,
-                _IsVector __is_vector,
-                /*parallel=*/std::false_type) noexcept
+__pattern_walk1(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Function __f) noexcept
 {
-    __internal::__brick_walk1(__first, __last, __f, __is_vector);
+    __internal::__brick_walk1(__first, __last, __f, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Function>
 void
-__pattern_walk1(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Function __f,
-                _IsVector __is_vector,
-                /*parallel=*/std::true_type)
+__pattern_walk1(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                _RandomAccessIterator __last, _Function __f)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __internal::__except_handler([&]() {
-        __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                      [__f, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                                          __internal::__brick_walk1(__i, __j, __f, __is_vector);
+        __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                      [__f](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                                          __internal::__brick_walk1(__i, __j, __f, _IsVector{});
                                       });
     });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Brick>
 void
-__pattern_walk_brick(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Brick __brick,
-                     /*parallel=*/std::false_type) noexcept
+__pattern_walk_brick(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                     _Brick __brick) noexcept
 {
     __brick(__first, __last);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Brick>
 void
-__pattern_walk_brick(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                     _Brick __brick,
-                     /*parallel=*/std::true_type)
+__pattern_walk_brick(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                     _RandomAccessIterator __last, _Brick __brick)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
             [__brick](_RandomAccessIterator __i, _RandomAccessIterator __j) { __brick(__i, __j); });
     });
 }
 
 //------------------------------------------------------------------------
 // walk1_n
 //------------------------------------------------------------------------
 template <class _ForwardIterator, class _Size, class _Function>
 _ForwardIterator
 __brick_walk1_n(_ForwardIterator __first, _Size __n, _Function __f, /*_IsVectorTag=*/std::false_type)
 {
     return __internal::__for_each_n_it_serial(__first, __n,
                                               [&__f](_ForwardIterator __it) { __f(*__it); }); // calling serial version
 }
 
 template <class _RandomAccessIterator, class _DifferenceType, class _Function>
 _RandomAccessIterator
 __brick_walk1_n(_RandomAccessIterator __first, _DifferenceType __n, _Function __f,
                 /*vectorTag=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_walk_1(__first, __n, __f);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Function>
 _ForwardIterator
-__pattern_walk1_n(_ExecutionPolicy&&, _ForwardIterator __first, _Size __n, _Function __f, _IsVector __is_vector,
-                  /*is_parallel=*/std::false_type) noexcept
+__pattern_walk1_n(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _Size __n, _Function __f) noexcept
 {
-    return __internal::__brick_walk1_n(__first, __n, __f, __is_vector);
+    return __internal::__brick_walk1_n(__first, __n, __f, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Function>
 _RandomAccessIterator
-__pattern_walk1_n(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _Size __n, _Function __f,
-                  _IsVector __is_vector,
-                  /*is_parallel=*/std::true_type)
+__pattern_walk1_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first, _Size __n,
+                  _Function __f)
 {
-    __internal::__pattern_walk1(std::forward<_ExecutionPolicy>(__exec), __first, __first + __n, __f, __is_vector,
-                                std::true_type());
+    __internal::__pattern_walk1(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __first + __n, __f);
+
     return __first + __n;
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Brick>
 _ForwardIterator
-__pattern_walk_brick_n(_ExecutionPolicy&&, _ForwardIterator __first, _Size __n, _Brick __brick,
-                       /*is_parallel=*/std::false_type) noexcept
+__pattern_walk_brick_n(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _Size __n, _Brick __brick) noexcept
 {
     return __brick(__first, __n);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Brick>
 _RandomAccessIterator
-__pattern_walk_brick_n(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _Size __n, _Brick __brick,
-                       /*is_parallel=*/std::true_type)
+__pattern_walk_brick_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                       _Size __n, _Brick __brick)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first, __first + __n,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __first + __n,
             [__brick](_RandomAccessIterator __i, _RandomAccessIterator __j) { __brick(__i, __j - __i); });
         return __first + __n;
     });
 }
 
 //------------------------------------------------------------------------
 // walk2 (pseudo)
 //

 template <class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2, class _Function>
 _RandomAccessIterator2
 __brick_walk2_n(_RandomAccessIterator1 __first1, _Size __n, _RandomAccessIterator2 __first2, _Function __f,
                 /*vector=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_walk_2(__first1, __n, __first2, __f);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Function>
 _ForwardIterator2
-__pattern_walk2(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
-                _Function __f, _IsVector __is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_walk2(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                _ForwardIterator2 __first2, _Function __f) noexcept
 {
-    return __internal::__brick_walk2(__first1, __last1, __first2, __f, __is_vector);
+    return __internal::__brick_walk2(__first1, __last1, __first2, __f, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Function,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Function>
 _RandomAccessIterator2
-__pattern_walk2(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                _RandomAccessIterator2 __first2, _Function __f, _IsVector __is_vector, /*parallel=*/std::true_type)
+__pattern_walk2(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _Function __f)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
-            [__f, __first1, __first2, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
-                __internal::__brick_walk2(__i, __j, __first2 + (__i - __first1), __f, __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            [__f, __first1, __first2](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+                __internal::__brick_walk2(__i, __j, __first2 + (__i - __first1), __f, _IsVector{});
             });
         return __first2 + (__last1 - __first1);
     });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2, class _Function,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2,
+          class _Function>
 _ForwardIterator2
-__pattern_walk2_n(_ExecutionPolicy&&, _ForwardIterator1 __first1, _Size __n, _ForwardIterator2 __first2, _Function __f,
-                  _IsVector is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_walk2_n(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _Size __n, _ForwardIterator2 __first2,
+                  _Function __f) noexcept
 {
-    return __internal::__brick_walk2_n(__first1, __n, __first2, __f, is_vector);
+    return __internal::__brick_walk2_n(__first1, __n, __first2, __f, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2,
-          class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _Size,
+          class _RandomAccessIterator2, class _Function>
 _RandomAccessIterator2
-__pattern_walk2_n(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _Size __n,
-                  _RandomAccessIterator2 __first2, _Function __f, _IsVector is_vector, /*parallel=*/std::true_type)
+__pattern_walk2_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                  _Size __n, _RandomAccessIterator2 __first2, _Function __f)
 {
-    return __internal::__pattern_walk2(std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n, __first2, __f,
-                                       is_vector, std::true_type());
+    return __internal::__pattern_walk2(__tag, std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
+                                       __first2, __f);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Brick>
 _ForwardIterator2
-__pattern_walk2_brick(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                      _ForwardIterator2 __first2, _Brick __brick, /*parallel=*/std::false_type) noexcept
+__pattern_walk2_brick(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                      _ForwardIterator2 __first2, _Brick __brick) noexcept
 {
     return __brick(__first1, __last1, __first2);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Brick>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Brick>
 _RandomAccessIterator2
-__pattern_walk2_brick(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                      _RandomAccessIterator2 __first2, _Brick __brick, /*parallel=*/std::true_type)
+__pattern_walk2_brick(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                      _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _Brick __brick)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
             [__first1, __first2, __brick](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                 __brick(__i, __j, __first2 + (__i - __first1));
             });
         return __first2 + (__last1 - __first1);
     });
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _Size, class _RandomAccessIterator2, class _Brick>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2,
+          class _Brick>
+_ForwardIterator2
+__pattern_walk2_brick_n(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _Size __n, _ForwardIterator2 __first2,
+                        _Brick __brick) noexcept
+{
+    return __brick(__first1, __n, __first2);
+}
+
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _Size,
+          class _RandomAccessIterator2, class _Brick>
 _RandomAccessIterator2
-__pattern_walk2_brick_n(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _Size __n,
-                        _RandomAccessIterator2 __first2, _Brick __brick, /*parallel=*/std::true_type)
+__pattern_walk2_brick_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                        _Size __n, _RandomAccessIterator2 __first2, _Brick __brick)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
             [__first1, __first2, __brick](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                 __brick(__i, __j - __i, __first2 + (__i - __first1));
             });
         return __first2 + __n;
     });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2, class _Brick>
-_ForwardIterator2
-__pattern_walk2_brick_n(_ExecutionPolicy&&, _ForwardIterator1 __first1, _Size __n, _ForwardIterator2 __first2,
-                        _Brick __brick, /*parallel=*/std::false_type) noexcept
-{
-    return __brick(__first1, __n, __first2);
-}
-
 //------------------------------------------------------------------------
 // walk3 (pseudo)
 //
 // walk3 evaluates f(x,y,z) for (x,y,z) drawn from [first1,last1), [first2,...), [first3,...)
 //------------------------------------------------------------------------
 template <class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3, class _Function>
 _ForwardIterator3
 __brick_walk3(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
               _ForwardIterator3 __first3, _Function __f, /*vector=*/std::false_type) noexcept
 {
     for (; __first1 != __last1; ++__first1, ++__first2, ++__first3)
         __f(*__first1, *__first2, *__first3);
     return __first3;
 }
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _RandomAccessIterator3, class _Function>
 _RandomAccessIterator3
 __brick_walk3(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
               _RandomAccessIterator3 __first3, _Function __f, /*vector=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_walk_3(__first1, __last1 - __first1, __first2, __first3, __f);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3,
-          class _Function, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator3,
+          class _Function>
 _ForwardIterator3
-__pattern_walk3(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
-                _ForwardIterator3 __first3, _Function __f, _IsVector __is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_walk3(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                _ForwardIterator2 __first2, _ForwardIterator3 __first3, _Function __f) noexcept
 {
-    return __internal::__brick_walk3(__first1, __last1, __first2, __first3, __f, __is_vector);
+    return __internal::__brick_walk3(__first1, __last1, __first2, __first3, __f, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Function, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Function>
 _RandomAccessIterator3
-__pattern_walk3(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                _RandomAccessIterator2 __first2, _RandomAccessIterator3 __first3, _Function __f, _IsVector __is_vector,
-                /*parallel=*/std::true_type)
+__pattern_walk3(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator3 __first3,
+                _Function __f)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
-            [__f, __first1, __first2, __first3, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            [__f, __first1, __first2, __first3](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                 __internal::__brick_walk3(__i, __j, __first2 + (__i - __first1), __first3 + (__i - __first1), __f,
-                                          __is_vector);
+                                          _IsVector{});
             });
         return __first3 + (__last1 - __first1);
     });
 }
 
 //------------------------------------------------------------------------
 // equal
 //------------------------------------------------------------------------
__brick_equal(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
               _RandomAccessIterator2 __last2, _BinaryPredicate __p, /* is_vector = */ std::true_type) noexcept
 {
     if (__last1 - __first1 != __last2 - __first2)
         return false;
 
     return __unseq_backend::__simd_first(__first1, __last1 - __first1, __first2, std::not_fn(__p)).first == __last1;
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
-                _ForwardIterator2 __last2, _BinaryPredicate __p, _IsVector __is_vector, /* is_parallel = */
-                std::false_type) noexcept
+__pattern_equal(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                _ForwardIterator2 __first2, _ForwardIterator2 __last2, _BinaryPredicate __p) noexcept
 {
-    return __internal::__brick_equal(__first1, __last1, __first2, __last2, __p, __is_vector);
+    return __internal::__brick_equal(__first1, __last1, __first2, __last2, __p, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _BinaryPredicate __p,
-                _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_equal(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                _BinaryPredicate __p)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__last1 - __first1 != __last2 - __first2)
         return false;
 
     return __internal::__except_handler([&]() {
         return !__internal::__parallel_or(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
-            [__first1, __first2, __p, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            [__first1, __first2, __p](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                 return !__internal::__brick_equal(__i, __j, __first2 + (__i - __first1), __first2 + (__j - __first1),
-                                                  __p, __is_vector);
+                                                  __p, _IsVector{});
             });
     });
 }
 
 //------------------------------------------------------------------------
 // equal version for sequences with equal length
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool
 __brick_equal(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2, _BinaryPredicate __p,
               /* IsVector = */ std::false_type) noexcept
 {
     return std::equal(__first1, __last1, __first2, __p);
 }
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 bool
 __brick_equal(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
               _BinaryPredicate __p, /* is_vector = */ std::true_type) noexcept
 {
     return __unseq_backend::__simd_first(__first1, __last1 - __first1, __first2, std::not_fn(__p)).first == __last1;
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
-                _BinaryPredicate __p, _IsVector __is_vector, /* is_parallel = */ std::false_type) noexcept
+__pattern_equal(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                _ForwardIterator2 __first2, _BinaryPredicate __p) noexcept
 {
-    return __internal::__brick_equal(__first1, __last1, __first2, __p, __is_vector);
+    return __internal::__brick_equal(__first1, __last1, __first2, __p, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 bool
-__pattern_equal(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                _RandomAccessIterator2 __first2, _BinaryPredicate __p, _IsVector __is_vector,
-                /*is_parallel=*/std::true_type)
+__pattern_equal(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _BinaryPredicate __p)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return !__internal::__parallel_or(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
-            [__first1, __first2, __p, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
-                return !__internal::__brick_equal(__i, __j, __first2 + (__i - __first1), __p, __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            [__first1, __first2, __p](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+                return !__internal::__brick_equal(__i, __j, __first2 + (__i - __first1), __p, _IsVector{});
             });
     });
 }
 
 //------------------------------------------------------------------------
 // find_if
 //------------------------------------------------------------------------
 template <class _ForwardIterator, class _Predicate>
__brick_find_if(_RandomAccessIterator __first, _RandomAccessIterator __last, _Predicate __pred,
                 /*is_vector=*/std::true_type) noexcept
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _SizeType;
     return __unseq_backend::__simd_first(
         __first, _SizeType(0), __last - __first,
         [&__pred](_RandomAccessIterator __it, _SizeType __i) { return __pred(__it[__i]); });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 _ForwardIterator
-__pattern_find_if(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred,
-                  _IsVector __is_vector,
-                  /*is_parallel=*/std::false_type) noexcept
+__pattern_find_if(_Tag __tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                  _Predicate __pred) noexcept
 {
-    return __internal::__brick_find_if(__first, __last, __pred, __is_vector);
+    return __internal::__brick_find_if(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate>
 _RandomAccessIterator
-__pattern_find_if(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                  _Predicate __pred, _IsVector __is_vector,
-                  /*is_parallel=*/std::true_type)
+__pattern_find_if(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                  _RandomAccessIterator __last, _Predicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __internal::__parallel_find(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
-            [__pred, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                return __internal::__brick_find_if(__i, __j, __pred, __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+            [__pred](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                return __internal::__brick_find_if(__i, __j, __pred, _IsVector{});
             },
             std::less<typename std::iterator_traits<_RandomAccessIterator>::difference_type>(),
             /*is_first=*/true);
     });
 }
 
 //------------------------------------------------------------------------
 // find_end

 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1
 __brick_find_end(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first,
                  _RandomAccessIterator2 __s_last, _BinaryPredicate __pred, /*__is_vector=*/std::true_type) noexcept
 {
     return __find_subrange(__first, __last, __last, __s_first, __s_last, __pred, false, std::true_type());
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_find_end(_ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
-                   _ForwardIterator2 __s_last, _BinaryPredicate __pred, _IsVector __is_vector,
-                   /*is_parallel=*/std::false_type) noexcept
+__pattern_find_end(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last,
+                   _ForwardIterator2 __s_first, _ForwardIterator2 __s_last, _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_find_end(__first, __last, __s_first, __s_last, __pred, __is_vector);
+    return __internal::__brick_find_end(__first, __last, __s_first, __s_last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_find_end(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                   _RandomAccessIterator2 __s_first, _RandomAccessIterator2 __s_last, _BinaryPredicate __pred,
-                   _IsVector __is_vector, /*is_parallel=*/std::true_type) noexcept
+__pattern_find_end(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                   _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first, _RandomAccessIterator2 __s_last,
+                   _BinaryPredicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__last - __first == __s_last - __s_first)
     {
-        const bool __res = __internal::__pattern_equal(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                                       __s_first, __pred, __is_vector, std::true_type());
+        const bool __res = __internal::__pattern_equal(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                       __s_first, __pred);
         return __res ? __first : __last;
     }
     else
     {
         return __internal::__except_handler([&]() {
             return __internal::__parallel_find(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [__last, __s_first, __s_last, __pred, __is_vector](_RandomAccessIterator1 __i,
-                                                                   _RandomAccessIterator1 __j) {
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [__last, __s_first, __s_last, __pred](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                     return __internal::__find_subrange(__i, __j, __last, __s_first, __s_last, __pred, false,
-                                                       __is_vector);
+                                                       _IsVector{});
                 },
                 std::greater<typename std::iterator_traits<_RandomAccessIterator1>::difference_type>(),
                 /*is_first=*/false);
         });
     }
 }
 
 //------------------------------------------------------------------------

 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1
 __brick_find_first_of(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first,
                       _RandomAccessIterator2 __s_last, _BinaryPredicate __pred, /*__is_vector=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_find_first_of(__first, __last, __s_first, __s_last, __pred);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_find_first_of(_ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last,
-                        _ForwardIterator2 __s_first, _ForwardIterator2 __s_last, _BinaryPredicate __pred,
-                        _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_find_first_of(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last,
+                        _ForwardIterator2 __s_first, _ForwardIterator2 __s_last, _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_find_first_of(__first, __last, __s_first, __s_last, __pred, __is_vector);
+    return __internal::__brick_find_first_of(__first, __last, __s_first, __s_last, __pred,
+                                             typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_find_first_of(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                        _RandomAccessIterator2 __s_first, _RandomAccessIterator2 __s_last, _BinaryPredicate __pred,
-                        _IsVector __is_vector, /*is_parallel=*/std::true_type) noexcept
+__pattern_find_first_of(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                        _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first,
+                        _RandomAccessIterator2 __s_last, _BinaryPredicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __internal::__parallel_find(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
-            [__s_first, __s_last, __pred, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
-                return __internal::__brick_find_first_of(__i, __j, __s_first, __s_last, __pred, __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+            [__s_first, __s_last, __pred](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+                return __internal::__brick_find_first_of(__i, __j, __s_first, __s_last, __pred, _IsVector{});
             },
             std::less<typename std::iterator_traits<_RandomAccessIterator1>::difference_type>(), /*is_first=*/true);
     });
 }
 
 //------------------------------------------------------------------------
 // search
 //------------------------------------------------------------------------
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1
 __brick_search(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first,
                _RandomAccessIterator2 __s_last, _BinaryPredicate __pred, /*vector=*/std::false_type) noexcept
 {
     return std::search(__first, __last, __s_first, __s_last, __pred);
 }
 
 template <class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate>
 _RandomAccessIterator1
 __brick_search(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first,
                _RandomAccessIterator2 __s_last, _BinaryPredicate __pred, /*vector=*/std::true_type) noexcept
 {
     return __internal::__find_subrange(__first, __last, __last, __s_first, __s_last, __pred, true, std::true_type());
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 _ForwardIterator1
-__pattern_search(_ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
-                 _ForwardIterator2 __s_last, _BinaryPredicate __pred, _IsVector __is_vector,
-                 /*is_parallel=*/std::false_type) noexcept
+__pattern_search(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first, _ForwardIterator1 __last,
+                 _ForwardIterator2 __s_first, _ForwardIterator2 __s_last, _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_search(__first, __last, __s_first, __s_last, __pred, __is_vector);
+    return __internal::__brick_search(__first, __last, __s_first, __s_last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator1
-__pattern_search(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                 _RandomAccessIterator2 __s_first, _RandomAccessIterator2 __s_last, _BinaryPredicate __pred,
-                 _IsVector __is_vector,
-                 /*is_parallel=*/std::true_type) noexcept
+__pattern_search(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                 _RandomAccessIterator1 __last, _RandomAccessIterator2 __s_first, _RandomAccessIterator2 __s_last,
+                 _BinaryPredicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__last - __first == __s_last - __s_first)
     {
-        const bool __res = __internal::__pattern_equal(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                                       __s_first, __pred, __is_vector, std::true_type());
+        const bool __res = __internal::__pattern_equal(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                       __s_first, __pred);
         return __res ? __first : __last;
     }
     else
     {
         return __internal::__except_handler([&]() {
             return __internal::__parallel_find(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [__last, __s_first, __s_last, __pred, __is_vector](_RandomAccessIterator1 __i,
-                                                                   _RandomAccessIterator1 __j) {
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [__last, __s_first, __s_last, __pred](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                     return __internal::__find_subrange(__i, __j, __last, __s_first, __s_last, __pred, true,
-                                                       __is_vector);
+                                                       _IsVector{});
                 },
                 std::less<typename std::iterator_traits<_RandomAccessIterator1>::difference_type>(), /*is_first=*/true);
         });
     }
 }
 
 //------------------------------------------------------------------------
 // search_n

 template <class _RandomAccessIterator, class _Size, class _Tp, class _BinaryPredicate>
 _RandomAccessIterator
 __brick_search_n(_RandomAccessIterator __first, _RandomAccessIterator __last, _Size __count, const _Tp& __value,
                  _BinaryPredicate __pred, /*vector=*/std::true_type) noexcept
 {
     return __internal::__find_subrange(__first, __last, __last, __count, __value, __pred, std::true_type());
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
 _ForwardIterator
-__pattern_search_n(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
-                   const _Tp& __value, _BinaryPredicate __pred, _IsVector __is_vector,
-                   /*is_parallel=*/std::false_type) noexcept
+__pattern_search_n(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
+                   const _Tp& __value, _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_search_n(__first, __last, __count, __value, __pred, __is_vector);
+    return __internal::__brick_search_n(__first, __last, __count, __value, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp,
+          class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_search_n(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                   _Size __count, const _Tp& __value, _BinaryPredicate __pred, _IsVector __is_vector,
-                   /*is_parallel=*/std::true_type) noexcept
+__pattern_search_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                   _RandomAccessIterator __last, _Size __count, const _Tp& __value, _BinaryPredicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (static_cast<_Size>(__last - __first) == __count)
     {
-        const bool __result = !__internal::__pattern_any_of(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
-            [&__value, &__pred](const _Tp& __val) { return !__pred(__val, __value); }, __is_vector,
-            /*is_parallel*/ std::true_type());
+        const bool __result =
+            !__internal::__pattern_any_of(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                          [&__value, &__pred](const _Tp& __val) { return !__pred(__val, __value); });
         return __result ? __first : __last;
     }
     else
     {
-        return __internal::__except_handler([&__exec, __first, __last, __count, &__value, __pred, __is_vector]() {
+        return __internal::__except_handler([&__exec, __first, __last, __count, &__value, __pred]() {
             return __internal::__parallel_find(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [__last, __count, &__value, __pred, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                    return __internal::__find_subrange(__i, __j, __last, __count, __value, __pred, __is_vector);
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [__last, __count, &__value, __pred](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                    return __internal::__find_subrange(__i, __j, __last, __count, __value, __pred, _IsVector{});
                 },
                 std::less<typename std::iterator_traits<_RandomAccessIterator>::difference_type>(), /*is_first=*/true);
         });
     }
 }
 
 //------------------------------------------------------------------------
 // copy_n

 template <class _DifferenceType, class _ForwardIterator, class _UnaryPredicate>
 std::pair<_DifferenceType, _DifferenceType>
 __brick_calc_mask_1(_ForwardIterator __first, _ForwardIterator __last, bool* __restrict __mask, _UnaryPredicate __pred,
                     /*vector=*/std::false_type) noexcept
 {
     auto __count_true = _DifferenceType(0);
     auto __size = __last - __first;
 
-    static_assert(__is_random_access_iterator<_ForwardIterator>::value,
+    static_assert(__are_random_access_iterators<_ForwardIterator>::value,
                   "Pattern-brick error. Should be a random access iterator.");
 
     for (; __first != __last; ++__first, ++__mask)
     {
         *__mask = __pred(*__first);
         if (*__mask)
         {
             ++__count_true;

 {
 #if defined(_PSTL_MONOTONIC_PRESENT)
     __unseq_backend::__simd_partition_by_mask(__first, __last - __first, __out_true, __out_false, __mask);
 #else
     __internal::__brick_partition_by_mask(__first, __last, __out_true, __out_false, __mask, std::false_type());
 #endif
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryPredicate>
 _OutputIterator
-__pattern_copy_if(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _OutputIterator __result,
-                  _UnaryPredicate __pred, _IsVector __is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_copy_if(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _OutputIterator __result,
+                  _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_copy_if(__first, __last, __result, __pred, __is_vector);
+    return __internal::__brick_copy_if(__first, __last, __result, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _UnaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _UnaryPredicate>
 _RandomAccessIterator2
-__pattern_copy_if(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                  _RandomAccessIterator2 __result, _UnaryPredicate __pred, _IsVector __is_vector,
-                  /*parallel=*/std::true_type)
+__pattern_copy_if(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                  _RandomAccessIterator1 __last, _RandomAccessIterator2 __result, _UnaryPredicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
     const _DifferenceType __n = __last - __first;
     if (_DifferenceType(1) < __n)
     {
         __par_backend::__buffer<bool> __mask_buf(__n);
-        return __internal::__except_handler([&__exec, __n, __first, __result, __is_vector, __pred, &__mask_buf]() {
+        return __internal::__except_handler([&__exec, __n, __first, __result, __pred, &__mask_buf]() {
             bool* __mask = __mask_buf.get();
             _DifferenceType __m{};
             __par_backend::__parallel_strict_scan(
-                std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
                 [=](_DifferenceType __i, _DifferenceType __len) { // Reduce
                     return __internal::__brick_calc_mask_1<_DifferenceType>(__first + __i, __first + (__i + __len),
-                                                                            __mask + __i, __pred, __is_vector)
+                                                                            __mask + __i, __pred, _IsVector{})
                         .first;
                 },
                 std::plus<_DifferenceType>(),                                                // Combine
                 [=](_DifferenceType __i, _DifferenceType __len, _DifferenceType __initial) { // Scan
                     __internal::__brick_copy_by_mask(
                         __first + __i, __first + (__i + __len), __result + __initial, __mask + __i,
-                        [](_RandomAccessIterator1 __x, _RandomAccessIterator2 __z) { *__z = *__x; }, __is_vector);
+                        [](_RandomAccessIterator1 __x, _RandomAccessIterator2 __z) { *__z = *__x; }, _IsVector{});
                 },
                 [&__m](_DifferenceType __total) { __m = __total; });
             return __result + __m;
         });
     }
     // trivial sequence - use serial algorithm
-    return __internal::__brick_copy_if(__first, __last, __result, __pred, __is_vector);
+    return __internal::__brick_copy_if(__first, __last, __result, __pred, _IsVector{});
 }
 
 //------------------------------------------------------------------------
 // count
 //------------------------------------------------------------------------
 template <class _RandomAccessIterator, class _Predicate>
 typename std::iterator_traits<_RandomAccessIterator>::difference_type
 __brick_count(_RandomAccessIterator __first, _RandomAccessIterator __last, _Predicate __pred,
               /* is_vector = */ std::true_type) noexcept
 {
     return __unseq_backend::__simd_count(__first, __last - __first, __pred);
 }
 
 template <class _ForwardIterator, class _Predicate>
 typename std::iterator_traits<_ForwardIterator>::difference_type
 __brick_count(_ForwardIterator __first, _ForwardIterator __last, _Predicate __pred,
               /* is_vector = */ std::false_type) noexcept
 {
     return std::count_if(__first, __last, __pred);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 typename std::iterator_traits<_ForwardIterator>::difference_type
-__pattern_count(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred,
-                /* is_parallel */ std::false_type, _IsVector __is_vector) noexcept
+__pattern_count(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred) noexcept
 {
-    return __internal::__brick_count(__first, __last, __pred, __is_vector);
+    return __internal::__brick_count(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Predicate>
 typename std::iterator_traits<_RandomAccessIterator>::difference_type
-__pattern_count(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                _Predicate __pred,
-                /* is_parallel */ std::true_type, _IsVector __is_vector)
+__pattern_count(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                _RandomAccessIterator __last, _Predicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _SizeType;
     return __internal::__except_handler([&]() {
         return __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last, _SizeType(0),
-            [__pred, __is_vector](_RandomAccessIterator __begin, _RandomAccessIterator __end, _SizeType __value)
-                -> _SizeType { return __value + __internal::__brick_count(__begin, __end, __pred, __is_vector); },
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, _SizeType(0),
+            [__pred](_RandomAccessIterator __begin, _RandomAccessIterator __end, _SizeType __value) -> _SizeType {
+                return __value + __internal::__brick_count(__begin, __end, __pred, _IsVector{});
+            },
             std::plus<_SizeType>());
     });
 }
 
 //------------------------------------------------------------------------
 // unique
 //------------------------------------------------------------------------
 

 _RandomAccessIterator
 __brick_unique(_RandomAccessIterator __first, _RandomAccessIterator __last, _BinaryPredicate __pred,
                /*is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::unique(__first, __last, __pred);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
-__pattern_unique(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred,
-                 _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_unique(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                 _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_unique(__first, __last, __pred, __is_vector);
+    return __internal::__brick_unique(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
 // That function is shared between two algorithms - remove_if (__pattern_remove_if) and unique (pattern unique). But a mask calculation is different.
 // So, a caller passes _CalcMask brick into remove_elements.
-template <class _ExecutionPolicy, class _ForwardIterator, class _CalcMask, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _ForwardIterator, class _CalcMask>
 _ForwardIterator
-__remove_elements(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _CalcMask __calc_mask,
-                  _IsVector __is_vector)
+__remove_elements(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _ForwardIterator __first,
+                  _ForwardIterator __last, _CalcMask __calc_mask)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_ForwardIterator>::difference_type _DifferenceType;
     typedef typename std::iterator_traits<_ForwardIterator>::value_type _Tp;
     _DifferenceType __n = __last - __first;
     __par_backend::__buffer<bool> __mask_buf(__n);
     // 1. find a first iterator that should be removed
     return __internal::__except_handler([&]() {
         bool* __mask = __mask_buf.get();
         _DifferenceType __min = __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), _DifferenceType(0), __n, __n,
-            [__first, __mask, &__calc_mask, __is_vector](_DifferenceType __i, _DifferenceType __j,
-                                                         _DifferenceType __local_min) -> _DifferenceType {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), _DifferenceType(0), __n, __n,
+            [__first, __mask, &__calc_mask](_DifferenceType __i, _DifferenceType __j,
                 // Create mask
                 __calc_mask(__mask + __i, __mask + __j, __first + __i);
 
                 // if minimum was found in a previous range we shouldn't do anymore
                 if (__local_min < __i)
                 {
                     return __local_min;
                 }
                 // find first iterator that should be removed
-                bool* __result = __internal::__brick_find_if(__mask + __i, __mask + __j,
-                                                             [](bool __val) { return !__val; }, __is_vector);
+                bool* __result = __internal::__brick_find_if(
+                    __mask + __i, __mask + __j, [](bool __val) { return !__val; }, _IsVector{});
                 if (__result - __mask == __j)
                 {
                     return __local_min;
                 }
                 return std::min(__local_min, _DifferenceType(__result - __mask));
             },
             [](_DifferenceType __local_min1, _DifferenceType __local_min2) -> _DifferenceType {
                 return std::min(__local_min1, __local_min2);
             });
 
         // No elements to remove - exit
         if (__min == __n)
         {
             return __last;
         }
         __n -= __min;
         __first += __min;
 
         __par_backend::__buffer<_Tp> __buf(__n);
         _Tp* __result = __buf.get();
         __mask += __min;
         _DifferenceType __m{};
         // 2. Elements that doesn't satisfy pred are moved to result
         __par_backend::__parallel_strict_scan(
-            std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
-            [__mask, __is_vector](_DifferenceType __i, _DifferenceType __len) {
-                return __internal::__brick_count(__mask + __i, __mask + __i + __len, [](bool __val) { return __val; },
-                                                 __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
+            [__mask](_DifferenceType __i, _DifferenceType __len) {
+                return __internal::__brick_count(
+                    __mask + __i, __mask + __i + __len, [](bool __val) { return __val; }, _IsVector{});
             },
             std::plus<_DifferenceType>(),
             [=](_DifferenceType __i, _DifferenceType __len, _DifferenceType __initial) {
                 __internal::__brick_copy_by_mask(
                     __first + __i, __first + __i + __len, __result + __initial, __mask + __i,
                     [](_ForwardIterator __x, _Tp* __z) {
-                        __internal::__invoke_if_else(std::is_trivial<_Tp>(), [&]() { *__z = std::move(*__x); },
-                                                     [&]() { ::new (std::addressof(*__z)) _Tp(std::move(*__x)); });
+                        __internal::__invoke_if_else(
+                            std::is_trivial<_Tp>(), [&]() { *__z = std::move(*__x); },
                     },
-                    __is_vector);
+                    _IsVector{});
             },
             [&__m](_DifferenceType __total) { __m = __total; });
 
         // 3. Elements from result are moved to [first, last)
         __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __result, __result + __m,
-            [__result, __first, __is_vector](_Tp* __i, _Tp* __j) {
-                __invoke_if_else(std::is_trivial<_Tp>(),
-                                 [&]() { __brick_move(__i, __j, __first + (__i - __result), __is_vector); },
-                                 [&]() { __brick_move_destroy()(__i, __j, __first + (__i - __result), __is_vector); });
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __result, __result + __m,
+            [__result, __first](_Tp* __i, _Tp* __j) {
+                __invoke_if_else(
+                    std::is_trivial<_Tp>(), [&]() { __brick_move(__i, __j, __first + (__i - __result), _IsVector{}); },
+                    [&]() { __brick_move_destroy()(__i, __j, __first + (__i - __result), _IsVector{}); });
             });
         return __first + __m;
     });
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_unique(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                 _BinaryPredicate __pred, _IsVector __is_vector, /*is_parallel=*/std::true_type) noexcept
+__pattern_unique(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                 _RandomAccessIterator __last, _BinaryPredicate __pred) noexcept
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::reference _ReferenceType;
 
     if (__first == __last)
     {
         return __last;
     }
     if (__first + 1 == __last || __first + 2 == __last)
     {
         // Trivial sequence - use serial algorithm
-        return __internal::__brick_unique(__first, __last, __pred, __is_vector);
+        return __internal::__brick_unique(__first, __last, __pred, _IsVector{});
     }
     return __internal::__remove_elements(
-        std::forward<_ExecutionPolicy>(__exec), ++__first, __last,
-        [&__pred, __is_vector](bool* __b, bool* __e, _RandomAccessIterator __it) {
+        __tag, std::forward<_ExecutionPolicy>(__exec), ++__first, __last,
+        [&__pred](bool* __b, bool* __e, _RandomAccessIterator __it) {
             __internal::__brick_walk3(
                 __b, __e, __it - 1, __it,
-                [&__pred](bool& __x, _ReferenceType __y, _ReferenceType __z) { __x = !__pred(__y, __z); }, __is_vector);
-        },
-        __is_vector);
+                [&__pred](bool& __x, _ReferenceType __y, _ReferenceType __z) { __x = !__pred(__y, __z); }, _IsVector{});
+        });
 }
 
 //------------------------------------------------------------------------
 // unique_copy
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class OutputIterator, class _BinaryPredicate>
 OutputIterator

 {
 #if defined(_PSTL_MONOTONIC_PRESENT)
     return __unseq_backend::__simd_unique_copy(__first, __last - __first, __result, __pred);
 #else
     return std::unique_copy(__first, __last, __result, __pred);
 #endif
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryPredicate,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryPredicate>
 _OutputIterator
-__pattern_unique_copy(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _OutputIterator __result,
-                      _BinaryPredicate __pred, _IsVector __is_vector, /*parallel=*/std::false_type) noexcept
+__pattern_unique_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                      _OutputIterator __result, _BinaryPredicate __pred) noexcept
 {
-    return __internal::__brick_unique_copy(__first, __last, __result, __pred, __is_vector);
+    return __internal::__brick_unique_copy(__first, __last, __result, __pred, typename _Tag::__is_vector{});
 }
 
 template <class _DifferenceType, class _RandomAccessIterator, class _BinaryPredicate>
 _DifferenceType
 __brick_calc_mask_2(_RandomAccessIterator __first, _RandomAccessIterator __last, bool* __restrict __mask,
                     _BinaryPredicate __pred, /*vector=*/std::false_type) noexcept
 {
     _DifferenceType __count = 0;
     for (; __first != __last; ++__first, ++__mask)
     {
         *__mask = !__pred(*__first, *(__first - 1));
         __count += *__mask;
     }
     return __count;
 }
 
 template <class _DifferenceType, class _RandomAccessIterator, class _BinaryPredicate>
 _DifferenceType
 __brick_calc_mask_2(_RandomAccessIterator __first, _RandomAccessIterator __last, bool* __restrict __mask,
                     _BinaryPredicate __pred, /*vector=*/std::true_type) noexcept
 {
     return __unseq_backend::__simd_calc_mask_2(__first, __last - __first, __mask, __pred);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryPredicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryPredicate>
 _RandomAccessIterator2
-__pattern_unique_copy(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                      _RandomAccessIterator2 __result, _BinaryPredicate __pred, _IsVector __is_vector,
-                      /*parallel=*/std::true_type)
+__pattern_unique_copy(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                      _RandomAccessIterator1 __last, _RandomAccessIterator2 __result, _BinaryPredicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
     const _DifferenceType __n = __last - __first;
     if (_DifferenceType(2) < __n)
     {
         __par_backend::__buffer<bool> __mask_buf(__n);
         if (_DifferenceType(2) < __n)
         {
-            return __internal::__except_handler([&__exec, __n, __first, __result, __pred, __is_vector, &__mask_buf]() {
+            return __internal::__except_handler([&__exec, __n, __first, __result, __pred, &__mask_buf]() {
                 bool* __mask = __mask_buf.get();
                 _DifferenceType __m{};
                 __par_backend::__parallel_strict_scan(
-                    std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
+                    __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n, _DifferenceType(0),
                     [=](_DifferenceType __i, _DifferenceType __len) -> _DifferenceType { // Reduce
                         _DifferenceType __extra = 0;
                         if (__i == 0)
                         {
                             // Special boundary case
                             __mask[__i] = true;
                             if (--__len == 0)
                                 return 1;
                             ++__i;
                             ++__extra;
                         }
                         return __internal::__brick_calc_mask_2<_DifferenceType>(__first + __i, __first + (__i + __len),
-                                                                                __mask + __i, __pred, __is_vector) +
+                                                                                __mask + __i, __pred, _IsVector{}) +
                                __extra;
                     },
                     std::plus<_DifferenceType>(),                                                // Combine
                     [=](_DifferenceType __i, _DifferenceType __len, _DifferenceType __initial) { // Scan
                         // Phase 2 is same as for __pattern_copy_if
                         __internal::__brick_copy_by_mask(
                             __first + __i, __first + (__i + __len), __result + __initial, __mask + __i,
-                            [](_RandomAccessIterator1 __x, _RandomAccessIterator2 __z) { *__z = *__x; }, __is_vector);
+                            [](_RandomAccessIterator1 __x, _RandomAccessIterator2 __z) { *__z = *__x; }, _IsVector{});
                     },
                     [&__m](_DifferenceType __total) { __m = __total; });
                 return __result + __m;
             });
         }
     }
     // trivial sequence - use serial algorithm
-    return __internal::__brick_unique_copy(__first, __last, __result, __pred, __is_vector);
+    return __internal::__brick_unique_copy(__first, __last, __result, __pred, _IsVector{});
 }
 
 //------------------------------------------------------------------------
 // reverse
 //------------------------------------------------------------------------
 template <class _BidirectionalIterator>
 void
 __brick_reverse(_BidirectionalIterator __first, _BidirectionalIterator __last, /*__is_vector=*/std::false_type) noexcept
__brick_reverse(_RandomAccessIterator __first, _RandomAccessIterator __last, _RandomAccessIterator __d_last,
 
     __unseq_backend::__simd_walk_2(__first, __last - __first, std::reverse_iterator<_RandomAccessIterator>(__d_last),
                                    [](_ReferenceType __x, _ReferenceType __y) {
                                        using std::swap;
                                        swap(__x, __y);
                                    });
 }
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator>
 void
-__pattern_reverse(_ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
-                  _IsVector _is_vector,
-                  /*is_parallel=*/std::false_type) noexcept
+__pattern_reverse(_Tag, _ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last) noexcept
 {
-    __internal::__brick_reverse(__first, __last, _is_vector);
+    __internal::__brick_reverse(__first, __last, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator>
 void
-__pattern_reverse(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                  _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_reverse(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                  _RandomAccessIterator __last)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __par_backend::__parallel_for(
-        std::forward<_ExecutionPolicy>(__exec), __first, __first + (__last - __first) / 2,
-        [__is_vector, __first, __last](_RandomAccessIterator __inner_first, _RandomAccessIterator __inner_last) {
-            __internal::__brick_reverse(__inner_first, __inner_last, __last - (__inner_first - __first), __is_vector);
+        __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __first + (__last - __first) / 2,
+        [__first, __last](_RandomAccessIterator __inner_first, _RandomAccessIterator __inner_last) {
+            __internal::__brick_reverse(__inner_first, __inner_last, __last - (__inner_first - __first), _IsVector{});
         });
 }
 
 //------------------------------------------------------------------------
 // reverse_copy
 //------------------------------------------------------------------------
 
 template <class _BidirectionalIterator, class _OutputIterator>

 {
     typedef typename std::iterator_traits<_RandomAccessIterator1>::reference _ReferenceType1;
     typedef typename std::iterator_traits<_RandomAccessIterator2>::reference _ReferenceType2;
 
     return __unseq_backend::__simd_walk_2(std::reverse_iterator<_RandomAccessIterator1>(__last), __last - __first,
                                           __d_first, [](_ReferenceType1 __x, _ReferenceType2 __y) { __y = __x; });
 }
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _OutputIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _OutputIterator>
 _OutputIterator
-__pattern_reverse_copy(_ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
-                       _OutputIterator __d_first, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_reverse_copy(_Tag, _ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
+                       _OutputIterator __d_first) noexcept
 {
-    return __internal::__brick_reverse_copy(__first, __last, __d_first, __is_vector);
+    return __internal::__brick_reverse_copy(__first, __last, __d_first, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2>
 _RandomAccessIterator2
-__pattern_reverse_copy(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                       _RandomAccessIterator2 __d_first, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_reverse_copy(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                       _RandomAccessIterator1 __last, _RandomAccessIterator2 __d_first)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     auto __len = __last - __first;
-    __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                  [__is_vector, __first, __len, __d_first](_RandomAccessIterator1 __inner_first,
-                                                                           _RandomAccessIterator1 __inner_last) {
-                                      __internal::__brick_reverse_copy(__inner_first, __inner_last,
-                                                                       __d_first + (__len - (__inner_last - __first)),
-                                                                       __is_vector);
-                                  });
+    __par_backend::__parallel_for(
+        __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+        [__first, __len, __d_first](_RandomAccessIterator1 __inner_first, _RandomAccessIterator1 __inner_last) {
+                                             __d_first + (__len - (__inner_last - __first)), _IsVector{});
     return __d_first + __len;
 }
 
 //------------------------------------------------------------------------
 // rotate
 //------------------------------------------------------------------------
 template <class _ForwardIterator>
 _ForwardIterator
while (__n > 1 && __m > 0)
         __is_left = !__is_left;
         __m = __n % __m;
         __n = __last - __first;
     }
 
     return __ret;
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator>
 _ForwardIterator
-__pattern_rotate(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last,
-                 _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_rotate(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __middle,
+                 _ForwardIterator __last) noexcept
 {
-    return __internal::__brick_rotate(__first, __middle, __last, __is_vector);
+    return __internal::__brick_rotate(__first, __middle, __last, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator>
 _RandomAccessIterator
-__pattern_rotate(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __middle,
-                 _RandomAccessIterator __last, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_rotate(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                 _RandomAccessIterator __middle, _RandomAccessIterator __last)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _Tp;
     auto __n = __last - __first;
     auto __m = __middle - __first;
     if (__m <= __n / 2)
     {
         __par_backend::__buffer<_Tp> __buf(__n - __m);
-        return __internal::__except_handler([&__exec, __n, __m, __first, __middle, __last, __is_vector, &__buf]() {
+        return __internal::__except_handler([&__exec, __n, __m, __first, __middle, __last, &__buf]() {
             _Tp* __result = __buf.get();
-            __par_backend::__parallel_for(
-                std::forward<_ExecutionPolicy>(__exec), __middle, __last,
-                [__middle, __result, __is_vector](_RandomAccessIterator __b, _RandomAccessIterator __e) {
-                    __internal::__brick_uninitialized_move(__b, __e, __result + (__b - __middle), __is_vector);
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __middle, __last,
+                                          [__middle, __result](_RandomAccessIterator __b, _RandomAccessIterator __e) {
+                                              __internal::__brick_uninitialized_move(
+                                                  __b, __e, __result + (__b - __middle), _IsVector{});
                                           });
 
-            __par_backend::__parallel_for(
-                std::forward<_ExecutionPolicy>(__exec), __first, __middle,
-                [__last, __middle, __is_vector](_RandomAccessIterator __b, _RandomAccessIterator __e) {
-                    __internal::__brick_move(__b, __e, __b + (__last - __middle), __is_vector);
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __middle,
+                                          [__last, __middle](_RandomAccessIterator __b, _RandomAccessIterator __e) {
+                                              __internal::__brick_move(__b, __e, __b + (__last - __middle),
+                                                                       _IsVector{});
                                           });
 
-            __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __result, __result + (__n - __m),
-                                          [__first, __result, __is_vector](_Tp* __b, _Tp* __e) {
-                                              __brick_move_destroy()(__b, __e, __first + (__b - __result), __is_vector);
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __result,
+                                          __result + (__n - __m), [__first, __result](_Tp* __b, _Tp* __e) {
+                                              __brick_move_destroy()(__b, __e, __first + (__b - __result), _IsVector{});
                                           });
 
             return __first + (__last - __middle);
         });
     }
     else
     {
         __par_backend::__buffer<_Tp> __buf(__m);
-        return __internal::__except_handler([&__exec, __n, __m, __first, __middle, __last, __is_vector, &__buf]() {
+        return __internal::__except_handler([&__exec, __n, __m, __first, __middle, __last, &__buf]() {
             _Tp* __result = __buf.get();
-            __par_backend::__parallel_for(
-                std::forward<_ExecutionPolicy>(__exec), __first, __middle,
-                [__first, __result, __is_vector](_RandomAccessIterator __b, _RandomAccessIterator __e) {
-                    __internal::__brick_uninitialized_move(__b, __e, __result + (__b - __first), __is_vector);
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __middle,
+                                          [__first, __result](_RandomAccessIterator __b, _RandomAccessIterator __e) {
+                                              __internal::__brick_uninitialized_move(
+                                                  __b, __e, __result + (__b - __first), _IsVector{});
                                           });
 
-            __par_backend::__parallel_for(
-                std::forward<_ExecutionPolicy>(__exec), __middle, __last,
-                [__first, __middle, __is_vector](_RandomAccessIterator __b, _RandomAccessIterator __e) {
-                    __internal::__brick_move(__b, __e, __first + (__b - __middle), __is_vector);
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __middle, __last,
+                                          [__first, __middle](_RandomAccessIterator __b, _RandomAccessIterator __e) {
+                                              __internal::__brick_move(__b, __e, __first + (__b - __middle),
+                                                                       _IsVector{});
                                           });
 
-            __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __result, __result + __m,
-                                          [__n, __m, __first, __result, __is_vector](_Tp* __b, _Tp* __e) {
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __result,
+                                          __result + __m, [__n, __m, __first, __result](_Tp* __b, _Tp* __e) {
                                               __brick_move_destroy()(
-                                                  __b, __e, __first + ((__n - __m) + (__b - __result)), __is_vector);
+                                                  __b, __e, __first + ((__n - __m) + (__b - __result)), _IsVector{});
                                           });
 
             return __first + (__last - __middle);
         });
     }
 }
 
 //------------------------------------------------------------------------

 _RandomAccessIterator2
 __brick_rotate_copy(_RandomAccessIterator1 __first, _RandomAccessIterator1 __middle, _RandomAccessIterator1 __last,
                     _RandomAccessIterator2 __result, /*__is_vector=*/std::true_type) noexcept
 {
     _RandomAccessIterator2 __res = __internal::__brick_copy(__middle, __last, __result, std::true_type());
     return __internal::__brick_copy(__first, __middle, __res, std::true_type());
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator>
 _OutputIterator
-__pattern_rotate_copy(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last,
-                      _OutputIterator __result, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_rotate_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __middle,
+                      _ForwardIterator __last, _OutputIterator __result) noexcept
 {
-    return __internal::__brick_rotate_copy(__first, __middle, __last, __result, __is_vector);
+    return __internal::__brick_rotate_copy(__first, __middle, __last, __result, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2>
 _RandomAccessIterator2
-__pattern_rotate_copy(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __middle,
-                      _RandomAccessIterator1 __last, _RandomAccessIterator2 __result, _IsVector __is_vector,
-                      /*is_parallel=*/std::true_type)
+__pattern_rotate_copy(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                      _RandomAccessIterator1 __middle, _RandomAccessIterator1 __last, _RandomAccessIterator2 __result)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __par_backend::__parallel_for(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last,
-        [__first, __last, __middle, __result, __is_vector](_RandomAccessIterator1 __b, _RandomAccessIterator1 __e) {
+        __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+        [__first, __last, __middle, __result](_RandomAccessIterator1 __b, _RandomAccessIterator1 __e) {
             if (__b > __middle)
             {
-                __internal::__brick_copy(__b, __e, __result + (__b - __middle), __is_vector);
+                __internal::__brick_copy(__b, __e, __result + (__b - __middle), _IsVector{});
             }
             else
             {
                 _RandomAccessIterator2 __new_result = __result + ((__last - __middle) + (__b - __first));
                 if (__e < __middle)
                 {
-                    __internal::__brick_copy(__b, __e, __new_result, __is_vector);
+                    __internal::__brick_copy(__b, __e, __new_result, _IsVector{});
                 }
                 else
                 {
-                    __internal::__brick_copy(__b, __middle, __new_result, __is_vector);
-                    __internal::__brick_copy(__middle, __e, __result, __is_vector);
+                    __internal::__brick_copy(__b, __middle, __new_result, _IsVector{});
+                    __internal::__brick_copy(__middle, __e, __result, _IsVector{});
                 }
             }
         });
     return __result + (__last - __first);
 }
 
 //------------------------------------------------------------------------
 // is_partitioned
else
         else
         {
             ++__result;
             return !__unseq_backend::__simd_or(__result, __last - __result, __pred);
         }
     }
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 bool
-__pattern_is_partitioned(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred,
-                         _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_is_partitioned(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                         _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_is_partitioned(__first, __last, __pred, __is_vector);
+    return __internal::__brick_is_partitioned(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 bool
-__pattern_is_partitioned(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                         _UnaryPredicate __pred, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_is_partitioned(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                         _RandomAccessIterator __last, _UnaryPredicate __pred)
 {
     if (__first == __last)
     {
         return true;
     }
     else
     {
+        using __backend_tag = typename decltype(__tag)::__backend_tag;
+
         return __internal::__except_handler([&]() {
             // State of current range:
             // broken     - current range is not partitioned by pred
             // all_true   - all elements in current range satisfy pred
             // all_false  - all elements in current range don't satisfy pred
             // true_false - elements satisfy pred are placed before elements that don't satisfy pred
             enum _ReduceType
             {
                 __not_init = -1,
                 __broken,
                 __all_true,
                 __all_false,
                 __true_false
             };
             _ReduceType __init = __not_init;
 
             // Array with states that we'll have when state from the left branch is merged with state from the right branch.
             // State is calculated by formula: new_state = table[left_state * 4 + right_state]
             _ReduceType __table[] = {__broken,     __broken,     __broken,     __broken, __broken,    __all_true,
                                      __true_false, __true_false, __broken,     __broken, __all_false, __broken,
                                      __broken,     __broken,     __true_false, __broken};
 
             __init = __par_backend::__parallel_reduce(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
-                [&__pred, &__table, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j,
-                                                 _ReduceType __value) -> _ReduceType {
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
+                [&__pred, &__table](_RandomAccessIterator __i, _RandomAccessIterator __j,
                     if (__value == __broken)
                     {
                         return __broken;
                     }
                     _ReduceType __res = __not_init;
                     // if first element satisfy pred
                     if (__pred(*__i))
                     {
                         // find first element that don't satisfy pred
                         _RandomAccessIterator __x =
-                            __internal::__brick_find_if(__i + 1, __j, std::not_fn(__pred), __is_vector);
+                            __internal::__brick_find_if(__i + 1, __j, std::not_fn(__pred), _IsVector{});
                         if (__x != __j)
                         {
                             // find first element after "x" that satisfy pred
-                            _RandomAccessIterator __y = __internal::__brick_find_if(__x + 1, __j, __pred, __is_vector);
+                            _RandomAccessIterator __y = __internal::__brick_find_if(__x + 1, __j, __pred, _IsVector{});
                             // if it was found then range isn't partitioned by pred
                             if (__y != __j)
                             {
                                 return __broken;
                             }
                             else
                             {
                                 __res = __true_false;
                             }
                         }
                         else
                         {
                             __res = __all_true;
                         }
                     }
                     else
                     { // if first element doesn't satisfy pred
                         // then we should find the first element that satisfy pred.
                         // If we found it then range isn't partitioned by pred
-                        if (__internal::__brick_find_if(__i + 1, __j, __pred, __is_vector) != __j)
+                        if (__internal::__brick_find_if(__i + 1, __j, __pred, _IsVector{}) != __j)
                         {
                             return __broken;
                         }
                         else
                         {
                             __res = __all_false;
                         }
                     }
                     // if we have value from left range then we should calculate the result
                     return (__value == -1) ? __res : __table[__value * 4 + __res];
                 },
 
                 [&__table](_ReduceType __val1, _ReduceType __val2) -> _ReduceType {
                     if (__val1 == __broken || __val2 == __broken)
                     {
                         return __broken;
                     }
                     // calculate the result for new big range
                     return __table[__val1 * 4 + __val2];
                 });
             return __init != __broken;

 _RandomAccessIterator
 __brick_partition(_RandomAccessIterator __first, _RandomAccessIterator __last, _UnaryPredicate __pred,
                   /*is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::partition(__first, __last, __pred);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator
-__pattern_partition(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred,
-                    _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+__pattern_partition(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                    _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_partition(__first, __last, __pred, __is_vector);
+    return __internal::__brick_partition(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_partition(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                    _UnaryPredicate __pred, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_partition(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                    _RandomAccessIterator __last, _UnaryPredicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
 
     // partitioned range: elements before pivot satisfy pred (true part),
     //                    elements after pivot don't satisfy pred (false part)
     struct _PartitionRange
     {
         _RandomAccessIterator __begin;
         _RandomAccessIterator __pivot;
         _RandomAccessIterator __end;
     };
 
     return __internal::__except_handler([&]() {
         _PartitionRange __init{__last, __last, __last};
 
         // lambda for merging two partitioned ranges to one partitioned range
-        auto __reductor = [&__exec, __is_vector](_PartitionRange __val1, _PartitionRange __val2) -> _PartitionRange {
+        auto __reductor = [&__exec](_PartitionRange __val1, _PartitionRange __val2) -> _PartitionRange {
             auto __size1 = __val1.__end - __val1.__pivot;
             auto __size2 = __val2.__pivot - __val2.__begin;
             auto __new_begin = __val2.__begin - (__val1.__end - __val1.__begin);
 
             // if all elements in left range satisfy pred then we can move new pivot to pivot of right range
             if (__val1.__end == __val1.__pivot)
             {
                 return {__new_begin, __val2.__pivot, __val2.__end};
             }
             // if true part of right range greater than false part of left range
             // then we should swap the false part of left range and last part of true part of right range
             else if (__size2 > __size1)
             {
                 __par_backend::__parallel_for(
-                    std::forward<_ExecutionPolicy>(__exec), __val1.__pivot, __val1.__pivot + __size1,
-                    [__val1, __val2, __size1, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                    __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __val1.__pivot, __val1.__pivot + __size1,
+                    [__val1, __val2, __size1](_RandomAccessIterator __i, _RandomAccessIterator __j) {
                         __internal::__brick_swap_ranges(__i, __j, (__val2.__pivot - __size1) + (__i - __val1.__pivot),
-                                                        __is_vector);
+                                                        _IsVector{});
                     });
                 return {__new_begin, __val2.__pivot - __size1, __val2.__end};
             }
             // else we should swap the first part of false part of left range and true part of right range
             else
             {
                 __par_backend::__parallel_for(
-                    std::forward<_ExecutionPolicy>(__exec), __val1.__pivot, __val1.__pivot + __size2,
-                    [__val1, __val2, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                        __internal::__brick_swap_ranges(__i, __j, __val2.__begin + (__i - __val1.__pivot), __is_vector);
+                    __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __val1.__pivot, __val1.__pivot + __size2,
+                    [__val1, __val2](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                        __internal::__brick_swap_ranges(__i, __j, __val2.__begin + (__i - __val1.__pivot), _IsVector{});
                     });
                 return {__new_begin, __val1.__pivot + __size2, __val2.__end};
             }
         };
 
         _PartitionRange __result = __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
-            [__pred, __is_vector, __reductor](_RandomAccessIterator __i, _RandomAccessIterator __j,
-                                              _PartitionRange __value) -> _PartitionRange {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
+            [__pred, __reductor](_RandomAccessIterator __i, _RandomAccessIterator __j,
                 //1. serial partition
-                _RandomAccessIterator __pivot = __internal::__brick_partition(__i, __j, __pred, __is_vector);
+                _RandomAccessIterator __pivot = __internal::__brick_partition(__i, __j, __pred, _IsVector{});
 
                 // 2. merging of two ranges (left and right respectively)
                 return __reductor(__value, {__i, __pivot, __j});
             },
             __reductor);
         return __result.__pivot;
     });
 }

 _RandomAccessIterator
 __brick_stable_partition(_RandomAccessIterator __first, _RandomAccessIterator __last, _UnaryPredicate __pred,
                          /*__is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::stable_partition(__first, __last, __pred);
 }
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _UnaryPredicate>
 _BidirectionalIterator
-__pattern_stable_partition(_ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
-                           _UnaryPredicate __pred, _IsVector __is_vector,
-                           /*is_parallelization=*/std::false_type) noexcept
+__pattern_stable_partition(_Tag, _ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __last,
+                           _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_stable_partition(__first, __last, __pred, __is_vector);
+    return __internal::__brick_stable_partition(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_stable_partition(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                           _UnaryPredicate __pred, _IsVector __is_vector,
-                           /*is_parallelization=*/std::true_type) noexcept
+__pattern_stable_partition(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                           _RandomAccessIterator __last, _UnaryPredicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     // partitioned range: elements before pivot satisfy pred (true part),
     //                    elements after pivot don't satisfy pred (false part)
     struct _PartitionRange
     {
         _RandomAccessIterator __begin;
         _RandomAccessIterator __pivot;
         _RandomAccessIterator __end;
     };
 
     return __internal::__except_handler([&]() {
         _PartitionRange __init{__last, __last, __last};
 
         // lambda for merging two partitioned ranges to one partitioned range
-        auto __reductor = [__is_vector](_PartitionRange __val1, _PartitionRange __val2) -> _PartitionRange {
+        auto __reductor = [](_PartitionRange __val1, _PartitionRange __val2) -> _PartitionRange {
             auto __size1 = __val1.__end - __val1.__pivot;
             auto __new_begin = __val2.__begin - (__val1.__end - __val1.__begin);
 
             // if all elements in left range satisfy pred then we can move new pivot to pivot of right range
             if (__val1.__end == __val1.__pivot)
             {
                 return {__new_begin, __val2.__pivot, __val2.__end};
             }
             // if true part of right range greater than false part of left range
             // then we should swap the false part of left range and last part of true part of right range
             else
             {
-                __internal::__brick_rotate(__val1.__pivot, __val2.__begin, __val2.__pivot, __is_vector);
+                __internal::__brick_rotate(__val1.__pivot, __val2.__begin, __val2.__pivot, _IsVector{});
                 return {__new_begin, __val2.__pivot - __size1, __val2.__end};
             }
         };
 
         _PartitionRange __result = __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
-            [&__pred, __is_vector, __reductor](_RandomAccessIterator __i, _RandomAccessIterator __j,
-                                               _PartitionRange __value) -> _PartitionRange {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, __init,
+            [&__pred, __reductor](_RandomAccessIterator __i, _RandomAccessIterator __j,
                 //1. serial stable_partition
-                _RandomAccessIterator __pivot = __internal::__brick_stable_partition(__i, __j, __pred, __is_vector);
+                _RandomAccessIterator __pivot = __internal::__brick_stable_partition(__i, __j, __pred, _IsVector{});
 
                 // 2. merging of two ranges (left and right respectively)
                 return __reductor(__value, {__i, __pivot, __j});
             },
             __reductor);
         return __result.__pivot;
     });
 }

 {
 #if defined(_PSTL_MONOTONIC_PRESENT)
     return __unseq_backend::__simd_partition_copy(__first, __last - __first, __out_true, __out_false, __pred);
 #else
     return std::partition_copy(__first, __last, __out_true, __out_false, __pred);
 #endif
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator1, class _OutputIterator2,
-          class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator1, class _OutputIterator2,
+          class _UnaryPredicate>
 std::pair<_OutputIterator1, _OutputIterator2>
-__pattern_partition_copy(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
-                         _OutputIterator1 __out_true, _OutputIterator2 __out_false, _UnaryPredicate __pred,
-                         _IsVector __is_vector, /*is_parallelization=*/std::false_type) noexcept
+__pattern_partition_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                         _OutputIterator1 __out_true, _OutputIterator2 __out_false, _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_partition_copy(__first, __last, __out_true, __out_false, __pred, __is_vector);
+    return __internal::__brick_partition_copy(__first, __last, __out_true, __out_false, __pred,
+                                              typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _UnaryPredicate>
 std::pair<_RandomAccessIterator2, _RandomAccessIterator3>
-__pattern_partition_copy(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                         _RandomAccessIterator2 __out_true, _RandomAccessIterator3 __out_false, _UnaryPredicate __pred,
-                         _IsVector __is_vector, /*is_parallelization=*/std::true_type)
+__pattern_partition_copy(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                         _RandomAccessIterator1 __last, _RandomAccessIterator2 __out_true,
+                         _RandomAccessIterator3 __out_false, _UnaryPredicate __pred)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
     typedef std::pair<_DifferenceType, _DifferenceType> _ReturnType;
     const _DifferenceType __n = __last - __first;
     if (_DifferenceType(1) < __n)
     {
         __par_backend::__buffer<bool> __mask_buf(__n);
-        return __internal::__except_handler([&__exec, __n, __first, __out_true, __out_false, __is_vector, __pred,
-                                             &__mask_buf]() {
+        return __internal::__except_handler([&__exec, __n, __first, __out_true, __out_false, __pred, &__mask_buf]() {
             bool* __mask = __mask_buf.get();
             _ReturnType __m{};
             __par_backend::__parallel_strict_scan(
-                std::forward<_ExecutionPolicy>(__exec), __n, std::make_pair(_DifferenceType(0), _DifferenceType(0)),
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n,
+                std::make_pair(_DifferenceType(0), _DifferenceType(0)),
                 [=](_DifferenceType __i, _DifferenceType __len) { // Reduce
                     return __internal::__brick_calc_mask_1<_DifferenceType>(__first + __i, __first + (__i + __len),
-                                                                            __mask + __i, __pred, __is_vector);
+                                                                            __mask + __i, __pred, _IsVector{});
                 },
                 [](const _ReturnType& __x, const _ReturnType& __y) -> _ReturnType {
                     return std::make_pair(__x.first + __y.first, __x.second + __y.second);
                 },                                                                       // Combine
                 [=](_DifferenceType __i, _DifferenceType __len, _ReturnType __initial) { // Scan
                     __internal::__brick_partition_by_mask(__first + __i, __first + (__i + __len),
                                                           __out_true + __initial.first, __out_false + __initial.second,
-                                                          __mask + __i, __is_vector);
+                                                          __mask + __i, _IsVector{});
                 },
                 [&__m](_ReturnType __total) { __m = __total; });
             return std::make_pair(__out_true + __m.first, __out_false + __m.second);
         });
     }
     // trivial sequence - use serial algorithm
-    return __internal::__brick_partition_copy(__first, __last, __out_true, __out_false, __pred, __is_vector);
+    return __internal::__brick_partition_copy(__first, __last, __out_true, __out_false, __pred, _IsVector{});
 }
 
 //------------------------------------------------------------------------
 // sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector,
-          class _IsMoveConstructible>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsMoveConstructible>
 void
-__pattern_sort(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
-               _IsVector /*is_vector*/, /*is_parallel=*/std::false_type, _IsMoveConstructible) noexcept
+__pattern_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
+               _IsMoveConstructible) noexcept
 {
     std::sort(__first, __last, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
-               _IsVector /*is_vector*/, /*is_parallel=*/std::true_type, /*is_move_constructible=*/std::true_type)
+__pattern_sort(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+               _RandomAccessIterator __last, _Compare __comp, /*is_move_constructible=*/std::true_type)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __internal::__except_handler([&]() {
-        __par_backend::__parallel_stable_sort(std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
+        __par_backend::__parallel_stable_sort(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                              __comp,
                                               [](_RandomAccessIterator __first, _RandomAccessIterator __last,
                                                  _Compare __comp) { std::sort(__first, __last, __comp); });
     });
 }
 
 //------------------------------------------------------------------------
 // stable_sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_stable_sort(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
-                      _IsVector /*is_vector*/, /*is_parallel=*/std::false_type) noexcept
+__pattern_stable_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last,
+                      _Compare __comp) noexcept
 {
     std::stable_sort(__first, __last, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_stable_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                      _Compare __comp, _IsVector /*is_vector*/, /*is_parallel=*/std::true_type)
+__pattern_stable_sort(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                      _RandomAccessIterator __last, _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __internal::__except_handler([&]() {
-        __par_backend::__parallel_stable_sort(std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
+        __par_backend::__parallel_stable_sort(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                              __comp,
                                               [](_RandomAccessIterator __first, _RandomAccessIterator __last,
                                                  _Compare __comp) { std::stable_sort(__first, __last, __comp); });
     });
 }
 
 //------------------------------------------------------------------------
 // partial_sort
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_partial_sort(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __middle,
-                       _RandomAccessIterator __last, _Compare __comp, _IsVector,
-                       /*is_parallel=*/std::false_type) noexcept
+__pattern_partial_sort(_Tag, _ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __middle,
+                       _RandomAccessIterator __last, _Compare __comp) noexcept
 {
     std::partial_sort(__first, __middle, __last, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_partial_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __middle,
-                       _RandomAccessIterator __last, _Compare __comp, _IsVector, /*is_parallel=*/std::true_type)
+__pattern_partial_sort(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                       _RandomAccessIterator __middle, _RandomAccessIterator __last, _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     const auto __n = __middle - __first;
     if (__n == 0)
         return;
 
     __internal::__except_handler([&]() {
         __par_backend::__parallel_stable_sort(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
             [__n](_RandomAccessIterator __begin, _RandomAccessIterator __end, _Compare __comp) {
                 if (__n < __end - __begin)
                     std::partial_sort(__begin, __begin + __n, __end, __comp);
                 else
                     std::sort(__begin, __end, __comp);
             },
             __n);
     });
 }
 
 //------------------------------------------------------------------------
 // partial_sort_copy
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
-__pattern_partial_sort_copy(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
-                            _RandomAccessIterator __d_first, _RandomAccessIterator __d_last, _Compare __comp, _IsVector,
-                            /*is_parallel=*/std::false_type) noexcept
+__pattern_partial_sort_copy(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                            _RandomAccessIterator __d_first, _RandomAccessIterator __d_last, _Compare __comp) noexcept
 {
     return std::partial_sort_copy(__first, __last, __d_first, __d_last, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 _RandomAccessIterator2
-__pattern_partial_sort_copy(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                            _RandomAccessIterator2 __d_first, _RandomAccessIterator2 __d_last, _Compare __comp,
-                            _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_partial_sort_copy(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first,
+                            _RandomAccessIterator1 __last, _RandomAccessIterator2 __d_first,
+                            _RandomAccessIterator2 __d_last, _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__last == __first || __d_last == __d_first)
     {
         return __d_first;
     }
     auto __n1 = __last - __first;
     auto __n2 = __d_last - __d_first;
     return __internal::__except_handler([&]() {
         if (__n2 >= __n1)
         {
             __par_backend::__parallel_stable_sort(
-                std::forward<_ExecutionPolicy>(__exec), __d_first, __d_first + __n1, __comp,
-                [__first, __d_first, __is_vector](_RandomAccessIterator2 __i, _RandomAccessIterator2 __j,
-                                                  _Compare __comp) {
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __d_first, __d_first + __n1, __comp,
+                [__first, __d_first](_RandomAccessIterator2 __i, _RandomAccessIterator2 __j, _Compare __comp) {
                     _RandomAccessIterator1 __i1 = __first + (__i - __d_first);
                     _RandomAccessIterator1 __j1 = __first + (__j - __d_first);
 
                 // 1. Copy elements from input to output
 #if !defined(_PSTL_ICC_18_OMP_SIMD_BROKEN)
-                    __internal::__brick_copy(__i1, __j1, __i, __is_vector);
+                    __internal::__brick_copy(__i1, __j1, __i, _IsVector{});
 #else
                     std::copy(__i1, __j1, __i);
 #endif
                     // 2. Sort elements in output sequence
                     std::sort(__i, __j, __comp);
                 },
                 __n1);
             return __d_first + __n1;
         }
         else
         {
             typedef typename std::iterator_traits<_RandomAccessIterator1>::value_type _T1;
             typedef typename std::iterator_traits<_RandomAccessIterator2>::value_type _T2;
             __par_backend::__buffer<_T1> __buf(__n1);
             _T1* __r = __buf.get();
 
-            __par_backend::__parallel_stable_sort(std::forward<_ExecutionPolicy>(__exec), __r, __r + __n1, __comp,
-                                                  [__n2, __first, __r](_T1* __i, _T1* __j, _Compare __comp) {
-                                                      _RandomAccessIterator1 __it = __first + (__i - __r);
+            __par_backend::__parallel_stable_sort(
+                __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __r, __r + __n1, __comp,
 
-                                                      // 1. Copy elements from input to raw memory
-                                                      for (_T1* __k = __i; __k != __j; ++__k, ++__it)
-                                                      {
-                                                          ::new (__k) _T2(*__it);
-                                                      }
 
-                                                      // 2. Sort elements in temporary __buffer
-                                                      if (__n2 < __j - __i)
-                                                          std::partial_sort(__i, __i + __n2, __j, __comp);
-                                                      else
-                                                          std::sort(__i, __j, __comp);
-                                                  },
-                                                  __n2);
 
             // 3. Move elements from temporary __buffer to output
-            __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __r, __r + __n2,
-                                          [__r, __d_first, __is_vector](_T1* __i, _T1* __j) {
-                                              __brick_move_destroy()(__i, __j, __d_first + (__i - __r), __is_vector);
-                                          });
-            __par_backend::__parallel_for(
-                std::forward<_ExecutionPolicy>(__exec), __r + __n2, __r + __n1,
-                [__is_vector](_T1* __i, _T1* __j) { __brick_destroy(__i, __j, __is_vector); });
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __r, __r + __n2,
+                                          [__r, __d_first](_T1* __i, _T1* __j) {
+                                              __brick_move_destroy()(__i, __j, __d_first + (__i - __r), _IsVector{});
+                                          });
+            __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __r + __n2,
+                                          __r + __n1,
+                                          [](_T1* __i, _T1* __j) { __brick_destroy(__i, __j, _IsVector{}); });
 
             return __d_first + __n2;
         }
     });
 }
 
 //------------------------------------------------------------------------
 // adjacent_find

 template <class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
 __brick_adjacent_find(_ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred,
                       /* IsVector = */ std::false_type, bool) noexcept
 {
     return std::adjacent_find(__first, __last, __pred);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 _ForwardIterator
-__pattern_adjacent_find(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred,
-                        /* is_parallel */ std::false_type, _IsVector __is_vector, bool __or_semantic) noexcept
+__pattern_adjacent_find(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                        _BinaryPredicate __pred, bool __or_semantic) noexcept
 {
-    return __internal::__brick_adjacent_find(__first, __last, __pred, __is_vector, __or_semantic);
+    return __internal::__brick_adjacent_find(__first, __last, __pred, typename _Tag::__is_vector{}, __or_semantic);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _BinaryPredicate>
 _RandomAccessIterator
-__pattern_adjacent_find(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                        _BinaryPredicate __pred, /* is_parallel */ std::true_type, _IsVector __is_vector,
-                        bool __or_semantic)
+__pattern_adjacent_find(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                        _RandomAccessIterator __last, _BinaryPredicate __pred, bool __or_semantic)
 {
     if (__last - __first < 2)
         return __last;
 
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last, __last,
-            [__last, __pred, __is_vector, __or_semantic](_RandomAccessIterator __begin, _RandomAccessIterator __end,
-                                                         _RandomAccessIterator __value) -> _RandomAccessIterator {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last, __last,
+            [__last, __pred, __or_semantic](_RandomAccessIterator __begin, _RandomAccessIterator __end,
                 // TODO: investigate performance benefits from the use of shared variable for the result,
                 // checking (compare_and_swap idiom) its __value at __first.
                 if (__or_semantic && __value < __last)
                 { //found
                     __par_backend::__cancel_execution();
                     return __value;
                 }
 
                 if (__value > __begin)
                 {
                     // modify __end to check the predicate on the boundary __values;
                     // TODO: to use a custom range with boundaries overlapping
                     // TODO: investigate what if we remove "if" below and run algorithm on range [__first, __last-1)
                     // then check the pair [__last-1, __last)
                     if (__end != __last)
                         ++__end;
 
                     //correct the global result iterator if the "brick" returns a local "__last"
                     const _RandomAccessIterator __res =
-                        __internal::__brick_adjacent_find(__begin, __end, __pred, __is_vector, __or_semantic);
+                        __internal::__brick_adjacent_find(__begin, __end, __pred, _IsVector{}, __or_semantic);
                     if (__res < __end)
                         __value = __res;
                 }
                 return __value;
             },
             [](_RandomAccessIterator __x, _RandomAccessIterator __y) -> _RandomAccessIterator {
                 return __x < __y ? __x : __y;
             } //reduce a __value
         );
     });
 }
 
 //------------------------------------------------------------------------
 // nth_element
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_nth_element(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __nth,
-                      _RandomAccessIterator __last, _Compare __comp, _IsVector,
-                      /*is_parallel=*/std::false_type) noexcept
+__pattern_nth_element(_Tag, _ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __nth,
+                      _RandomAccessIterator __last, _Compare __comp) noexcept
 {
     std::nth_element(__first, __nth, __last, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_nth_element(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __nth,
-                      _RandomAccessIterator __last, _Compare __comp, _IsVector __is_vector,
-                      /*is_parallel=*/std::true_type) noexcept
+__pattern_nth_element(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                      _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) noexcept
 {
     if (__first == __last || __nth == __last)
     {
         return;
     }
 
     using std::iter_swap;
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _Tp;
     _RandomAccessIterator __x;
     do
     {
-        __x = __internal::__pattern_partition(std::forward<_ExecutionPolicy>(__exec), __first + 1, __last,
-                                              [&__comp, __first](const _Tp& __x) { return __comp(__x, *__first); },
-                                              __is_vector,
-                                              /*is_parallel=*/std::true_type());
+        __x = __internal::__pattern_partition(__tag, std::forward<_ExecutionPolicy>(__exec), __first + 1, __last,
+                                              [&__comp, __first](const _Tp& __x) { return __comp(__x, *__first); });
         --__x;
         if (__x != __first)
         {
             iter_swap(__first, __x);
         }
         // if x > nth then our new range for partition is [first, x)
         if (__x - __nth > 0)
         {

 template <class _ForwardIterator, class _Tp>
 void
 __brick_fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value,
              /* __is_vector = */ std::false_type) noexcept
 {
     std::fill(__first, __last, __value);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 void
-__pattern_fill(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value,
-               /*is_parallel=*/std::false_type, _IsVector __is_vector) noexcept
+__pattern_fill(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) noexcept
 {
-    __internal::__brick_fill(__first, __last, __value, __is_vector);
+    __internal::__brick_fill(__first, __last, __value, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Tp>
 _RandomAccessIterator
-__pattern_fill(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-               const _Tp& __value,
-               /*is_parallel=*/std::true_type, _IsVector __is_vector)
+__pattern_fill(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+               _RandomAccessIterator __last, const _Tp& __value)
 {
-    return __internal::__except_handler([&__exec, __first, __last, &__value, __is_vector]() {
-        __par_backend::__parallel_for(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
-            [&__value, __is_vector](_RandomAccessIterator __begin, _RandomAccessIterator __end) {
-                __internal::__brick_fill(__begin, __end, __value, __is_vector);
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
+    return __internal::__except_handler([&__exec, __first, __last, &__value]() {
+        __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                      [&__value](_RandomAccessIterator __begin, _RandomAccessIterator __end) {
+                                          __internal::__brick_fill(__begin, __end, __value, _IsVector{});
                                       });
         return __last;
     });
 }
 
 template <class _RandomAccessIterator, class _Size, class _Tp>
 _RandomAccessIterator
 __brick_fill_n(_RandomAccessIterator __first, _Size __count, const _Tp& __value,
                /* __is_vector = */ std::true_type) noexcept
 {
     return __unseq_backend::__simd_fill_n(__first, __count, __value);
 }
 
 template <class _OutputIterator, class _Size, class _Tp>
 _OutputIterator
 __brick_fill_n(_OutputIterator __first, _Size __count, const _Tp& __value, /* __is_vector = */ std::false_type) noexcept
 {
     return std::fill_n(__first, __count, __value);
 }
 
-template <class _ExecutionPolicy, class _OutputIterator, class _Size, class _Tp, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _OutputIterator, class _Size, class _Tp>
 _OutputIterator
-__pattern_fill_n(_ExecutionPolicy&&, _OutputIterator __first, _Size __count, const _Tp& __value,
-                 /*is_parallel=*/std::false_type, _IsVector __is_vector) noexcept
+__pattern_fill_n(_Tag, _ExecutionPolicy&&, _OutputIterator __first, _Size __count, const _Tp& __value) noexcept
 {
-    return __internal::__brick_fill_n(__first, __count, __value, __is_vector);
+    return __internal::__brick_fill_n(__first, __count, __value, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Tp>
 _RandomAccessIterator
-__pattern_fill_n(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _Size __count, const _Tp& __value,
-                 /*is_parallel=*/std::true_type, _IsVector __is_vector)
+__pattern_fill_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                 _Size __count, const _Tp& __value)
 {
-    return __internal::__pattern_fill(std::forward<_ExecutionPolicy>(__exec), __first, __first + __count, __value,
-                                      std::true_type(), __is_vector);
+    return __internal::__pattern_fill(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __first + __count,
+                                      __value);
 }
 
 //------------------------------------------------------------------------
 // generate, generate_n
 //------------------------------------------------------------------------
 template <class _RandomAccessIterator, class _Generator>
 void
 __brick_generate(_RandomAccessIterator __first, _RandomAccessIterator __last, _Generator __g,
                  /* is_vector = */ std::true_type) noexcept
 {
     __unseq_backend::__simd_generate_n(__first, __last - __first, __g);
 }
 
 template <class _ForwardIterator, class _Generator>
 void
 __brick_generate(_ForwardIterator __first, _ForwardIterator __last, _Generator __g,
                  /* is_vector = */ std::false_type) noexcept
 {
     std::generate(__first, __last, __g);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Generator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Generator>
 void
-__pattern_generate(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Generator __g,
-                   /*is_parallel=*/std::false_type, _IsVector __is_vector) noexcept
+__pattern_generate(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Generator __g) noexcept
 {
-    __internal::__brick_generate(__first, __last, __g, __is_vector);
+    __internal::__brick_generate(__first, __last, __g, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Generator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Generator>
 _RandomAccessIterator
-__pattern_generate(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                   _Generator __g,
-                   /*is_parallel=*/std::true_type, _IsVector __is_vector)
+__pattern_generate(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                   _RandomAccessIterator __last, _Generator __g)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
-        __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                      [__g, __is_vector](_RandomAccessIterator __begin, _RandomAccessIterator __end) {
-                                          __internal::__brick_generate(__begin, __end, __g, __is_vector);
+        __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                      [__g](_RandomAccessIterator __begin, _RandomAccessIterator __end) {
+                                          __internal::__brick_generate(__begin, __end, __g, _IsVector{});
                                       });
         return __last;
     });
 }
 
 template <class _RandomAccessIterator, class Size, class _Generator>
 _RandomAccessIterator
 __brick_generate_n(_RandomAccessIterator __first, Size __count, _Generator __g,
                    /* is_vector = */ std::true_type) noexcept
 {
     return __unseq_backend::__simd_generate_n(__first, __count, __g);
 }
 
 template <class OutputIterator, class Size, class _Generator>
 OutputIterator
 __brick_generate_n(OutputIterator __first, Size __count, _Generator __g, /* is_vector = */ std::false_type) noexcept
 {
     return std::generate_n(__first, __count, __g);
 }
 
-template <class _ExecutionPolicy, class _OutputIterator, class _Size, class _Generator, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _OutputIterator, class _Size, class _Generator>
 _OutputIterator
-__pattern_generate_n(_ExecutionPolicy&&, _OutputIterator __first, _Size __count, _Generator __g,
-                     /*is_parallel=*/std::false_type, _IsVector __is_vector) noexcept
+__pattern_generate_n(_Tag, _ExecutionPolicy&&, _OutputIterator __first, _Size __count, _Generator __g) noexcept
 {
-    return __internal::__brick_generate_n(__first, __count, __g, __is_vector);
+    return __internal::__brick_generate_n(__first, __count, __g, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Generator, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Size, class _Generator>
 _RandomAccessIterator
-__pattern_generate_n(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _Size __count, _Generator __g,
-                     /*is_parallel=*/std::true_type, _IsVector __is_vector)
+__pattern_generate_n(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                     _Size __count, _Generator __g)
 {
-    static_assert(__is_random_access_iterator<_RandomAccessIterator>::value,
+    static_assert(__are_random_access_iterators<_RandomAccessIterator>::value,
                   "Pattern-brick error. Should be a random access iterator.");
-    return __internal::__pattern_generate(std::forward<_ExecutionPolicy>(__exec), __first, __first + __count, __g,
-                                          std::true_type(), __is_vector);
+    return __internal::__pattern_generate(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __first + __count,
+                                          __g);
 }
 
 //------------------------------------------------------------------------
 // remove
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator

 {
 #if defined(_PSTL_MONOTONIC_PRESENT)
     return __unseq_backend::__simd_remove_if(__first, __last - __first, __pred);
 #else
     return std::remove_if(__first, __last, __pred);
 #endif
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 _ForwardIterator
-__pattern_remove_if(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred,
-                    _IsVector __is_vector, /*is_parallel*/ std::false_type) noexcept
+__pattern_remove_if(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                    _UnaryPredicate __pred) noexcept
 {
-    return __internal::__brick_remove_if(__first, __last, __pred, __is_vector);
+    return __internal::__brick_remove_if(__first, __last, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryPredicate>
 _RandomAccessIterator
-__pattern_remove_if(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                    _UnaryPredicate __pred, _IsVector __is_vector, /*is_parallel*/ std::true_type) noexcept
+__pattern_remove_if(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                    _RandomAccessIterator __last, _UnaryPredicate __pred) noexcept
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::reference _ReferenceType;
 
     if (__first == __last || __first + 1 == __last)
     {
         // Trivial sequence - use serial algorithm
-        return __internal::__brick_remove_if(__first, __last, __pred, __is_vector);
+        return __internal::__brick_remove_if(__first, __last, __pred, _IsVector{});
     }
 
     return __internal::__remove_elements(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last,
-        [&__pred, __is_vector](bool* __b, bool* __e, _RandomAccessIterator __it) {
-            __internal::__brick_walk2(__b, __e, __it, [&__pred](bool& __x, _ReferenceType __y) { __x = !__pred(__y); },
-                                      __is_vector);
-        },
-        __is_vector);
+        __tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+        [&__pred](bool* __b, bool* __e, _RandomAccessIterator __it) {
+            __internal::__brick_walk2(
+                __b, __e, __it, [&__pred](bool& __x, _ReferenceType __y) { __x = !__pred(__y); }, _IsVector{});
+        });
 }
 
 //------------------------------------------------------------------------
 // merge
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator

 __brick_merge(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
               _RandomAccessIterator2 __last2, _RandomAccessIterator3 __d_first, _Compare __comp,
               /* __is_vector = */ std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::merge(__first1, __last1, __first2, __last2, __d_first, __comp);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_merge(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
-                _ForwardIterator2 __last2, _OutputIterator __d_first, _Compare __comp, _IsVector __is_vector,
-                /* is_parallel = */ std::false_type) noexcept
+__pattern_merge(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __d_first,
+                _Compare __comp) noexcept
 {
-    return __internal::__brick_merge(__first1, __last1, __first2, __last2, __d_first, __comp, __is_vector);
+    return __internal::__brick_merge(__first1, __last1, __first2, __last2, __d_first, __comp,
+                                     typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Compare>
 _RandomAccessIterator3
-__pattern_merge(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _RandomAccessIterator3 __d_first,
-                _Compare __comp, _IsVector __is_vector, /* is_parallel = */ std::true_type)
+__pattern_merge(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                _RandomAccessIterator3 __d_first, _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     __par_backend::__parallel_merge(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __d_first, __comp,
-        [__is_vector](_RandomAccessIterator1 __f1, _RandomAccessIterator1 __l1, _RandomAccessIterator2 __f2,
-                      _RandomAccessIterator2 __l2, _RandomAccessIterator3 __f3, _Compare __comp) {
-            return __internal::__brick_merge(__f1, __l1, __f2, __l2, __f3, __comp, __is_vector);
-        });
+        __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __d_first,
+        __comp,
+        [](_RandomAccessIterator1 __f1, _RandomAccessIterator1 __l1, _RandomAccessIterator2 __f2,
+           _RandomAccessIterator2 __l2, _RandomAccessIterator3 __f3,
+           _Compare __comp) { return __internal::__brick_merge(__f1, __l1, __f2, __l2, __f3, __comp, _IsVector{}); });
     return __d_first + (__last1 - __first1) + (__last2 - __first2);
 }
 
 //------------------------------------------------------------------------
 // inplace_merge
 //------------------------------------------------------------------------
 template <class _BidirectionalIterator, class _Compare>
 void
 __brick_inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator __middle, _BidirectionalIterator __last,
                       _Compare __comp, /* __is_vector = */ std::false_type) noexcept
 {
     std::inplace_merge(__first, __middle, __last, __comp);
 }
 
 template <class _RandomAccessIterator, class _Compare>
 void
 __brick_inplace_merge(_RandomAccessIterator __first, _RandomAccessIterator __middle, _RandomAccessIterator __last,
                       _Compare __comp, /* __is_vector = */ std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial")
     std::inplace_merge(__first, __middle, __last, __comp);
 }
 
-template <class _ExecutionPolicy, class _BidirectionalIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _BidirectionalIterator, class _Compare>
 void
-__pattern_inplace_merge(_ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __middle,
-                        _BidirectionalIterator __last, _Compare __comp, _IsVector __is_vector,
-                        /* is_parallel = */ std::false_type) noexcept
+__pattern_inplace_merge(_Tag, _ExecutionPolicy&&, _BidirectionalIterator __first, _BidirectionalIterator __middle,
+                        _BidirectionalIterator __last, _Compare __comp) noexcept
 {
-    __internal::__brick_inplace_merge(__first, __middle, __last, __comp, __is_vector);
+    __internal::__brick_inplace_merge(__first, __middle, __last, __comp, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 void
-__pattern_inplace_merge(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __middle,
-                        _RandomAccessIterator __last, _Compare __comp, _IsVector __is_vector,
-                        /*is_parallel=*/std::true_type)
+__pattern_inplace_merge(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                        _RandomAccessIterator __middle, _RandomAccessIterator __last, _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__first == __last || __first == __middle || __middle == __last)
     {
         return;
     }
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _Tp;
     auto __n = __last - __first;
     __par_backend::__buffer<_Tp> __buf(__n);
     _Tp* __r = __buf.get();
     __internal::__except_handler([&]() {
         auto __move_values = [](_RandomAccessIterator __x, _Tp* __z) {
-            __internal::__invoke_if_else(std::is_trivial<_Tp>(), [&]() { *__z = std::move(*__x); },
-                                         [&]() { ::new (std::addressof(*__z)) _Tp(std::move(*__x)); });
+            __internal::__invoke_if_else(
+                std::is_trivial<_Tp>(), [&]() { *__z = std::move(*__x); },
         };
 
         auto __move_sequences = [](_RandomAccessIterator __first1, _RandomAccessIterator __last1, _Tp* __first2) {
             return __internal::__brick_uninitialized_move(__first1, __last1, __first2, _IsVector());
         };
 
         __par_backend::__parallel_merge(
-            std::forward<_ExecutionPolicy>(__exec), __first, __middle, __middle, __last, __r, __comp,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __middle, __middle, __last, __r, __comp,
             [__n, __move_values, __move_sequences](_RandomAccessIterator __f1, _RandomAccessIterator __l1,
                                                    _RandomAccessIterator __f2, _RandomAccessIterator __l2, _Tp* __f3,
                                                    _Compare __comp) {
                 (__utils::__serial_move_merge(__n))(__f1, __l1, __f2, __l2, __f3, __comp, __move_values, __move_values,
                                                     __move_sequences, __move_sequences);
                 return __f3 + (__l1 - __f1) + (__l2 - __f2);
             });
-        __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __r, __r + __n,
-                                      [__r, __first, __is_vector](_Tp* __i, _Tp* __j) {
-                                          __brick_move_destroy()(__i, __j, __first + (__i - __r), __is_vector);
+        __par_backend::__parallel_for(__backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __r, __r + __n,
+                                      [__r, __first](_Tp* __i, _Tp* __j) {
+                                          __brick_move_destroy()(__i, __j, __first + (__i - __r), _IsVector{});
                                       });
     });
 }
 
 //------------------------------------------------------------------------
 // includes
 //------------------------------------------------------------------------
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 bool
-__pattern_includes(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                   _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Compare __comp, _IsVector,
-                   /*is_parallel=*/std::false_type) noexcept
+__pattern_includes(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                   _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Compare __comp) noexcept
 {
     return std::includes(__first1, __last1, __first2, __last2, __comp);
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 bool
-__pattern_includes(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                   _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _Compare __comp, _IsVector,
-                   /*is_parallel=*/std::true_type)
+__pattern_includes(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                   _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                   _Compare __comp)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__first2 >= __last2)
         return true;
 
     if (__first1 >= __last1 || __comp(*__first2, *__first1) || __comp(*(__last1 - 1), *(__last2 - 1)))
         return false;
 
     __first1 = std::lower_bound(__first1, __last1, *__first2, __comp);
     if (__first1 == __last1)
         return false;
 
     if (__last2 - __first2 == 1)
         return !__comp(*__first1, *__first2) && !__comp(*__first2, *__first1);
 
     return __internal::__except_handler([&]() {
         return !__internal::__parallel_or(
-            std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
             [__first1, __last1, __first2, __last2, &__comp](_RandomAccessIterator2 __i, _RandomAccessIterator2 __j) {
                 _PSTL_ASSERT(__j > __i);
                 //_PSTL_ASSERT(__j - __i > 1);
 
                 //1. moving boundaries to "consume" subsequence of equal elements
                 auto __is_equal = [&__comp](_RandomAccessIterator2 __a, _RandomAccessIterator2 __b) -> bool {
                     return !__comp(*__a, *__b) && !__comp(*__b, *__a);
                 };
 
                 //1.1 left bound, case "aaa[aaaxyz...]" - searching "x"
                 if (__i > __first2 && __is_equal(__i, __i - 1))
                 {
                     //whole subrange continues to content equal elements - return "no op"
                     if (__is_equal(__i, __j - 1))
                         return false;
 
                     __i = std::upper_bound(__i, __last2, *__i, __comp);
                 }
 
                 //1.2 right bound, case "[...aaa]aaaxyz" - searching "x"
                 if (__j < __last2 && __is_equal(__j - 1, __j))
                     __j = std::upper_bound(__j, __last2, *__j, __comp);
 
                 //2. testing is __a subsequence of the second range included into the first range
                 auto __b = std::lower_bound(__first1, __last1, *__i, __comp);
 
                 _PSTL_ASSERT(!__comp(*(__last1 - 1), *__b));
                 _PSTL_ASSERT(!__comp(*(__j - 1), *__i));
                 return !std::includes(__b, __last1, __i, __j, __comp);
             });
     });
 }
 
 constexpr auto __set_algo_cut_off = 1000;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector, class _SizeFunction, class _SetOP>
+template <class _IsVector, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2,
+          class _OutputIterator, class _Compare, class _SizeFunction, class _SetOP>
 _OutputIterator
-__parallel_set_op(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                  _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result, _Compare __comp,
-                  _SizeFunction __size_func, _SetOP __set_op, _IsVector __is_vector)
+__parallel_set_op(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _ForwardIterator1 __first1,
+                  _ForwardIterator1 __last1, _ForwardIterator2 __first2, _ForwardIterator2 __last2,
+                  _OutputIterator __result, _Compare __comp, _SizeFunction __size_func, _SetOP __set_op)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_ForwardIterator1>::difference_type _DifferenceType;
     typedef typename std::iterator_traits<_OutputIterator>::value_type _Tp;
 
     struct _SetRange
     {
         _DifferenceType __pos, __len, __buf_pos;
         bool
         empty() const
         {
             return __len == 0;
         }
     };
 
     const _DifferenceType __n1 = __last1 - __first1;
     const _DifferenceType __n2 = __last2 - __first2;
 
     __par_backend::__buffer<_Tp> __buf(__size_func(__n1, __n2));
 
-    return __internal::__except_handler([&__exec, __n1, __first1, __last1, __first2, __last2, __result, __is_vector,
-                                         __comp, __size_func, __set_op, &__buf]() {
+    return __internal::__except_handler([&__exec, __n1, __first1, __last1, __first2, __last2, __result, __comp,
+                                         __size_func, __set_op, &__buf]() {
         auto __buffer = __buf.get();
         _DifferenceType __m{};
         auto __scan = [=](_DifferenceType, _DifferenceType, const _SetRange& __s) { // Scan
             if (!__s.empty())
                 __brick_move_destroy()(__buffer + __s.__buf_pos, __buffer + (__s.__buf_pos + __s.__len),
-                                       __result + __s.__pos, __is_vector);
+                                       __result + __s.__pos, _IsVector{});
         };
         __par_backend::__parallel_strict_scan(
-            std::forward<_ExecutionPolicy>(__exec), __n1, _SetRange{0, 0, 0}, //-1, 0},
-            [=](_DifferenceType __i, _DifferenceType __len) {                 // Reduce
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n1, _SetRange{0, 0, 0}, //-1, 0},
+            [=](_DifferenceType __i, _DifferenceType __len) {                                  // Reduce
                 //[__b; __e) - a subrange of the first sequence, to reduce
                 _ForwardIterator1 __b = __first1 + __i, __e = __first1 + (__i + __len);
 
                 //try searching for the first element which not equal to *__b
                 if (__b != __first1)
                     __b = std::upper_bound(__b, __last1, *__b, __comp);
 
                 //try searching for the first element which not equal to *__e
                 if (__e != __last1)
                     __e = std::upper_bound(__e, __last1, *__e, __comp);
 
                 //check is [__b; __e) empty
                 if (__e - __b < 1)
                 {
                     _ForwardIterator2 __bb = __last2;
                     if (__b != __last1)
                         __bb = std::lower_bound(__first2, __last2, *__b, __comp);
 
                     const _DifferenceType __buf_pos = __size_func((__b - __first1), (__bb - __first2));
                     return _SetRange{0, 0, __buf_pos};
                 }
 
                 //try searching for "corresponding" subrange [__bb; __ee) in the second sequence
                 _ForwardIterator2 __bb = __first2;
                 if (__b != __first1)
                     __bb = std::lower_bound(__first2, __last2, *__b, __comp);
 
                 _ForwardIterator2 __ee = __last2;
                 if (__e != __last1)
                     __ee = std::lower_bound(__bb, __last2, *__e, __comp);
 
                 const _DifferenceType __buf_pos = __size_func((__b - __first1), (__bb - __first2));
                 auto __buffer_b = __buffer + __buf_pos;
                 auto __res = __set_op(__b, __e, __bb, __ee, __buffer_b, __comp);
 
                 return _SetRange{0, __res - __buffer_b, __buf_pos};
             },
             [](const _SetRange& __a, const _SetRange& __b) { // Combine
                 if (__b.__buf_pos > __a.__buf_pos || ((__b.__buf_pos == __a.__buf_pos) && !__b.empty()))
                     return _SetRange{__a.__pos + __a.__len + __b.__pos, __b.__len, __b.__buf_pos};
                 return _SetRange{__b.__pos + __b.__len + __a.__pos, __a.__len, __a.__buf_pos};
             },
             __scan,                                     // Scan
             [&__m, &__scan](const _SetRange& __total) { // Apex
                 //final scan
                 __scan(0, 0, __total);
                 __m = __total.__pos + __total.__len;
             });
         return __result + __m;
     });
 }
 
 //a shared parallel pattern for '__pattern_set_union' and '__pattern_set_symmetric_difference'
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _SetUnionOp, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare, class _SetUnionOp>
 _OutputIterator
-__parallel_set_union_op(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+__parallel_set_union_op(_Tag __tag, _ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                         _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result,
-                        _Compare __comp, _SetUnionOp __set_union_op, _IsVector __is_vector)
+                        _Compare __comp, _SetUnionOp __set_union_op)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_ForwardIterator1>::difference_type _DifferenceType;
 
     const auto __n1 = __last1 - __first1;
     const auto __n2 = __last2 - __first2;
 
-    auto copy_range1 = [__is_vector](_ForwardIterator1 __begin, _ForwardIterator1 __end, _OutputIterator __res) {
-        return __internal::__brick_copy(__begin, __end, __res, __is_vector);
+    auto copy_range1 = [](_ForwardIterator1 __begin, _ForwardIterator1 __end, _OutputIterator __res) {
+        return __internal::__brick_copy(__begin, __end, __res, typename _Tag::__is_vector{});
     };
-    auto copy_range2 = [__is_vector](_ForwardIterator2 __begin, _ForwardIterator2 __end, _OutputIterator __res) {
-        return __internal::__brick_copy(__begin, __end, __res, __is_vector);
+    auto copy_range2 = [](_ForwardIterator2 __begin, _ForwardIterator2 __end, _OutputIterator __res) {
+        return __internal::__brick_copy(__begin, __end, __res, typename _Tag::__is_vector{});
     };
 
     // {1} {}: parallel copying just first sequence
     if (__n2 == 0)
-        return __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
-                                                 copy_range1, std::true_type());
+        return __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+                                                 __result, copy_range1);
 
     // {} {2}: parallel copying justmake  second sequence
     if (__n1 == 0)
-        return __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first2, __last2, __result,
-                                                 copy_range2, std::true_type());
+        return __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
+                                                 __result, copy_range2);
 
     // testing  whether the sequences are intersected
     _ForwardIterator1 __left_bound_seq_1 = std::lower_bound(__first1, __last1, *__first2, __comp);
 
     if (__left_bound_seq_1 == __last1)
     {
         //{1} < {2}: seq2 is wholly greater than seq1, so, do parallel copying seq1 and seq2
         __par_backend::__parallel_invoke(
-            std::forward<_ExecutionPolicy>(__exec),
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec),
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
-                                                  copy_range1, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+                                                  __result, copy_range1);
             },
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
-                                                  __result + __n1, copy_range2, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
+                                                  __result + __n1, copy_range2);
             });
         return __result + __n1 + __n2;
     }
 
     // testing  whether the sequences are intersected
     _ForwardIterator2 __left_bound_seq_2 = std::lower_bound(__first2, __last2, *__first1, __comp);
 
     if (__left_bound_seq_2 == __last2)
     {
         //{2} < {1}: seq2 is wholly greater than seq1, so, do parallel copying seq1 and seq2
         __par_backend::__parallel_invoke(
-            std::forward<_ExecutionPolicy>(__exec),
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec),
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first2, __last2, __result,
-                                                  copy_range2, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first2, __last2,
+                                                  __result, copy_range2);
             },
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
-                                                  __result + __n2, copy_range1, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+                                                  __result + __n2, copy_range1);
             });
         return __result + __n1 + __n2;
     }
 
     const auto __m1 = __left_bound_seq_1 - __first1;
     if (__m1 > __set_algo_cut_off)
     {
         auto __res_or = __result;
         __result += __m1; //we know proper offset due to [first1; left_bound_seq_1) < [first2; last2)
         __par_backend::__parallel_invoke(
-            std::forward<_ExecutionPolicy>(__exec),
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec),
             //do parallel copying of [first1; left_bound_seq_1)
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first1, __left_bound_seq_1,
-                                                  __res_or, copy_range1, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                                  __left_bound_seq_1, __res_or, copy_range1);
             },
             [=, &__result] {
                 __result = __internal::__parallel_set_op(
-                    std::forward<_ExecutionPolicy>(__exec), __left_bound_seq_1, __last1, __first2, __last2, __result,
-                    __comp, [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; }, __set_union_op,
-                    __is_vector);
+                    __tag, std::forward<_ExecutionPolicy>(__exec), __left_bound_seq_1, __last1, __first2, __last2,
+                    __result, __comp, [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; },
+                    __set_union_op);
             });
         return __result;
     }
 
     const auto __m2 = __left_bound_seq_2 - __first2;
     _PSTL_ASSERT(__m1 == 0 || __m2 == 0);
     if (__m2 > __set_algo_cut_off)
     {
         auto __res_or = __result;
         __result += __m2; //we know proper offset due to [first2; left_bound_seq_2) < [first1; last1)
         __par_backend::__parallel_invoke(
-            std::forward<_ExecutionPolicy>(__exec),
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec),
             //do parallel copying of [first2; left_bound_seq_2)
             [=] {
-                __internal::__pattern_walk2_brick(std::forward<_ExecutionPolicy>(__exec), __first2, __left_bound_seq_2,
-                                                  __res_or, copy_range2, std::true_type());
+                __internal::__pattern_walk2_brick(__tag, std::forward<_ExecutionPolicy>(__exec), __first2,
+                                                  __left_bound_seq_2, __res_or, copy_range2);
             },
             [=, &__result] {
                 __result = __internal::__parallel_set_op(
-                    std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __left_bound_seq_2, __last2, __result,
-                    __comp, [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; }, __set_union_op,
-                    __is_vector);
+                    __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __left_bound_seq_2, __last2,
+                    __result, __comp, [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; },
+                    __set_union_op);
             });
         return __result;
     }
 
     return __internal::__parallel_set_op(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
-        [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; }, __set_union_op, __is_vector);
+        __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
+        [](_DifferenceType __n, _DifferenceType __m) { return __n + __m; }, __set_union_op);
 }
 
 //------------------------------------------------------------------------
 // set_union
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator

 __brick_set_union(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                   _RandomAccessIterator2 __last2, _OutputIterator __result, _Compare __comp,
                   /*__is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::set_union(__first1, __last1, __first2, __last2, __result, __comp);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_union(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                    _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result, _Compare __comp,
-                    _IsVector __is_vector,
-                    /*is_parallel=*/std::false_type) noexcept
+__pattern_set_union(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                    _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result,
+                    _Compare __comp) noexcept
 {
-    return __internal::__brick_set_union(__first1, __last1, __first2, __last2, __result, __comp, __is_vector);
+    return __internal::__brick_set_union(__first1, __last1, __first2, __last2, __result, __comp,
+                                         typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _OutputIterator, class _Compare>
 _OutputIterator
-__pattern_set_union(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                    _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _OutputIterator __result,
-                    _Compare __comp, _IsVector __is_vector, /*__is_parallel=*/std::true_type)
+__pattern_set_union(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                    _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                    _OutputIterator __result, _Compare __comp)
 {
 
     const auto __n1 = __last1 - __first1;
     const auto __n2 = __last2 - __first2;
 
     // use serial algorithm
     if (__n1 + __n2 <= __set_algo_cut_off)
         return std::set_union(__first1, __last1, __first2, __last2, __result, __comp);
 
     typedef typename std::iterator_traits<_OutputIterator>::value_type _Tp;
     return __parallel_set_union_op(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
+        __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
         [](_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
            _RandomAccessIterator2 __last2, _Tp* __result, _Compare __comp) {
             return __pstl::__utils::__set_union_construct(__first1, __last1, __first2, __last2, __result, __comp,
                                                           __BrickCopyConstruct<_IsVector>());
-        },
-        __is_vector);
+        });
 }
 
 //------------------------------------------------------------------------
 // set_intersection
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator, class _Compare>
 _OutputIterator
__brick_set_intersection(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
                          _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
                          _RandomAccessIterator3 __result, _Compare __comp,
                          /*__is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::set_intersection(__first1, __last1, __first2, __last2, __result, __comp);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_intersection(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+__pattern_set_intersection(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                            _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result,
-                           _Compare __comp, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+                           _Compare __comp) noexcept
 {
-    return __internal::__brick_set_intersection(__first1, __last1, __first2, __last2, __result, __comp, __is_vector);
+    return __internal::__brick_set_intersection(__first1, __last1, __first2, __last2, __result, __comp,
+                                                typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Compare>
 _RandomAccessIterator3
-__pattern_set_intersection(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                           _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
-                           _RandomAccessIterator3 __result, _Compare __comp, _IsVector __is_vector,
-                           /*is_parallel=*/std::true_type)
+__pattern_set_intersection(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                           _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
+                           _RandomAccessIterator2 __last2, _RandomAccessIterator3 __result, _Compare __comp)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator3>::value_type _Tp;
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
 
     const auto __n1 = __last1 - __first1;
     const auto __n2 = __last2 - __first2;
 
     // intersection is empty
__pattern_set_intersection(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
     if (__left_bound_seq_2 == __last2)
         return __result;
 
     const auto __m1 = __last1 - __left_bound_seq_1 + __n2;
     if (__m1 > __set_algo_cut_off)
     {
         //we know proper offset due to [first1; left_bound_seq_1) < [first2; last2)
         return __internal::__parallel_set_op(
-            std::forward<_ExecutionPolicy>(__exec), __left_bound_seq_1, __last1, __first2, __last2, __result, __comp,
-            [](_DifferenceType __n, _DifferenceType __m) { return std::min(__n, __m); },
+            __tag, std::forward<_ExecutionPolicy>(__exec), __left_bound_seq_1, __last1, __first2, __last2, __result,
+            __comp, [](_DifferenceType __n, _DifferenceType __m) { return std::min(__n, __m); },
             [](_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                _RandomAccessIterator2 __last2, _Tp* __result, _Compare __comp) {
                 return __pstl::__utils::__set_intersection_construct(__first1, __last1, __first2, __last2, __result,
                                                                      __comp);
-            },
-            __is_vector);
+            });
     }
 
     const auto __m2 = __last2 - __left_bound_seq_2 + __n1;
     if (__m2 > __set_algo_cut_off)
     {
         //we know proper offset due to [first2; left_bound_seq_2) < [first1; last1)
         __result = __internal::__parallel_set_op(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __left_bound_seq_2, __last2, __result, __comp,
-            [](_DifferenceType __n, _DifferenceType __m) { return std::min(__n, __m); },
+            __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __left_bound_seq_2, __last2, __result,
+            __comp, [](_DifferenceType __n, _DifferenceType __m) { return std::min(__n, __m); },
             [](_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                _RandomAccessIterator2 __last2, _Tp* __result, _Compare __comp) {
                 return __pstl::__utils::__set_intersection_construct(__first2, __last2, __first1, __last1, __result,
                                                                      __comp);
-            },
-            __is_vector);
+            });
         return __result;
     }
 
     // [left_bound_seq_1; last1) and [left_bound_seq_2; last2) - use serial algorithm
     return std::set_intersection(__left_bound_seq_1, __last1, __left_bound_seq_2, __last2, __result, __comp);
 }
 
 //------------------------------------------------------------------------

 __brick_set_difference(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                        _RandomAccessIterator2 __last2, _RandomAccessIterator3 __result, _Compare __comp,
                        /*__is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::set_difference(__first1, __last1, __first2, __last2, __result, __comp);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_difference(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+__pattern_set_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                          _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result,
-                         _Compare __comp, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+                         _Compare __comp) noexcept
 {
-    return __internal::__brick_set_difference(__first1, __last1, __first2, __last2, __result, __comp, __is_vector);
+    return __internal::__brick_set_difference(__first1, __last1, __first2, __last2, __result, __comp,
+                                              typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Compare>
 _RandomAccessIterator3
-__pattern_set_difference(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                         _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
-                         _RandomAccessIterator3 __result, _Compare __comp, _IsVector __is_vector,
-                         /*is_parallel=*/std::true_type)
+__pattern_set_difference(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                         _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
+                         _RandomAccessIterator2 __last2, _RandomAccessIterator3 __result, _Compare __comp)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator3>::value_type _Tp;
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
 
     const auto __n1 = __last1 - __first1;
     const auto __n2 = __last2 - __first2;
 
     // {} \ {2}: the difference is empty
     if (__n1 == 0)
         return __result;
 
     // {1} \ {}: parallel copying just first sequence
     if (__n2 == 0)
         return __internal::__pattern_walk2_brick(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
-            [__is_vector](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
-                return __internal::__brick_copy(__begin, __end, __res, __is_vector);
-            },
-            std::true_type());
+            __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
+            [](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
+                return __internal::__brick_copy(__begin, __end, __res, _IsVector{});
+            });
 
     // testing  whether the sequences are intersected
     _RandomAccessIterator1 __left_bound_seq_1 = std::lower_bound(__first1, __last1, *__first2, __comp);
     //{1} < {2}: seq 2 is wholly greater than seq 1, so, parallel copying just first sequence
     if (__left_bound_seq_1 == __last1)
         return __internal::__pattern_walk2_brick(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
-            [__is_vector](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
-                return __internal::__brick_copy(__begin, __end, __res, __is_vector);
-            },
-            std::true_type());
+            __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
+            [](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
+                return __internal::__brick_copy(__begin, __end, __res, _IsVector{});
+            });
 
     // testing  whether the sequences are intersected
     _RandomAccessIterator2 __left_bound_seq_2 = std::lower_bound(__first2, __last2, *__first1, __comp);
     //{2} < {1}: seq 1 is wholly greater than seq 2, so, parallel copying just first sequence
     if (__left_bound_seq_2 == __last2)
         return __internal::__pattern_walk2_brick(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
-            [__is_vector](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
-                return __internal::__brick_copy(__begin, __end, __res, __is_vector);
-            },
-            std::true_type());
+            __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __result,
+            [](_RandomAccessIterator1 __begin, _RandomAccessIterator1 __end, _RandomAccessIterator3 __res) {
+                return __internal::__brick_copy(__begin, __end, __res, _IsVector{});
+            });
 
     if (__n1 + __n2 > __set_algo_cut_off)
         return __parallel_set_op(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
+            __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
             [](_DifferenceType __n, _DifferenceType) { return __n; },
             [](_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                _RandomAccessIterator2 __last2, _Tp* __result, _Compare __comp) {
                 return __pstl::__utils::__set_difference_construct(__first1, __last1, __first2, __last2, __result,
                                                                    __comp, __BrickCopyConstruct<_IsVector>());
-            },
-            __is_vector);
+            });
 
     // use serial algorithm
     return std::set_difference(__first1, __last1, __first2, __last2, __result, __comp);
 }
 
 //------------------------------------------------------------------------
 // set_symmetric_difference
 //------------------------------------------------------------------------
__brick_set_symmetric_difference(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
                                  _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
                                  _RandomAccessIterator3 __result, _Compare __comp,
                                  /*__is_vector=*/std::true_type) noexcept
 {
     _PSTL_PRAGMA_MESSAGE("Vectorized algorithm unimplemented, redirected to serial");
     return std::set_symmetric_difference(__first1, __last1, __first2, __last2, __result, __comp);
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
-          class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _OutputIterator,
+          class _Compare>
 _OutputIterator
-__pattern_set_symmetric_difference(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+__pattern_set_symmetric_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                                    _ForwardIterator2 __first2, _ForwardIterator2 __last2, _OutputIterator __result,
-                                   _Compare __comp, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+                                   _Compare __comp) noexcept
 {
     return __internal::__brick_set_symmetric_difference(__first1, __last1, __first2, __last2, __result, __comp,
-                                                        __is_vector);
+                                                        typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
-          class _RandomAccessIterator3, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _RandomAccessIterator3, class _Compare>
 _RandomAccessIterator3
-__pattern_set_symmetric_difference(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
-                                   _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
-                                   _RandomAccessIterator2 __last2, _RandomAccessIterator3 __result, _Compare __comp,
-                                   _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_set_symmetric_difference(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec,
+                                   _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
+                                   _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                                   _RandomAccessIterator3 __result, _Compare __comp)
 {
 
     const auto __n1 = __last1 - __first1;
     const auto __n2 = __last2 - __first2;
 
     // use serial algorithm
     if (__n1 + __n2 <= __set_algo_cut_off)
         return std::set_symmetric_difference(__first1, __last1, __first2, __last2, __result, __comp);
 
     typedef typename std::iterator_traits<_RandomAccessIterator3>::value_type _Tp;
     return __internal::__parallel_set_union_op(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
+        __tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
         [](_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
            _RandomAccessIterator2 __last2, _Tp* __result, _Compare __comp) {
             return __pstl::__utils::__set_symmetric_difference_construct(__first1, __last1, __first2, __last2, __result,
                                                                          __comp, __BrickCopyConstruct<_IsVector>());
-        },
-        __is_vector);
+        });
 }
 
 //------------------------------------------------------------------------
 // is_heap_until
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
__brick_is_heap_until(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
     if (__last - __first < 2)
         return __last;
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _SizeType;
     return __unseq_backend::__simd_first(
         __first, _SizeType(0), __last - __first,
         [&__comp](_RandomAccessIterator __it, _SizeType __i) { return __comp(__it[(__i - 1) / 2], __it[__i]); });
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
-__pattern_is_heap_until(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                        _Compare __comp, _IsVector __is_vector, /* is_parallel = */ std::false_type) noexcept
+__pattern_is_heap_until(_Tag, _ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last,
+                        _Compare __comp) noexcept
 {
-    return __internal::__brick_is_heap_until(__first, __last, __comp, __is_vector);
+    return __internal::__brick_is_heap_until(__first, __last, __comp, typename _Tag::__is_vector{});
 }
 
 template <class _RandomAccessIterator, class _DifferenceType, class _Compare>
 _RandomAccessIterator
 __is_heap_until_local(_RandomAccessIterator __first, _DifferenceType __begin, _DifferenceType __end, _Compare __comp,
                       /* __is_vector = */ std::false_type) noexcept
 {
     _DifferenceType __i = __begin;

 __is_heap_until_local(_RandomAccessIterator __first, _DifferenceType __begin, _DifferenceType __end, _Compare __comp,
                       /* __is_vector = */ std::true_type) noexcept
 {
     return __unseq_backend::__simd_first(
         __first, __begin, __end,
         [&__comp](_RandomAccessIterator __it, _DifferenceType __i) { return __comp(__it[(__i - 1) / 2], __it[__i]); });
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 _RandomAccessIterator
-__pattern_is_heap_until(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                        _Compare __comp, _IsVector __is_vector, /* is_parallel = */ std::true_type) noexcept
+__pattern_is_heap_until(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                        _RandomAccessIterator __last, _Compare __comp) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__last - __first < 2)
         return __last;
 
     return __internal::__except_handler([&]() {
         return __parallel_find(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
-            [__first, __comp, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j) {
-                return __internal::__is_heap_until_local(__first, __i - __first, __j - __first, __comp, __is_vector);
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+            [__first, __comp](_RandomAccessIterator __i, _RandomAccessIterator __j) {
+                return __internal::__is_heap_until_local(__first, __i - __first, __j - __first, __comp, _IsVector{});
             },
             std::less<typename std::iterator_traits<_RandomAccessIterator>::difference_type>(), /*is_first=*/true);
     });
 }
 
 //------------------------------------------------------------------------
 // min_element
 //------------------------------------------------------------------------

 {
 #if defined(_PSTL_UDR_PRESENT)
     return __unseq_backend::__simd_min_element(__first, __last - __first, __comp);
 #else
     return std::min_element(__first, __last, __comp);
 #endif
 }
 
-template <typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare, typename _IsVector>
+template <typename _Tag, typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare>
 _ForwardIterator
-__pattern_min_element(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp,
-                      _IsVector __is_vector, /* is_parallel = */ std::false_type) noexcept
+__pattern_min_element(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                      _Compare __comp) noexcept
 {
-    return __internal::__brick_min_element(__first, __last, __comp, __is_vector);
+    return __internal::__brick_min_element(__first, __last, __comp, typename _Tag::__is_vector{});
 }
 
-template <typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _IsVector>
+template <typename _IsVector, typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare>
 _RandomAccessIterator
-__pattern_min_element(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                      _Compare __comp, _IsVector __is_vector, /* is_parallel = */ std::true_type)
+__pattern_min_element(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                      _RandomAccessIterator __last, _Compare __comp)
 {
     if (__first == __last)
         return __last;
 
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first + 1, __last, __first,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first + 1, __last, __first,
             [=](_RandomAccessIterator __begin, _RandomAccessIterator __end,
                 _RandomAccessIterator __init) -> _RandomAccessIterator {
                 const _RandomAccessIterator subresult =
-                    __internal::__brick_min_element(__begin, __end, __comp, __is_vector);
+                    __internal::__brick_min_element(__begin, __end, __comp, _IsVector{});
                 return __internal::__cmp_iterators_by_values(__init, subresult, __comp);
             },
             [=](_RandomAccessIterator __it1, _RandomAccessIterator __it2) -> _RandomAccessIterator {
                 return __internal::__cmp_iterators_by_values(__it1, __it2, __comp);
             });
     });
 }
 

 {
 #if defined(_PSTL_UDR_PRESENT)
     return __unseq_backend::__simd_minmax_element(__first, __last - __first, __comp);
 #else
     return std::minmax_element(__first, __last, __comp);
 #endif
 }
 
-template <typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare, typename _IsVector>
+template <typename _Tag, typename _ExecutionPolicy, typename _ForwardIterator, typename _Compare>
 std::pair<_ForwardIterator, _ForwardIterator>
-__pattern_minmax_element(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp,
-                         _IsVector __is_vector, /* is_parallel = */ std::false_type) noexcept
+__pattern_minmax_element(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                         _Compare __comp) noexcept
 {
-    return __internal::__brick_minmax_element(__first, __last, __comp, __is_vector);
+    return __internal::__brick_minmax_element(__first, __last, __comp, typename _Tag::__is_vector{});
 }
 
-template <typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _IsVector>
+template <typename _IsVector, typename _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare>
 std::pair<_RandomAccessIterator, _RandomAccessIterator>
-__pattern_minmax_element(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                         _Compare __comp, _IsVector __is_vector, /* is_parallel = */ std::true_type)
+__pattern_minmax_element(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                         _RandomAccessIterator __last, _Compare __comp)
 {
     if (__first == __last)
         return std::make_pair(__first, __first);
 
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         typedef std::pair<_RandomAccessIterator, _RandomAccessIterator> _Result;
 
         return __par_backend::__parallel_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first + 1, __last, std::make_pair(__first, __first),
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first + 1, __last,
+            std::make_pair(__first, __first),
             [=](_RandomAccessIterator __begin, _RandomAccessIterator __end, _Result __init) -> _Result {
-                const _Result __subresult = __internal::__brick_minmax_element(__begin, __end, __comp, __is_vector);
+                const _Result __subresult = __internal::__brick_minmax_element(__begin, __end, __comp, _IsVector{});
                 return std::make_pair(
                     __internal::__cmp_iterators_by_values(__subresult.first, __init.first, __comp),
                     __internal::__cmp_iterators_by_values(__init.second, __subresult.second, std::not_fn(__comp)));
             },
             [=](_Result __p1, _Result __p2) -> _Result {
                 return std::make_pair(
                     __internal::__cmp_iterators_by_values(__p1.first, __p2.first, __comp),
                     __internal::__cmp_iterators_by_values(__p2.second, __p1.second, std::not_fn(__comp)));
             });
     });
 }
 
 //------------------------------------------------------------------------

 std::pair<_RandomAccessIterator1, _RandomAccessIterator2>
 __brick_mismatch(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
                  _RandomAccessIterator2 __last2, _Predicate __pred, /* __is_vector = */ std::true_type) noexcept
 {
     auto __n = std::min(__last1 - __first1, __last2 - __first2);
     return __unseq_backend::__simd_first(__first1, __n, __first2, std::not_fn(__pred));
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
 std::pair<_ForwardIterator1, _ForwardIterator2>
-__pattern_mismatch(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                   _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Predicate __pred, _IsVector __is_vector,
-                   /* is_parallel = */ std::false_type) noexcept
+__pattern_mismatch(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                   _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Predicate __pred) noexcept
 {
-    return __internal::__brick_mismatch(__first1, __last1, __first2, __last2, __pred, __is_vector);
+    return __internal::__brick_mismatch(__first1, __last1, __first2, __last2, __pred, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Predicate,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Predicate>
 std::pair<_RandomAccessIterator1, _RandomAccessIterator2>
-__pattern_mismatch(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                   _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _Predicate __pred,
-                   _IsVector __is_vector, /* is_parallel = */ std::true_type) noexcept
+__pattern_mismatch(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                   _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                   _Predicate __pred) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         auto __n = std::min(__last1 - __first1, __last2 - __first2);
         auto __result = __internal::__parallel_find(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
-            [__first1, __first2, __pred, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
+            [__first1, __first2, __pred](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
                 return __internal::__brick_mismatch(__i, __j, __first2 + (__i - __first1), __first2 + (__j - __first1),
-                                                    __pred, __is_vector)
+                                                    __pred, _IsVector{})
                     .first;
             },
             std::less<typename std::iterator_traits<_RandomAccessIterator1>::difference_type>(), /*is_first=*/true);
         return std::make_pair(__result, __first2 + (__result - __first1));
     });
 }
 
 //------------------------------------------------------------------------
else
         }
         else
         { // if second sequence shorter than first or both have the same number of elements
             return __comp(*__result.first, *__result.second);
         }
     }
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 bool
-__pattern_lexicographical_compare(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
-                                  _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Compare __comp,
-                                  _IsVector __is_vector, /* is_parallel = */ std::false_type) noexcept
+__pattern_lexicographical_compare(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+                                  _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Compare __comp) noexcept
 {
-    return __internal::__brick_lexicographical_compare(__first1, __last1, __first2, __last2, __comp, __is_vector);
+    return __internal::__brick_lexicographical_compare(__first1, __last1, __first2, __last2, __comp,
+                                                       typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Compare,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Compare>
 bool
-__pattern_lexicographical_compare(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
-                                  _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2,
-                                  _RandomAccessIterator2 __last2, _Compare __comp, _IsVector __is_vector,
-                                  /* is_parallel = */ std::true_type) noexcept
+__pattern_lexicographical_compare(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec,
+                                  _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
+                                  _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                                  _Compare __comp) noexcept
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     if (__first2 == __last2)
     { // if second sequence is empty
         return false;
     }
     else if (__first1 == __last1)
     { // if first sequence is empty
         return true;
     }
     else
     {
         typedef typename std::iterator_traits<_RandomAccessIterator1>::reference _RefType1;
         typedef typename std::iterator_traits<_RandomAccessIterator2>::reference _RefType2;
         --__last1;
         --__last2;
         auto __n = std::min(__last1 - __first1, __last2 - __first2);
         auto __result = __internal::__parallel_find(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
-            [__first1, __first2, &__comp, __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
-                return __internal::__brick_mismatch(__i, __j, __first2 + (__i - __first1), __first2 + (__j - __first1),
-                                                    [&__comp](const _RefType1 __x, const _RefType2 __y) {
-                                                        return !__comp(__x, __y) && !__comp(__y, __x);
-                                                    },
-                                                    __is_vector)
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __first1 + __n,
+            [__first1, __first2, &__comp](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j) {
+                return __internal::__brick_mismatch(
+                           __i, __j, __first2 + (__i - __first1), __first2 + (__j - __first1),
+                           _IsVector{})
                     .first;
             },
             std::less<typename std::iterator_traits<_RandomAccessIterator1>::difference_type>(), /*is_first=*/true);
 
         if (__result == __last1 && __first2 + (__result - __first1) != __last2)
         { // if first sequence shorter than second
             return !__comp(*(__first2 + (__result - __first1)), *__result);
         }

pstl/include/pstl/internal/execution_defs.h

 namespace execution
 {
 inline namespace v1
 {
 
 // 2.4, Sequential execution policy
 class sequenced_policy
 {
-  public:
-    // For internal use only
-    static constexpr std::false_type
-    __allow_unsequenced()
-    {
-        return std::false_type{};
-    }
-    static constexpr std::false_type
-    __allow_vector()
-    {
-        return std::false_type{};
-    }
-    static constexpr std::false_type
-    __allow_parallel()
-    {
-        return std::false_type{};
-    }
 };
 
 // 2.5, Parallel execution policy
 class parallel_policy
 {
-  public:
-    // For internal use only
-    static constexpr std::false_type
-    __allow_unsequenced()
-    {
-        return std::false_type{};
-    }
-    static constexpr std::false_type
-    __allow_vector()
-    {
-        return std::false_type{};
-    }
-    static constexpr std::true_type
-    __allow_parallel()
-    {
-        return std::true_type{};
-    }
 };
 
 // 2.6, Parallel+Vector execution policy
 class parallel_unsequenced_policy
 {
-  public:
-    // For internal use only
-    static constexpr std::true_type
-    __allow_unsequenced()
-    {
-        return std::true_type{};
-    }
-    static constexpr std::true_type
-    __allow_vector()
-    {
-        return std::true_type{};
-    }
-    static constexpr std::true_type
-    __allow_parallel()
-    {
-        return std::true_type{};
-    }
 };
 
 class unsequenced_policy
 {
-  public:
-    // For internal use only
-    static constexpr std::true_type
-    __allow_unsequenced()
-    {
-        return std::true_type{};
-    }
-    static constexpr std::true_type
-    __allow_vector()
-    {
-        return std::true_type{};
-    }
-    static constexpr std::false_type
-    __allow_parallel()
-    {
-        return std::false_type{};
-    }
 };
 
 // 2.8, Execution policy objects
 constexpr sequenced_policy seq{};
 constexpr parallel_policy par{};
 constexpr parallel_unsequenced_policy par_unseq{};
 constexpr unsequenced_policy unseq{};
 

 } // namespace execution
 
 namespace __internal
 {
 template <class ExecPolicy, class T>
 using __enable_if_execution_policy =
     typename std::enable_if<__pstl::execution::is_execution_policy<typename std::decay<ExecPolicy>::type>::value,
                             T>::type;
+
+template <class _IsVector>
+struct __serial_tag;
+template <class _IsVector>
+struct __parallel_tag;
+
 } // namespace __internal
 
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_EXECUTION_POLICY_DEFS_H */

pstl/include/pstl/internal/execution_impl.h

 
 _PSTL_HIDE_FROM_ABI_PUSH
 
 namespace __pstl
 {
 namespace __internal
 {
 
-using namespace __pstl::execution;
+template <typename _IteratorTag, typename... _IteratorTypes>
+using __are_iterators_of = std::conjunction<
+    std::is_base_of<_IteratorTag, typename std::iterator_traits<std::decay_t<_IteratorTypes>>::iterator_category>...>;
 
-template <typename _IteratorType>
-struct __is_random_access_iterator
-    : std::is_same<typename std::iterator_traits<_IteratorType>::iterator_category,
-                   std::random_access_iterator_tag>
+template <typename... _IteratorTypes>
+using __are_random_access_iterators = __are_iterators_of<std::random_access_iterator_tag, _IteratorTypes...>;
+
+struct __serial_backend
 {
 };
-
-template <typename Policy>
-struct __policy_traits
+struct __tbb_backend
 {
 };
-
-template <>
-struct __policy_traits<sequenced_policy>
+struct __openmp_backend
 {
-    typedef std::false_type __allow_parallel;
-    typedef std::false_type __allow_unsequenced;
-    typedef std::false_type __allow_vector;
 };
 
-template <>
-struct __policy_traits<unsequenced_policy>
+#if defined(_PSTL_PAR_BACKEND_TBB)
+using __par_backend_tag = __tbb_backend;
+#elif defined(_PSTL_PAR_BACKEND_OPENMP)
+using __par_backend_tag = __openmp_backend;
+#elif defined(_PSTL_PAR_BACKEND_SERIAL)
+using __par_backend_tag = __serial_backend;
+#else
+#    error "A parallel backend must be specified";
+#endif
+
+template <class _IsVector>
+struct __serial_tag
 {
-    typedef std::false_type __allow_parallel;
-    typedef std::true_type __allow_unsequenced;
-    typedef std::true_type __allow_vector;
+    using __is_vector = _IsVector;
 };
 
-template <>
-struct __policy_traits<parallel_policy>
+template <class _IsVector>
+struct __parallel_tag
 {
-    typedef std::true_type __allow_parallel;
-    typedef std::false_type __allow_unsequenced;
-    typedef std::false_type __allow_vector;
+    using __is_vector = _IsVector;
+    // backend tag can be change depending on
+    // TBB availability in the environment
+    using __backend_tag = __par_backend_tag;
 };
 
-template <>
-struct __policy_traits<parallel_unsequenced_policy>
+template <class _IsVector, class... _IteratorTypes>
+using __tag_type = typename std::conditional<__internal::__are_random_access_iterators<_IteratorTypes...>::value,
+                                             __parallel_tag<_IsVector>, __serial_tag<_IsVector>>::type;
+
+template <class... _IteratorTypes>
+__serial_tag</*_IsVector = */ std::false_type>
+__select_backend(__pstl::execution::sequenced_policy, _IteratorTypes&&...)
 {
-    typedef std::true_type __allow_parallel;
-    typedef std::true_type __allow_unsequenced;
-    typedef std::true_type __allow_vector;
-};
+    return {};
+}
+
+template <class... _IteratorTypes>
+__serial_tag<__internal::__are_random_access_iterators<_IteratorTypes...>>
+__select_backend(__pstl::execution::unsequenced_policy, _IteratorTypes&&...)
+{
+    return {};
+}
 
-template <typename _ExecutionPolicy>
-using __allow_vector =
-    typename __internal::__policy_traits<typename std::decay<_ExecutionPolicy>::type>::__allow_vector;
-
-template <typename _ExecutionPolicy>
-using __allow_unsequenced =
-    typename __internal::__policy_traits<typename std::decay<_ExecutionPolicy>::type>::__allow_unsequenced;
-
-template <typename _ExecutionPolicy>
-using __allow_parallel =
-    typename __internal::__policy_traits<typename std::decay<_ExecutionPolicy>::type>::__allow_parallel;
-
-template <typename _ExecutionPolicy, typename... _IteratorTypes>
-typename std::conjunction<__allow_vector<_ExecutionPolicy>,
-                          __is_random_access_iterator<_IteratorTypes>...>::type
-__is_vectorization_preferred(_ExecutionPolicy&&)
+template <class... _IteratorTypes>
+__tag_type</*_IsVector = */ std::false_type, _IteratorTypes...>
+__select_backend(__pstl::execution::parallel_policy, _IteratorTypes&&...)
 {
     return {};
 }
 
-template <typename _ExecutionPolicy, typename... _IteratorTypes>
-typename std::conjunction<__allow_parallel<_ExecutionPolicy>,
-                          __is_random_access_iterator<_IteratorTypes>...>::type
-__is_parallelization_preferred(_ExecutionPolicy&&)
+template <class... _IteratorTypes>
+__tag_type<__internal::__are_random_access_iterators<_IteratorTypes...>, _IteratorTypes...>
+__select_backend(__pstl::execution::parallel_unsequenced_policy, _IteratorTypes&&...)
 {
     return {};
 }
 
 } // namespace __internal
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_EXECUTION_IMPL_H */

pstl/include/pstl/internal/glue_algorithm_impl.h

 {
 
 // [alg.any_of]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 any_of(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred)
 {
-    return __pstl::__internal::__pattern_any_of(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_any_of(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                __pred);
 }
 
 // [alg.all_of]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Pred>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 all_of(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Pred __pred)
 {

 }
 
 // [alg.foreach]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Function>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 for_each(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Function __f)
 {
-    __pstl::__internal::__pattern_walk1(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __f,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_walk1(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __f);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Function>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 for_each_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __n, _Function __f)
 {
-    return __pstl::__internal::__pattern_walk1_n(
-        std::forward<_ExecutionPolicy>(__exec), __first, __n, __f,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_walk1_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                                                 __f);
 }
 
 // [alg.find]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 find_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred)
 {
-    return __pstl::__internal::__pattern_find_if(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_find_if(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                 __last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 find_if_not(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred)
 {
     return std::find_if(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::not_fn(__pred));
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 find(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
 {
     return std::find_if(std::forward<_ExecutionPolicy>(__exec), __first, __last,
                         __pstl::__internal::__equal_value<_Tp>(__value));
 }
 
 // [alg.find.end]
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 find_end(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
          _ForwardIterator2 __s_last, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_find_end(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __s_first);
+
+    return __pstl::__internal::__pattern_find_end(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                  __last, __s_first, __s_last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 find_end(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
          _ForwardIterator2 __s_last)
 {
     return std::find_end(std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last,
                          std::equal_to<>());
 }
 
 // [alg.find_first_of]
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 find_first_of(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
               _ForwardIterator2 __s_first, _ForwardIterator2 __s_last, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_find_first_of(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __s_first);
+
+    return __pstl::__internal::__pattern_find_first_of(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, __s_first, __s_last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 find_first_of(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
               _ForwardIterator2 __s_first, _ForwardIterator2 __s_last)
 {
     return std::find_first_of(std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last,
                               std::equal_to<>());
 }
 
 // [alg.adjacent_find]
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 adjacent_find(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
-    return __pstl::__internal::__pattern_adjacent_find(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, std::equal_to<_ValueType>(),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        /*first_semantic*/ false);
+    return __pstl::__internal::__pattern_adjacent_find(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, std::equal_to<_ValueType>(), /*first_semantic*/ false);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 adjacent_find(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_adjacent_find(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        /*first_semantic*/ false);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_adjacent_find(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, __pred, /*first_semantic*/ false);
 }
 
 // [alg.count]
 
 // Implementation note: count and count_if call the pattern directly instead of calling std::transform_reduce
 // so that we do not have to include <numeric>.
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy,
                                                  typename iterator_traits<_ForwardIterator>::difference_type>
 count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
-    return __pstl::__internal::__pattern_count(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last,
-        [&__value](const _ValueType& __x) { return __value == __x; },
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    return __pstl::__internal::__pattern_count(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                               [&__value](const _ValueType& __x) { return __value == __x; });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy,
                                                  typename iterator_traits<_ForwardIterator>::difference_type>
 count_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred)
 {
-    return __pstl::__internal::__pattern_count(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_count(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                               __pred);
 }
 
 // [alg.search]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 search(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
        _ForwardIterator2 __s_last, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_search(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __s_first);
+
+    return __pstl::__internal::__pattern_search(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                __s_first, __s_last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator1>
 search(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
        _ForwardIterator2 __s_last)
 {
     return std::search(std::forward<_ExecutionPolicy>(__exec), __first, __last, __s_first, __s_last, std::equal_to<>());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 search_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
          const _Tp& __value, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_search_n(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __count, __value, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_search_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                  __last, __count, __value, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 search_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
          const _Tp& __value)
 {
     return std::search_n(std::forward<_ExecutionPolicy>(__exec), __first, __last, __count, __value,
                          std::equal_to<typename iterator_traits<_ForwardIterator>::value_type>());
 }
 
 // [alg.copy]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result)
 {
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__pattern_walk2_brick(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
-        [__is_vector](_ForwardIterator1 __begin, _ForwardIterator1 __end, _ForwardIterator2 __res) {
-            return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector);
-        },
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
+        [](_ForwardIterator1 __begin, _ForwardIterator1 __end, _ForwardIterator2 __res) {
+            return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector{});
+        });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _Size, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 copy_n(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _Size __n, _ForwardIterator2 __result)
 {
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__pattern_walk2_brick_n(
-        std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
-        [__is_vector](_ForwardIterator1 __begin, _Size __sz, _ForwardIterator2 __res) {
-            return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector);
-        },
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
+        [](_ForwardIterator1 __begin, _Size __sz, _ForwardIterator2 __res) {
+            return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector{});
+        });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 copy_if(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
         _Predicate __pred)
 {
-    return __pstl::__internal::__pattern_copy_if(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_copy_if(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                 __last, __result, __pred);
 }
 
 // [alg.swap]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 swap_ranges(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
             _ForwardIterator2 __first2)
 {
     typedef typename iterator_traits<_ForwardIterator1>::reference _ReferenceType1;
     typedef typename iterator_traits<_ForwardIterator2>::reference _ReferenceType2;
-    return __pstl::__internal::__pattern_walk2(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2,
-        [](_ReferenceType1 __x, _ReferenceType2 __y) {
+
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_walk2(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                               __last1, __first2, [](_ReferenceType1 __x, _ReferenceType2 __y) {
                                                    using std::swap;
                                                    swap(__x, __y);
-        },
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+                                               });
 }
 
 // [alg.transform]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
           _UnaryOperation __op)
 {
     typedef typename iterator_traits<_ForwardIterator1>::reference _InputType;
     typedef typename iterator_traits<_ForwardIterator2>::reference _OutputType;
-    return __pstl::__internal::__pattern_walk2(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
-        [__op](_InputType __x, _OutputType __y) mutable { __y = __op(__x); },
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_walk2(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                               __result,
+                                               [__op](_InputType __x, _OutputType __y) mutable { __y = __op(__x); });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _BinaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
           _ForwardIterator __result, _BinaryOperation __op)
 {
     typedef typename iterator_traits<_ForwardIterator1>::reference _Input1Type;
     typedef typename iterator_traits<_ForwardIterator2>::reference _Input2Type;
     typedef typename iterator_traits<_ForwardIterator>::reference _OutputType;
+
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __result);
+
     return __pstl::__internal::__pattern_walk3(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __result,
-        [__op](_Input1Type x, _Input2Type y, _OutputType z) mutable { z = __op(x, y); },
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __result,
+        [__op](_Input1Type x, _Input2Type y, _OutputType z) mutable { z = __op(x, y); });
 }
 
 // [alg.replace]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 replace_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred,
            const _Tp& __new_value)
 {
     typedef typename iterator_traits<_ForwardIterator>::reference _ElementType;
-    __pstl::__internal::__pattern_walk1(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last,
+
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_walk1(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
                                         [&__pred, &__new_value](_ElementType __elem) {
                                             if (__pred(__elem))
                                             {
                                                 __elem = __new_value;
                                             }
-        },
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+                                        });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 replace(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __old_value,
         const _Tp& __new_value)
 {
     std::replace_if(std::forward<_ExecutionPolicy>(__exec), __first, __last,
                     __pstl::__internal::__equal_value<_Tp>(__old_value), __new_value);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryPredicate, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 replace_copy_if(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                 _ForwardIterator2 __result, _UnaryPredicate __pred, const _Tp& __new_value)
 {
     typedef typename iterator_traits<_ForwardIterator1>::reference _InputType;
     typedef typename iterator_traits<_ForwardIterator2>::reference _OutputType;
+
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
     return __pstl::__internal::__pattern_walk2(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
-        [__pred, &__new_value](_InputType __x, _OutputType __y) mutable { __y = __pred(__x) ? __new_value : __x; },
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
+        [__pred, &__new_value](_InputType __x, _OutputType __y) mutable { __y = __pred(__x) ? __new_value : __x; });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 replace_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
              const _Tp& __old_value, const _Tp& __new_value)
 {
     return std::replace_copy_if(std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
                                 __pstl::__internal::__equal_value<_Tp>(__old_value), __new_value);
 }
 
 // [alg.fill]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 fill(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
 {
-    __pstl::__internal::__pattern_fill(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __value,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_fill(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                       __value);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 fill_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __count, const _Tp& __value)
 {
     if (__count <= 0)
         return __first;
 
-    return __pstl::__internal::__pattern_fill_n(
-        std::forward<_ExecutionPolicy>(__exec), __first, __count, __value,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_fill_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                __count, __value);
 }
 
 // [alg.generate]
 template <class _ExecutionPolicy, class _ForwardIterator, class _Generator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 generate(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Generator __g)
 {
-    __pstl::__internal::__pattern_generate(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __g,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_generate(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                           __g);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Generator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 generate_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __count, _Generator __g)
 {
     if (__count <= 0)
         return __first;
 
-    return __pstl::__internal::__pattern_generate_n(
-        std::forward<_ExecutionPolicy>(__exec), __first, __count, __g,
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_generate_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                    __count, __g);
 }
 
 // [alg.remove]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 remove_copy_if(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Predicate __pred)
remove_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
     return std::copy_if(std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
                         __pstl::__internal::__not_equal_value<_Tp>(__value));
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 remove_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_remove_if(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_remove_if(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                   __last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 remove(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
 {
     return std::remove_if(std::forward<_ExecutionPolicy>(__exec), __first, __last,
                           __pstl::__internal::__equal_value<_Tp>(__value));
 }
 
 // [alg.unique]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 unique(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_unique(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_unique(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 unique(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     return std::unique(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::equal_to<>());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 unique_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
             _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_unique_copy(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_unique_copy(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                     __last, __result, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 unique_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result)
 {
     return std::unique_copy(__exec, __first, __last, __result, std::equal_to<>());
 }
 
 // [alg.reverse]
 
 template <class _ExecutionPolicy, class _BidirectionalIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 reverse(_ExecutionPolicy&& __exec, _BidirectionalIterator __first, _BidirectionalIterator __last)
 {
-    __pstl::__internal::__pattern_reverse(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_reverse(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last);
 }
 
 template <class _ExecutionPolicy, class _BidirectionalIterator, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 reverse_copy(_ExecutionPolicy&& __exec, _BidirectionalIterator __first, _BidirectionalIterator __last,
              _ForwardIterator __d_first)
 {
-    return __pstl::__internal::__pattern_reverse_copy(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _BidirectionalIterator, _ForwardIterator>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _BidirectionalIterator, _ForwardIterator>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __d_first);
+
+    return __pstl::__internal::__pattern_reverse_copy(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                      __last, __d_first);
 }
 
 // [alg.rotate]
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 rotate(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __middle, _ForwardIterator __last)
 {
-    return __pstl::__internal::__pattern_rotate(
-        std::forward<_ExecutionPolicy>(__exec), __first, __middle, __last,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_rotate(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                __middle, __last);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 rotate_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __middle, _ForwardIterator1 __last,
             _ForwardIterator2 __result)
 {
-    return __pstl::__internal::__pattern_rotate_copy(
-        std::forward<_ExecutionPolicy>(__exec), __first, __middle, __last, __result,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_rotate_copy(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                     __middle, __last, __result);
 }
 
 // [alg.partitions]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 is_partitioned(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_is_partitioned(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_is_partitioned(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                        __last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 partition(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_partition(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_partition(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                   __last, __pred);
 }
 
 template <class _ExecutionPolicy, class _BidirectionalIterator, class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _BidirectionalIterator>
 stable_partition(_ExecutionPolicy&& __exec, _BidirectionalIterator __first, _BidirectionalIterator __last,
                  _UnaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_stable_partition(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_stable_partition(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                          __first, __last, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _ForwardIterator1, class _ForwardIterator2,
           class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, std::pair<_ForwardIterator1, _ForwardIterator2>>
 partition_copy(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last,
                _ForwardIterator1 __out_true, _ForwardIterator2 __out_false, _UnaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_partition_copy(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __out_true, __out_false, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator, _ForwardIterator1,
-                                                         _ForwardIterator2>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator, _ForwardIterator1,
-                                                           _ForwardIterator2>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __out_true, __out_false);
+
+    return __pstl::__internal::__pattern_partition_copy(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                        __last, __out_true, __out_false, __pred);
 }
 
 // [alg.sort]
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
     typedef typename iterator_traits<_RandomAccessIterator>::value_type _InputType;
-    return __pstl::__internal::__pattern_sort(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        typename std::is_move_constructible<_InputType>::type());
+    return __pstl::__internal::__pattern_sort(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                              __comp, typename std::is_move_constructible<_InputType>::type());
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _InputType;
     std::sort(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_InputType>());
 }
 
 // [stable.sort]
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 stable_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_stable_sort(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_stable_sort(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                     __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 stable_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _InputType;
     std::stable_sort(__exec, __first, __last, std::less<_InputType>());
 }
 
 // [mismatch]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, std::pair<_ForwardIterator1, _ForwardIterator2>>
 mismatch(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
          _ForwardIterator2 __last2, _BinaryPredicate __pred)
 {
-    return __pstl::__internal::__pattern_mismatch(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __pred,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_mismatch(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                                  __last1, __first2, __last2, __pred);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, std::pair<_ForwardIterator1, _ForwardIterator2>>
 mismatch(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
          _BinaryPredicate __pred)
 {
     return std::mismatch(__exec, __first1, __last1, __first2, std::next(__first2, std::distance(__first1, __last1)),

 
 // [alg.equal]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 equal(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
       _BinaryPredicate __p)
 {
-    return __pstl::__internal::__pattern_equal(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __p,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_equal(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                               __last1, __first2, __p);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 equal(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2)
 {
     return std::equal(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, std::equal_to<>());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 equal(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
       _ForwardIterator2 __last2, _BinaryPredicate __p)
 {
-    return __pstl::__internal::__pattern_equal(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __p,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_equal(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                               __last1, __first2, __last2, __p);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 equal(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
       _ForwardIterator2 __last2)
 {
     return equal(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, std::equal_to<>());
 }
 
 // [alg.move]
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 move(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __d_first)
 {
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __d_first);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__pattern_walk2_brick(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first,
-        [__is_vector](_ForwardIterator1 __begin, _ForwardIterator1 __end, _ForwardIterator2 __res) {
-            return __pstl::__internal::__brick_move(__begin, __end, __res, __is_vector);
-        },
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first,
+        [](_ForwardIterator1 __begin, _ForwardIterator1 __end, _ForwardIterator2 __res) {
+            return __pstl::__internal::__brick_move(__begin, __end, __res, __is_vector{});
+        });
 }
 
 // [partial.sort]
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 partial_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __middle,
              _RandomAccessIterator __last, _Compare __comp)
 {
-    __pstl::__internal::__pattern_partial_sort(
-        std::forward<_ExecutionPolicy>(__exec), __first, __middle, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_partial_sort(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                               __middle, __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 partial_sort(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __middle,
              _RandomAccessIterator __last)
 {
     typedef typename iterator_traits<_RandomAccessIterator>::value_type _InputType;
     std::partial_sort(__exec, __first, __middle, __last, std::less<_InputType>());
 }
 
 // [partial.sort.copy]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _RandomAccessIterator>
 partial_sort_copy(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last,
                   _RandomAccessIterator __d_first, _RandomAccessIterator __d_last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_partial_sort_copy(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first, __d_last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator, _RandomAccessIterator>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator, _RandomAccessIterator>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __d_first);
+
+    return __pstl::__internal::__pattern_partial_sort_copy(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                           __first, __last, __d_first, __d_last, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _RandomAccessIterator>
 partial_sort_copy(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last,
                   _RandomAccessIterator __d_first, _RandomAccessIterator __d_last)
 {
     return std::partial_sort_copy(std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first, __d_last,
                                   std::less<>());
 }
 
 // [is.sorted]
 template <class _ExecutionPolicy, class _ForwardIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 is_sorted_until(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
-    const _ForwardIterator __res = __pstl::__internal::__pattern_adjacent_find(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __pstl::__internal::__reorder_pred<_Compare>(__comp),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    const _ForwardIterator __res =
+        __pstl::__internal::__pattern_adjacent_find(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                    __last, __pstl::__internal::__reorder_pred<_Compare>(__comp),
                                                     /*first_semantic*/ false);
     return __res == __last ? __last : std::next(__res);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 is_sorted_until(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename std::iterator_traits<_ForwardIterator>::value_type _InputType;
     return is_sorted_until(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_InputType>());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 is_sorted(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_adjacent_find(
-               std::forward<_ExecutionPolicy>(__exec), __first, __last,
-               __pstl::__internal::__reorder_pred<_Compare>(__comp),
-               __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-               __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_adjacent_find(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, __pstl::__internal::__reorder_pred<_Compare>(__comp),
                                                        /*or_semantic*/ true) == __last;
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 is_sorted(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename std::iterator_traits<_ForwardIterator>::value_type _InputType;
     return std::is_sorted(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_InputType>());
 }
 
 // [alg.merge]
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 merge(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
       _ForwardIterator2 __last2, _ForwardIterator __d_first, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_merge(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __d_first, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __d_first);
+
+    return __pstl::__internal::__pattern_merge(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                               __last1, __first2, __last2, __d_first, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 merge(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
       _ForwardIterator2 __last2, _ForwardIterator __d_first)
 {
     return std::merge(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __d_first,
                       std::less<>());
 }
 
 template <class _ExecutionPolicy, class _BidirectionalIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 inplace_merge(_ExecutionPolicy&& __exec, _BidirectionalIterator __first, _BidirectionalIterator __middle,
               _BidirectionalIterator __last, _Compare __comp)
 {
-    __pstl::__internal::__pattern_inplace_merge(
-        std::forward<_ExecutionPolicy>(__exec), __first, __middle, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _BidirectionalIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_inplace_merge(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                __middle, __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _BidirectionalIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 inplace_merge(_ExecutionPolicy&& __exec, _BidirectionalIterator __first, _BidirectionalIterator __middle,
               _BidirectionalIterator __last)
 {
     typedef typename std::iterator_traits<_BidirectionalIterator>::value_type _InputType;
     std::inplace_merge(__exec, __first, __middle, __last, std::less<_InputType>());
 }
 
 // [includes]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 includes(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
          _ForwardIterator2 __last2, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_includes(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_includes(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                                  __last1, __first2, __last2, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 includes(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
          _ForwardIterator2 __last2)
 {
     return std::includes(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, std::less<>());
 }
 
 // [set.union]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_union(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
           _ForwardIterator2 __last2, _ForwardIterator __result, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_set_union(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __result);
+
+    return __pstl::__internal::__pattern_set_union(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1,
+                                                   __last1, __first2, __last2, __result, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_union(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
           _ForwardIterator2 __last2, _ForwardIterator __result)
 {
     return std::set_union(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result,
                           std::less<>());
 }
 
 // [set.intersection]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_intersection(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                  _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_set_intersection(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __result);
+
+    return __pstl::__internal::__pattern_set_intersection(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                          __first1, __last1, __first2, __last2, __result, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_intersection(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                  _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result)
 {
     return std::set_intersection(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result,
                                  std::less<>());
 }
 
 // [set.difference]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_set_difference(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __result);
+
+    return __pstl::__internal::__pattern_set_difference(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                        __first1, __last1, __first2, __last2, __result, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result)
 {
     return std::set_difference(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result,
                                std::less<>());
 }
 
 // [set.symmetric.difference]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator,
           class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_symmetric_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                          _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result,
                          _Compare __comp)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2, __result);
+
     return __pstl::__internal::__pattern_set_symmetric_difference(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                         _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2,
-                                                           _ForwardIterator>(__exec));
+        __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __result, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 set_symmetric_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                          _ForwardIterator2 __first2, _ForwardIterator2 __last2, _ForwardIterator __result)
 {
     return std::set_symmetric_difference(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2,
                                          __result, std::less<>());
 }
 
 // [is.heap]
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _RandomAccessIterator>
 is_heap_until(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_is_heap_until(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    return __pstl::__internal::__pattern_is_heap_until(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _RandomAccessIterator>
 is_heap_until(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _InputType;
     return std::is_heap_until(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_InputType>());

 }
 
 // [alg.min.max]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 min_element(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_min_element(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_min_element(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                     __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 min_element(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename std::iterator_traits<_ForwardIterator>::value_type _InputType;
     return std::min_element(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_InputType>());
max_element(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
     return std::min_element(std::forward<_ExecutionPolicy>(__exec), __first, __last,
                             __pstl::__internal::__reorder_pred<std::less<_InputType>>(std::less<_InputType>()));
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, std::pair<_ForwardIterator, _ForwardIterator>>
 minmax_element(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_minmax_element(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_minmax_element(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                        __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, std::pair<_ForwardIterator, _ForwardIterator>>
 minmax_element(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     return std::minmax_element(std::forward<_ExecutionPolicy>(__exec), __first, __last, std::less<_ValueType>());
 }
 
 // [alg.nth.element]
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 nth_element(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __nth,
             _RandomAccessIterator __last, _Compare __comp)
 {
-    __pstl::__internal::__pattern_nth_element(
-        std::forward<_ExecutionPolicy>(__exec), __first, __nth, __last, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _RandomAccessIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    __pstl::__internal::__pattern_nth_element(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __nth,
+                                              __last, __comp);
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 nth_element(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __nth,
             _RandomAccessIterator __last)
 {
     typedef typename iterator_traits<_RandomAccessIterator>::value_type _InputType;
     std::nth_element(std::forward<_ExecutionPolicy>(__exec), __first, __nth, __last, std::less<_InputType>());
 }
 
 // [alg.lex.comparison]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Compare>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 lexicographical_compare(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                         _ForwardIterator2 __first2, _ForwardIterator2 __last2, _Compare __comp)
 {
-    return __pstl::__internal::__pattern_lexicographical_compare(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2, __comp,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
+    return __pstl::__internal::__pattern_lexicographical_compare(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                                 __first1, __last1, __first2, __last2, __comp);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, bool>
 lexicographical_compare(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                         _ForwardIterator2 __first2, _ForwardIterator2 __last2)
 {
     return std::lexicographical_compare(std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __last2,
                                         std::less<>());
 }
 
 } // namespace std
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_GLUE_ALGORITHM_IMPL_H */

pstl/include/pstl/internal/glue_memory_impl.h

 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_copy(_ExecutionPolicy&& __exec, _InputIterator __first, _InputIterator __last, _ForwardIterator __result)
 {
     typedef typename iterator_traits<_InputIterator>::value_type _ValueType1;
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType2;
     typedef typename iterator_traits<_InputIterator>::reference _ReferenceType1;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType2;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::integral_constant < bool, std::is_trivial<_ValueType1>::value&& std::is_trivial<_ValueType2>::value > (),
         [&]() {
             return __pstl::__internal::__pattern_walk2_brick(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
-                [__is_vector](_InputIterator __begin, _InputIterator __end, _ForwardIterator __res) {
-                    return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
+                [](_InputIterator __begin, _InputIterator __end, _ForwardIterator __res) {
+                    return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector{});
+                });
         },
         [&]() {
-            return __pstl::__internal::__pattern_walk2(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                                       __result,
+            return __pstl::__internal::__pattern_walk2(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                       __last, __result,
                                                        [](_ReferenceType1 __val1, _ReferenceType2 __val2) {
                                                            ::new (std::addressof(__val2)) _ValueType2(__val1);
-                                                       },
-                                                       __is_vector, __is_parallel);
+                                                       });
         });
 }
 
 template <class _ExecutionPolicy, class _InputIterator, class _Size, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_copy_n(_ExecutionPolicy&& __exec, _InputIterator __first, _Size __n, _ForwardIterator __result)
 {
     typedef typename iterator_traits<_InputIterator>::value_type _ValueType1;
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType2;
     typedef typename iterator_traits<_InputIterator>::reference _ReferenceType1;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType2;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::integral_constant < bool, std::is_trivial<_ValueType1>::value&& std::is_trivial<_ValueType2>::value > (),
         [&]() {
             return __pstl::__internal::__pattern_walk2_brick_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
-                [__is_vector](_InputIterator __begin, _Size __sz, _ForwardIterator __res) {
-                    return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
+                [](_InputIterator __begin, _Size __sz, _ForwardIterator __res) {
+                    return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector{});
+                });
         },
         [&]() {
-            return __pstl::__internal::__pattern_walk2_n(std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
+            return __pstl::__internal::__pattern_walk2_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                         __first, __n, __result,
                                                          [](_ReferenceType1 __val1, _ReferenceType2 __val2) {
                                                              ::new (std::addressof(__val2)) _ValueType2(__val1);
-                                                         },
-                                                         __is_vector, __is_parallel);
+                                                         });
         });
 }
 
 // [uninitialized.move]
 
 template <class _ExecutionPolicy, class _InputIterator, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_move(_ExecutionPolicy&& __exec, _InputIterator __first, _InputIterator __last, _ForwardIterator __result)
 {
     typedef typename iterator_traits<_InputIterator>::value_type _ValueType1;
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType2;
     typedef typename iterator_traits<_InputIterator>::reference _ReferenceType1;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType2;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::integral_constant < bool, std::is_trivial<_ValueType1>::value&& std::is_trivial<_ValueType2>::value > (),
         [&]() {
             return __pstl::__internal::__pattern_walk2_brick(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
-                [__is_vector](_InputIterator __begin, _InputIterator __end, _ForwardIterator __res) {
-                    return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
+                [](_InputIterator __begin, _InputIterator __end, _ForwardIterator __res) {
+                    return __pstl::__internal::__brick_copy(__begin, __end, __res, __is_vector{});
+                });
         },
         [&]() {
             return __pstl::__internal::__pattern_walk2(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last, __result,
                 [](_ReferenceType1 __val1, _ReferenceType2 __val2) {
                     ::new (std::addressof(__val2)) _ValueType2(std::move(__val1));
-                },
-                __is_vector, __is_parallel);
+                });
         });
 }
 
 template <class _ExecutionPolicy, class _InputIterator, class _Size, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_move_n(_ExecutionPolicy&& __exec, _InputIterator __first, _Size __n, _ForwardIterator __result)
 {
     typedef typename iterator_traits<_InputIterator>::value_type _ValueType1;
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType2;
     typedef typename iterator_traits<_InputIterator>::reference _ReferenceType1;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType2;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _InputIterator, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::integral_constant < bool, std::is_trivial<_ValueType1>::value&& std::is_trivial<_ValueType2>::value > (),
         [&]() {
             return __pstl::__internal::__pattern_walk2_brick_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
-                [__is_vector](_InputIterator __begin, _Size __sz, _ForwardIterator __res) {
-                    return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
+                [](_InputIterator __begin, _Size __sz, _ForwardIterator __res) {
+                    return __pstl::__internal::__brick_copy_n(__begin, __sz, __res, __is_vector{});
+                });
         },
         [&]() {
-            return __pstl::__internal::__pattern_walk2_n(std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
-                                                         [](_ReferenceType1 __val1, _ReferenceType2 __val2) {
-                                                             ::new (std::addressof(__val2))
-                                                                 _ValueType2(std::move(__val1));
-                                                         },
-                                                         __is_vector, __is_parallel);
+            return __pstl::__internal::__pattern_walk2_n(
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n, __result,
+                    ::new (std::addressof(__val2)) _ValueType2(std::move(__val1));
+                });
         });
 }
 
 // [uninitialized.fill]
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 uninitialized_fill(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     __pstl::__internal::__invoke_if_else(
         std::is_arithmetic<_ValueType>(),
         [&]() {
             __pstl::__internal::__pattern_walk_brick(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [&__value, &__is_vector](_ForwardIterator __begin, _ForwardIterator __end) {
-                    __pstl::__internal::__brick_fill(__begin, __end, _ValueType(__value), __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [&__value](_ForwardIterator __begin, _ForwardIterator __end) {
+                    __pstl::__internal::__brick_fill(__begin, __end, _ValueType(__value), __is_vector{});
+                });
         },
         [&]() {
             __pstl::__internal::__pattern_walk1(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [&__value](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(__value); }, __is_vector,
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [&__value](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(__value); });
         });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_fill_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __n, const _Tp& __value)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::is_arithmetic<_ValueType>(),
         [&]() {
             return __pstl::__internal::__pattern_walk_brick_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                [&__value, &__is_vector](_ForwardIterator __begin, _Size __count) {
-                    return __pstl::__internal::__brick_fill_n(__begin, __count, _ValueType(__value), __is_vector);
-                },
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                [&__value](_ForwardIterator __begin, _Size __count) {
+                    return __pstl::__internal::__brick_fill_n(__begin, __count, _ValueType(__value), __is_vector{});
+                });
         },
         [&]() {
             return __pstl::__internal::__pattern_walk1_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                [&__value](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(__value); }, __is_vector,
-                __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                [&__value](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(__value); });
         });
 }
 
 // [specialized.destroy]
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 destroy(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
 
     __pstl::__internal::__invoke_if_not(std::is_trivially_destructible<_ValueType>(), [&]() {
-        __pstl::__internal::__pattern_walk1(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                            [](_ReferenceType __val) { __val.~_ValueType(); }, __is_vector,
-                                            __is_parallel);
+        __pstl::__internal::__pattern_walk1(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                            [](_ReferenceType __val) { __val.~_ValueType(); });
     });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 destroy_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __n)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
 
     return __pstl::__internal::__invoke_if_else(
         std::is_trivially_destructible<_ValueType>(), [&]() { return std::next(__first, __n); },
         [&]() {
-            return __pstl::__internal::__pattern_walk1_n(std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                                                         [](_ReferenceType __val) { __val.~_ValueType(); }, __is_vector,
-                                                         __is_parallel);
+            return __pstl::__internal::__pattern_walk1_n(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                         __first, __n,
+                                                         [](_ReferenceType __val) { __val.~_ValueType(); });
         });
 }
 
 // [uninitialized.construct.default]
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 uninitialized_default_construct(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
 
     __pstl::__internal::__invoke_if_not(std::is_trivial<_ValueType>(), [&]() {
-        __pstl::__internal::__pattern_walk1(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                            [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType; },
-                                            __is_vector, __is_parallel);
+        __pstl::__internal::__pattern_walk1(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                            [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType; });
     });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_default_construct_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __n)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
 
     return __pstl::__internal::__invoke_if_else(
         std::is_trivial<_ValueType>(), [&]() { return std::next(__first, __n); },
         [&]() {
             return __pstl::__internal::__pattern_walk1_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType; }, __is_vector, __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType; });
         });
 }
 
 // [uninitialized.construct.value]
 
 template <class _ExecutionPolicy, class _ForwardIterator>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 uninitialized_value_construct(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     __pstl::__internal::__invoke_if_else(
         std::is_trivial<_ValueType>(),
         [&]() {
-            __pstl::__internal::__pattern_walk_brick(std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                                                     [__is_vector](_ForwardIterator __begin, _ForwardIterator __end) {
+            __pstl::__internal::__pattern_walk_brick(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                     __last, [](_ForwardIterator __begin, _ForwardIterator __end) {
                                                          __pstl::__internal::__brick_fill(__begin, __end, _ValueType(),
-                                                                                          __is_vector);
-                                                     },
-                                                     __is_parallel);
+                                                                                          __is_vector{});
+                                                     });
         },
         [&]() {
             __pstl::__internal::__pattern_walk1(
-                std::forward<_ExecutionPolicy>(__exec), __first, __last,
-                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(); }, __is_vector, __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(); });
         });
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Size>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 uninitialized_value_construct_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __n)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type _ValueType;
     typedef typename iterator_traits<_ForwardIterator>::reference _ReferenceType;
 
-    const auto __is_parallel =
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
-    const auto __is_vector =
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec);
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+
+    using __is_vector = typename decltype(__dispatch_tag)::__is_vector;
 
     return __pstl::__internal::__invoke_if_else(
         std::is_trivial<_ValueType>(),
         [&]() {
-            return __pstl::__internal::__pattern_walk_brick_n(std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                                                              [__is_vector](_ForwardIterator __begin, _Size __count) {
-                                                                  return __pstl::__internal::__brick_fill_n(
-                                                                      __begin, __count, _ValueType(), __is_vector);
-                                                              },
-                                                              __is_parallel);
+            return __pstl::__internal::__pattern_walk_brick_n(
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                [](_ForwardIterator __begin, _Size __count) {
+                    return __pstl::__internal::__brick_fill_n(__begin, __count, _ValueType(), __is_vector{});
+                });
         },
         [&]() {
             return __pstl::__internal::__pattern_walk1_n(
-                std::forward<_ExecutionPolicy>(__exec), __first, __n,
-                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(); }, __is_vector, __is_parallel);
+                __dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __n,
+                [](_ReferenceType __val) { ::new (std::addressof(__val)) _ValueType(); });
         });
 }
 
 } // namespace std
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_GLUE_MEMORY_IMPL_H */

pstl/include/pstl/internal/glue_numeric_impl.h

 
 // [transform.reduce]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _Tp>
 transform_reduce(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                  _ForwardIterator2 __first2, _Tp __init)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+
     typedef typename iterator_traits<_ForwardIterator1>::value_type _InputType;
-    return __pstl::__internal::__pattern_transform_reduce(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __init, std::plus<_InputType>(),
-        std::multiplies<_InputType>(),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    return __pstl::__internal::__pattern_transform_reduce(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                          __first1, __last1, __first2, __init, std::plus<_InputType>(),
+                                                          std::multiplies<_InputType>());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation1,
           class _BinaryOperation2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _Tp>
 transform_reduce(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                  _ForwardIterator2 __first2, _Tp __init, _BinaryOperation1 __binary_op1, _BinaryOperation2 __binary_op2)
 {
-    return __pstl::__internal::__pattern_transform_reduce(
-        std::forward<_ExecutionPolicy>(__exec), __first1, __last1, __first2, __init, __binary_op1, __binary_op2,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first1, __first2);
+    return __pstl::__internal::__pattern_transform_reduce(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                          __first1, __last1, __first2, __init, __binary_op1,
+                                                          __binary_op2);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation, class _UnaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _Tp>
 transform_reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
                  _BinaryOperation __binary_op, _UnaryOperation __unary_op)
 {
-    return __pstl::__internal::__pattern_transform_reduce(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __init, __binary_op, __unary_op,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator>(__exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first);
+    return __pstl::__internal::__pattern_transform_reduce(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                          __first, __last, __init, __binary_op, __unary_op);
 }
 
 // [exclusive.scan]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Tp __init)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
     using namespace __pstl;
-    return __internal::__pattern_transform_scan(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __pstl::__internal::__no_op(), __init,
-        std::plus<_Tp>(), /*inclusive=*/std::false_type(),
-        __internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(__exec),
-        __internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(__exec));
+    return __internal::__pattern_transform_scan(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                __result, __pstl::__internal::__no_op(), __init, std::plus<_Tp>(),
+                                                /*inclusive=*/std::false_type());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Tp __init, _BinaryOperation __binary_op)
 {
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
     using namespace __pstl;
-    return __internal::__pattern_transform_scan(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __pstl::__internal::__no_op(), __init,
-        __binary_op, /*inclusive=*/std::false_type(),
-        __internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(__exec),
-        __internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(__exec));
+    return __internal::__pattern_transform_scan(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
+                                                __result, __pstl::__internal::__no_op(), __init, __binary_op,
+                                                /*inclusive=*/std::false_type());
 }
 
 // [inclusive.scan]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 inclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result)

 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation,
           class _UnaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 transform_exclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                          _ForwardIterator2 __result, _Tp __init, _BinaryOperation __binary_op,
                          _UnaryOperation __unary_op)
 {
-    return __pstl::__internal::__pattern_transform_scan(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __unary_op, __init, __binary_op,
-        /*inclusive=*/std::false_type(),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_transform_scan(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                        __last, __result, __unary_op, __init, __binary_op,
+                                                        /*inclusive=*/std::false_type());
 }
 
 // [transform.inclusive.scan]
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _BinaryOperation,
           class _UnaryOperation, class _Tp>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 transform_inclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                          _ForwardIterator2 __result, _BinaryOperation __binary_op, _UnaryOperation __unary_op,
                          _Tp __init)
 {
-    return __pstl::__internal::__pattern_transform_scan(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __result, __unary_op, __init, __binary_op,
-        /*inclusive=*/std::true_type(),
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __result);
+
+    return __pstl::__internal::__pattern_transform_scan(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec), __first,
+                                                        __last, __result, __unary_op, __init, __binary_op,
+                                                        /*inclusive=*/std::true_type());
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryOperation,
           class _BinaryOperation>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 transform_inclusive_scan(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                          _ForwardIterator2 __result, _BinaryOperation __binary_op, _UnaryOperation __unary_op)
 {

 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 adjacent_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                     _ForwardIterator2 __d_first, _BinaryOperation __op)
 {
 
     if (__first == __last)
         return __d_first;
 
-    return __pstl::__internal::__pattern_adjacent_difference(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last, __d_first, __op,
-        __pstl::__internal::__is_vectorization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec),
-        __pstl::__internal::__is_parallelization_preferred<_ExecutionPolicy, _ForwardIterator1, _ForwardIterator2>(
-            __exec));
+    auto __dispatch_tag = __pstl::__internal::__select_backend(__exec, __first, __d_first);
+
+    return __pstl::__internal::__pattern_adjacent_difference(__dispatch_tag, std::forward<_ExecutionPolicy>(__exec),
+                                                             __first, __last, __d_first, __op);
 }
 
 template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 adjacent_difference(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                     _ForwardIterator2 __d_first)
 {
     typedef typename iterator_traits<_ForwardIterator1>::value_type _ValueType;

pstl/include/pstl/internal/numeric_fwd.h

_Tp __brick_transform_reduce(_RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2, _Tp,
                              _BinaryOperation1, _BinaryOperation2,
                              /*__is_vector=*/std::true_type) noexcept;
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation1, class _BinaryOperation2>
 _Tp __brick_transform_reduce(_ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Tp, _BinaryOperation1,
                              _BinaryOperation2,
                              /*__is_vector=*/std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation1,
-          class _BinaryOperation2, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp,
+          class _BinaryOperation1, class _BinaryOperation2>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Tp,
-                           _BinaryOperation1, _BinaryOperation2, _IsVector,
-                           /*is_parallel=*/std::false_type) noexcept;
+__pattern_transform_reduce(_Tag, _ExecutionPolicy&&, _ForwardIterator1, _ForwardIterator1, _ForwardIterator2, _Tp,
+                           _BinaryOperation1, _BinaryOperation2) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Tp,
-          class _BinaryOperation1, class _BinaryOperation2, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Tp, class _BinaryOperation1, class _BinaryOperation2>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _RandomAccessIterator1, _RandomAccessIterator1, _RandomAccessIterator2,
-                           _Tp, _BinaryOperation1, _BinaryOperation2, _IsVector __is_vector,
-                           /*is_parallel=*/std::true_type);
+__pattern_transform_reduce(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator1,
+                           _RandomAccessIterator1, _RandomAccessIterator2, _Tp, _BinaryOperation1, _BinaryOperation2);
 
 //------------------------------------------------------------------------
 // transform_reduce (version with unary and binary functions)
 //------------------------------------------------------------------------
 
 template <class _RandomAccessIterator, class _Tp, class _UnaryOperation, class _BinaryOperation>
 _Tp __brick_transform_reduce(_RandomAccessIterator, _RandomAccessIterator, _Tp, _BinaryOperation, _UnaryOperation,
                              /*is_vector=*/std::true_type) noexcept;
 
 template <class _ForwardIterator, class _Tp, class _BinaryOperation, class _UnaryOperation>
 _Tp __brick_transform_reduce(_ForwardIterator, _ForwardIterator, _Tp, _BinaryOperation, _UnaryOperation,
                              /*is_vector=*/std::false_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation, class _UnaryOperation,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation,
+          class _UnaryOperation>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp, _BinaryOperation,
-                           _UnaryOperation, _IsVector,
-                           /*is_parallel=*/std::false_type) noexcept;
+__pattern_transform_reduce(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp, _BinaryOperation,
+                           _UnaryOperation) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _BinaryOperation, class _UnaryOperation,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _BinaryOperation,
+          class _UnaryOperation>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _Tp, _BinaryOperation,
-                           _UnaryOperation, _IsVector,
-                           /*is_parallel=*/std::true_type);
+__pattern_transform_reduce(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                           _Tp, _BinaryOperation, _UnaryOperation);
 
 //------------------------------------------------------------------------
 // transform_exclusive_scan
 //
 // walk3 evaluates f(x,y,z) for (x,y,z) drawn from [first1,last1), [first2,...), [first3,...)
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _OutputIterator, class _UnaryOperation, class _Tp, class _BinaryOperation>
 std::pair<_OutputIterator, _Tp> __brick_transform_scan(_ForwardIterator, _ForwardIterator, _OutputIterator,
                                                        _UnaryOperation, _Tp, _BinaryOperation,
                                                        /*Inclusive*/ std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator, class _UnaryOperation, class _Tp, class _BinaryOperation>
 std::pair<_OutputIterator, _Tp> __brick_transform_scan(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
                                                        _UnaryOperation, _Tp, _BinaryOperation,
                                                        /*Inclusive*/ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryOperation,
+          class _Tp, class _BinaryOperation, class _Inclusive>
 _OutputIterator
-__pattern_transform_scan(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator, _UnaryOperation, _Tp,
-                         _BinaryOperation, _Inclusive, _IsVector,
-                         /*is_parallel=*/std::false_type) noexcept;
+__pattern_transform_scan(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator, _UnaryOperation,
+                         _Tp, _BinaryOperation, _Inclusive) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _UnaryOperation, class _Tp, class _BinaryOperation, class _Inclusive>
 typename std::enable_if<!std::is_floating_point<_Tp>::value, _OutputIterator>::type
-__pattern_transform_scan(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
-                         _UnaryOperation, _Tp, _BinaryOperation, _Inclusive, _IsVector, /*is_parallel=*/std::true_type);
+__pattern_transform_scan(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&&, _RandomAccessIterator,
+                         _RandomAccessIterator, _OutputIterator, _UnaryOperation, _Tp, _BinaryOperation, _Inclusive);
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _UnaryOperation, class _Tp, class _BinaryOperation, class _Inclusive>
 typename std::enable_if<std::is_floating_point<_Tp>::value, _OutputIterator>::type
-__pattern_transform_scan(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
-                         _UnaryOperation, _Tp, _BinaryOperation, _Inclusive, _IsVector, /*is_parallel=*/std::true_type);
+__pattern_transform_scan(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator,
+                         _OutputIterator, _UnaryOperation, _Tp, _BinaryOperation, _Inclusive);
 
 //------------------------------------------------------------------------
 // adjacent_difference
 //------------------------------------------------------------------------
 
 template <class _ForwardIterator, class _OutputIterator, class _BinaryOperation>
 _OutputIterator __brick_adjacent_difference(_ForwardIterator, _ForwardIterator, _OutputIterator, _BinaryOperation,
                                             /*is_vector*/ std::false_type) noexcept;
 
 template <class _RandomAccessIterator, class _OutputIterator, class _BinaryOperation>
 _OutputIterator __brick_adjacent_difference(_RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
                                             _BinaryOperation,
                                             /*is_vector*/ std::true_type) noexcept;
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryOperation,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryOperation>
 _OutputIterator
-__pattern_adjacent_difference(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator, _BinaryOperation,
-                              _IsVector, /*is_parallel*/ std::false_type) noexcept;
+__pattern_adjacent_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _OutputIterator,
+                              _BinaryOperation) noexcept;
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _BinaryOperation,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _BinaryOperation>
 _OutputIterator
-__pattern_adjacent_difference(_ExecutionPolicy&&, _RandomAccessIterator, _RandomAccessIterator, _OutputIterator,
-                              _BinaryOperation, _IsVector, /*is_parallel*/ std::true_type);
+__pattern_adjacent_difference(__parallel_tag<_IsVector>, _ExecutionPolicy&&, _RandomAccessIterator,
+                              _RandomAccessIterator, _OutputIterator, _BinaryOperation);
 
 } // namespace __internal
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_NUMERIC_FWD_H */

pstl/include/pstl/internal/numeric_impl.h

__brick_transform_reduce(_RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
                          /*is_vector=*/std::true_type) noexcept
 {
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType;
     return __unseq_backend::__simd_transform_reduce(
         __last1 - __first1, __init, __binary_op1,
         [=, &__binary_op2](_DifferenceType __i) { return __binary_op2(__first1[__i], __first2[__i]); });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp, class _BinaryOperation1,
-          class _BinaryOperation2, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp,
+          class _BinaryOperation1, class _BinaryOperation2>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
+__pattern_transform_reduce(_Tag, _ExecutionPolicy&&, _ForwardIterator1 __first1, _ForwardIterator1 __last1,
                            _ForwardIterator2 __first2, _Tp __init, _BinaryOperation1 __binary_op1,
-                           _BinaryOperation2 __binary_op2, _IsVector __is_vector,
-                           /*is_parallel=*/std::false_type) noexcept
+                           _BinaryOperation2 __binary_op2) noexcept
 {
-    return __brick_transform_reduce(__first1, __last1, __first2, __init, __binary_op1, __binary_op2, __is_vector);
+    return __brick_transform_reduce(__first1, __last1, __first2, __init, __binary_op1, __binary_op2,
+                                    typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _Tp,
-          class _BinaryOperation1, class _BinaryOperation2, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _Tp, class _BinaryOperation1, class _BinaryOperation2>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                           _RandomAccessIterator2 __first2, _Tp __init, _BinaryOperation1 __binary_op1,
-                           _BinaryOperation2 __binary_op2, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_transform_reduce(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator1 __first1,
+                           _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _Tp __init,
+                           _BinaryOperation1 __binary_op1, _BinaryOperation2 __binary_op2)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __par_backend::__parallel_transform_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first1, __last1,
             [__first1, __first2, __binary_op2](_RandomAccessIterator1 __i) mutable {
                 return __binary_op2(*__i, *(__first2 + (__i - __first1)));
             },
             __init,
             __binary_op1, // Combine
-            [__first1, __first2, __binary_op1, __binary_op2,
-             __is_vector](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j, _Tp __init) -> _Tp {
+            [__first1, __first2, __binary_op1, __binary_op2](_RandomAccessIterator1 __i, _RandomAccessIterator1 __j,
+                                                             _Tp __init) -> _Tp {
                 return __internal::__brick_transform_reduce(__i, __j, __first2 + (__i - __first1), __init, __binary_op1,
-                                                            __binary_op2, __is_vector);
+                                                            __binary_op2, _IsVector{});
             });
     });
 }
 
 //------------------------------------------------------------------------
 // transform_reduce (version with unary and binary functions)
 //------------------------------------------------------------------------
 
__brick_transform_reduce(_RandomAccessIterator __first, _RandomAccessIterator __last, _Tp __init,
                          /*is_vector=*/std::true_type) noexcept
 {
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _DifferenceType;
     return __unseq_backend::__simd_transform_reduce(
         __last - __first, __init, __binary_op,
         [=, &__unary_op](_DifferenceType __i) { return __unary_op(__first[__i]); });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation, class _UnaryOperation,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation,
+          class _UnaryOperation>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
-                           _BinaryOperation __binary_op, _UnaryOperation __unary_op, _IsVector __is_vector,
-                           /*is_parallel=*/std::false_type) noexcept
+__pattern_transform_reduce(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
+                           _BinaryOperation __binary_op, _UnaryOperation __unary_op) noexcept
 {
-    return __internal::__brick_transform_reduce(__first, __last, __init, __binary_op, __unary_op, __is_vector);
+    return __internal::__brick_transform_reduce(__first, __last, __init, __binary_op, __unary_op,
+                                                typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _BinaryOperation, class _UnaryOperation,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _Tp, class _BinaryOperation,
+          class _UnaryOperation>
 _Tp
-__pattern_transform_reduce(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                           _Tp __init, _BinaryOperation __binary_op, _UnaryOperation __unary_op, _IsVector __is_vector,
-                           /*is_parallel=*/std::true_type)
+__pattern_transform_reduce(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                           _RandomAccessIterator __last, _Tp __init, _BinaryOperation __binary_op,
+                           _UnaryOperation __unary_op)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     return __internal::__except_handler([&]() {
         return __par_backend::__parallel_transform_reduce(
-            std::forward<_ExecutionPolicy>(__exec), __first, __last,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last,
             [__unary_op](_RandomAccessIterator __i) mutable { return __unary_op(*__i); }, __init, __binary_op,
-            [__unary_op, __binary_op, __is_vector](_RandomAccessIterator __i, _RandomAccessIterator __j, _Tp __init) {
-                return __internal::__brick_transform_reduce(__i, __j, __init, __binary_op, __unary_op, __is_vector);
+            [__unary_op, __binary_op](_RandomAccessIterator __i, _RandomAccessIterator __j, _Tp __init) {
+                return __internal::__brick_transform_reduce(__i, __j, __init, __binary_op, __unary_op, _IsVector{});
             });
     });
 }
 
 //------------------------------------------------------------------------
 // transform_exclusive_scan
 //
 // walk3 evaluates f(x,y,z) for (x,y,z) drawn from [first1,last1), [first2,...), [first3,...)

 __brick_transform_scan(_RandomAccessIterator __first, _RandomAccessIterator __last, _OutputIterator __result,
                        _UnaryOperation __unary_op, _Tp __init, _BinaryOperation __binary_op, _Inclusive,
                        /*is_vector=*/std::true_type) noexcept
 {
     return __internal::__brick_transform_scan(__first, __last, __result, __unary_op, __init, __binary_op, _Inclusive(),
                                               /*is_vector=*/std::false_type());
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _UnaryOperation,
+          class _Tp, class _BinaryOperation, class _Inclusive>
 _OutputIterator
-__pattern_transform_scan(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+__pattern_transform_scan(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
                          _OutputIterator __result, _UnaryOperation __unary_op, _Tp __init, _BinaryOperation __binary_op,
-                         _Inclusive, _IsVector __is_vector, /*is_parallel=*/std::false_type) noexcept
+                         _Inclusive) noexcept
 {
     return __internal::__brick_transform_scan(__first, __last, __result, __unary_op, __init, __binary_op, _Inclusive(),
-                                              __is_vector)
+                                              typename _Tag::__is_vector{})
         .first;
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _UnaryOperation, class _Tp, class _BinaryOperation, class _Inclusive>
 typename std::enable_if<!std::is_floating_point<_Tp>::value, _OutputIterator>::type
-__pattern_transform_scan(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                         _OutputIterator __result, _UnaryOperation __unary_op, _Tp __init, _BinaryOperation __binary_op,
-                         _Inclusive, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_transform_scan(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                         _RandomAccessIterator __last, _OutputIterator __result, _UnaryOperation __unary_op, _Tp __init,
+                         _BinaryOperation __binary_op, _Inclusive)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _DifferenceType;
 
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_transform_scan(
-            std::forward<_ExecutionPolicy>(__exec), __last - __first,
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __last - __first,
             [__first, __unary_op](_DifferenceType __i) mutable { return __unary_op(__first[__i]); }, __init,
             __binary_op,
             [__first, __unary_op, __binary_op](_DifferenceType __i, _DifferenceType __j, _Tp __init) {
                 // Execute serial __brick_transform_reduce, due to the explicit SIMD vectorization (reduction) requires a commutative operation for the guarantee of correct scan.
                 return __internal::__brick_transform_reduce(__first + __i, __first + __j, __init, __binary_op,
                                                             __unary_op,
                                                             /*__is_vector*/ std::false_type());
             },
-            [__first, __unary_op, __binary_op, __result, __is_vector](_DifferenceType __i, _DifferenceType __j,
-                                                                      _Tp __init) {
+            [__first, __unary_op, __binary_op, __result](_DifferenceType __i, _DifferenceType __j, _Tp __init) {
                 return __internal::__brick_transform_scan(__first + __i, __first + __j, __result + __i, __unary_op,
-                                                          __init, __binary_op, _Inclusive(), __is_vector)
+                                                          __init, __binary_op, _Inclusive(), _IsVector{})
                     .second;
             });
         return __result + (__last - __first);
     });
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator, class _UnaryOperation, class _Tp,
-          class _BinaryOperation, class _Inclusive, class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator, class _OutputIterator,
+          class _UnaryOperation, class _Tp, class _BinaryOperation, class _Inclusive>
 typename std::enable_if<std::is_floating_point<_Tp>::value, _OutputIterator>::type
-__pattern_transform_scan(_ExecutionPolicy&& __exec, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                         _OutputIterator __result, _UnaryOperation __unary_op, _Tp __init, _BinaryOperation __binary_op,
-                         _Inclusive, _IsVector __is_vector, /*is_parallel=*/std::true_type)
+__pattern_transform_scan(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec, _RandomAccessIterator __first,
+                         _RandomAccessIterator __last, _OutputIterator __result, _UnaryOperation __unary_op, _Tp __init,
+                         _BinaryOperation __binary_op, _Inclusive)
 {
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _DifferenceType;
     _DifferenceType __n = __last - __first;
 
     if (__n <= 0)
     {
         return __result;
     }
     return __internal::__except_handler([&]() {
         __par_backend::__parallel_strict_scan(
-            std::forward<_ExecutionPolicy>(__exec), __n, __init,
-            [__first, __unary_op, __binary_op, __result, __is_vector](_DifferenceType __i, _DifferenceType __len) {
+            __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __n, __init,
+            [__first, __unary_op, __binary_op, __result](_DifferenceType __i, _DifferenceType __len) {
                 return __internal::__brick_transform_scan(__first + __i, __first + (__i + __len), __result + __i,
-                                                          __unary_op, _Tp{}, __binary_op, _Inclusive(), __is_vector)
+                                                          __unary_op, _Tp{}, __binary_op, _Inclusive(), _IsVector{})
                     .second;
             },
             __binary_op,
             [__result, &__binary_op](_DifferenceType __i, _DifferenceType __len, _Tp __initial) {
                 return *(std::transform(__result + __i, __result + __i + __len, __result + __i,
                                         [&__initial, &__binary_op](const _Tp& __x) {
                                             _PSTL_PRAGMA_FORCEINLINE
                                             return __binary_op(__initial, __x);
__brick_adjacent_difference(_RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
 
     auto __n = __last - __first;
     *__d_first = *__first;
     return __unseq_backend::__simd_walk_3(
         __first + 1, __n - 1, __first, __d_first + 1,
         [&__op](_ReferenceType1 __x, _ReferenceType1 __y, _ReferenceType2 __z) { __z = __op(__x, __y); });
 }
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryOperation,
-          class _IsVector>
+template <class _Tag, class _ExecutionPolicy, class _ForwardIterator, class _OutputIterator, class _BinaryOperation>
 _OutputIterator
-__pattern_adjacent_difference(_ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
-                              _OutputIterator __d_first, _BinaryOperation __op, _IsVector __is_vector,
-                              /*is_parallel*/ std::false_type) noexcept
+__pattern_adjacent_difference(_Tag, _ExecutionPolicy&&, _ForwardIterator __first, _ForwardIterator __last,
+                              _OutputIterator __d_first, _BinaryOperation __op) noexcept
 {
-    return __internal::__brick_adjacent_difference(__first, __last, __d_first, __op, __is_vector);
+    return __internal::__brick_adjacent_difference(__first, __last, __d_first, __op, typename _Tag::__is_vector{});
 }
 
-template <class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2, class _BinaryOperation,
-          class _IsVector>
+template <class _IsVector, class _ExecutionPolicy, class _RandomAccessIterator1, class _RandomAccessIterator2,
+          class _BinaryOperation>
 _RandomAccessIterator2
-__pattern_adjacent_difference(_ExecutionPolicy&& __exec, _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
-                              _RandomAccessIterator2 __d_first, _BinaryOperation __op, _IsVector __is_vector,
-                              /*is_parallel=*/std::true_type)
+__pattern_adjacent_difference(__parallel_tag<_IsVector> __tag, _ExecutionPolicy&& __exec,
+                              _RandomAccessIterator1 __first, _RandomAccessIterator1 __last,
+                              _RandomAccessIterator2 __d_first, _BinaryOperation __op)
 {
     _PSTL_ASSERT(__first != __last);
     typedef typename std::iterator_traits<_RandomAccessIterator1>::reference _ReferenceType1;
     typedef typename std::iterator_traits<_RandomAccessIterator2>::reference _ReferenceType2;
 
+    using __backend_tag = typename decltype(__tag)::__backend_tag;
+
     *__d_first = *__first;
     __par_backend::__parallel_for(
-        std::forward<_ExecutionPolicy>(__exec), __first, __last - 1,
-        [&__op, __is_vector, __d_first, __first](_RandomAccessIterator1 __b, _RandomAccessIterator1 __e) {
+        __backend_tag{}, std::forward<_ExecutionPolicy>(__exec), __first, __last - 1,
+        [&__op, __d_first, __first](_RandomAccessIterator1 __b, _RandomAccessIterator1 __e) {
             _RandomAccessIterator2 __d_b = __d_first + (__b - __first);
             __internal::__brick_walk3(
                 __b, __e, __b + 1, __d_b + 1,
                 [&__op](_ReferenceType1 __x, _ReferenceType1 __y, _ReferenceType2 __z) { __z = __op(__y, __x); },
-                __is_vector);
+                _IsVector{});
         });
     return __d_first + (__last - __first);
 }
 
 } // namespace __internal
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_NUMERIC_IMPL_H */

pstl/include/pstl/internal/omp/parallel_for.h

 
 //------------------------------------------------------------------------
 // Notation:
 // Evaluation of brick f[i,j) for each subrange [i,j) of [first, last)
 //------------------------------------------------------------------------
 
 template <class _ExecutionPolicy, class _Index, class _Fp>
 void
-__parallel_for(_ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
+__parallel_for(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
 {
     if (omp_in_parallel())
     {
         // we don't create a nested parallel region in an existing parallel
         // region: just create tasks
         __pstl::__omp_backend::__parallel_for_body(__first, __last, __f);
     }
     else

pstl/include/pstl/internal/omp/parallel_invoke.h

__parallel_invoke_body(_F1&& __f1, _F2&& __f2)
     {
         _PSTL_PRAGMA(omp task untied mergeable) { std::forward<_F1>(__f1)(); }
         _PSTL_PRAGMA(omp task untied mergeable) { std::forward<_F2>(__f2)(); }
     }
 }
 
 template <class _ExecutionPolicy, typename _F1, typename _F2>
 void
-__parallel_invoke(_ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
+__parallel_invoke(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
 {
     if (omp_in_parallel())
     {
         __pstl::__omp_backend::__parallel_invoke_body(std::forward<_F1>(__f1), std::forward<_F2>(__f2));
     }
     else
     {
         _PSTL_PRAGMA(omp parallel)
         _PSTL_PRAGMA(omp single nowait)
         __pstl::__omp_backend::__parallel_invoke_body(std::forward<_F1>(__f1), std::forward<_F2>(__f2));
     }
 }
 
 } // namespace __omp_backend
 } // namespace __pstl
 #endif // _PSTL_INTERNAL_OMP_PARALLEL_INVOKE_H

pstl/include/pstl/internal/omp/parallel_merge.h

__pstl::__omp_backend::__parallel_merge_body(__xe - __xm, __ye - __ym, __xm, __xe, __ym, __ye, __zm, __comp,
                                                       __leaf_merge);
 
     _PSTL_PRAGMA(omp taskwait)
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator1, typename _RandomAccessIterator2,
           typename _RandomAccessIterator3, typename _Compare, typename _LeafMerge>
 void
-__parallel_merge(_ExecutionPolicy&& /*__exec*/, _RandomAccessIterator1 __xs, _RandomAccessIterator1 __xe,
-                 _RandomAccessIterator2 __ys, _RandomAccessIterator2 __ye, _RandomAccessIterator3 __zs, _Compare __comp,
-                 _LeafMerge __leaf_merge)
+__parallel_merge(__pstl::__internal::__openmp_backend, _ExecutionPolicy&& /*__exec*/, _RandomAccessIterator1 __xs,
+                 _RandomAccessIterator1 __xe, _RandomAccessIterator2 __ys, _RandomAccessIterator2 __ye,
+                 _RandomAccessIterator3 __zs, _Compare __comp, _LeafMerge __leaf_merge)
 
 {
     std::size_t __size_x = __xe - __xs;
     std::size_t __size_y = __ye - __ys;
 
     /*
      * Run the merge in parallel by chunking it up. Use the smaller range (if any) as the iteration range, and the
      * larger range as the search range.

pstl/include/pstl/internal/omp/parallel_reduce.h

 //------------------------------------------------------------------------
 // Notation:
 //      r(i,j,init) returns reduction of init with reduction over [i,j)
 //      c(x,y) combines values x and y that were the result of r
 //------------------------------------------------------------------------
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _Value, typename _RealBody, typename _Reduction>
 _Value
-__parallel_reduce(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last, _Value __identity,
-                  _RealBody __real_body, _Reduction __reduction)
+__parallel_reduce(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _RandomAccessIterator __first,
+                  _RandomAccessIterator __last, _Value __identity, _RealBody __real_body, _Reduction __reduction)
 {
     // We don't create a nested parallel region in an existing parallel region:
     // just create tasks.
     if (omp_in_parallel())
     {
         return __pstl::__omp_backend::__parallel_reduce_body(__first, __last, __identity, __real_body, __reduction);
     }
 

pstl/include/pstl/internal/omp/parallel_scan.h

__parallel_strict_scan_body(_Index __n, _Tp __initial, _Rp __reduce, _Cp __combine, _Sp __scan, _Ap __apex)
 
     __apex(__combine(__initial, __t));
     __omp_backend::__downsweep(_Index(0), _Index(__m + 1), __tilesize, __r, __n - __m * __tilesize, __initial,
                                __combine, __scan);
 }
 
 template <class _ExecutionPolicy, typename _Index, typename _Tp, typename _Rp, typename _Cp, typename _Sp, typename _Ap>
 void
-__parallel_strict_scan(_ExecutionPolicy&&, _Index __n, _Tp __initial, _Rp __reduce, _Cp __combine, _Sp __scan,
-                       _Ap __apex)
+__parallel_strict_scan(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _Index __n, _Tp __initial,
+                       _Rp __reduce, _Cp __combine, _Sp __scan, _Ap __apex)
 {
     if (__n <= __default_chunk_size)
     {
         _Tp __sum = __initial;
         if (__n)
         {
             __sum = __combine(__sum, __reduce(_Index(0), __n));
         }

pstl/include/pstl/internal/omp/parallel_stable_partial_sort.h

 
 namespace __pstl
 {
 namespace __omp_backend
 {
 
 template <typename _RandomAccessIterator, typename _Compare, typename _LeafSort>
 void
-__parallel_stable_partial_sort(_RandomAccessIterator __xs, _RandomAccessIterator __xe, _Compare __comp,
-                               _LeafSort __leaf_sort, std::size_t /* __nsort */)
+__parallel_stable_partial_sort(__pstl::__internal::__openmp_backend, _RandomAccessIterator __xs,
+                               _RandomAccessIterator __xe, _Compare __comp, _LeafSort __leaf_sort,
+                               std::size_t /* __nsort */)
 {
     // TODO: "Parallel partial sort needs to be implemented.");
     __leaf_sort(__xs, __xe, __comp);
 }
 
 } // namespace __omp_backend
 } // namespace __pstl
 #endif // _PSTL_INTERNAL_OMP_PARALLEL_STABLE_PARTIAL_SORT_H

pstl/include/pstl/internal/omp/parallel_stable_sort.h

else
 
         // Move the values from __output_data back in the original source range.
         __pstl::__omp_backend::__sort_details::__parallel_move_range(__output_data.begin(), __output_data.end(), __xs);
     }
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _LeafSort>
 void
-__parallel_stable_sort(_ExecutionPolicy&& /*__exec*/, _RandomAccessIterator __xs, _RandomAccessIterator __xe,
-                       _Compare __comp, _LeafSort __leaf_sort, std::size_t __nsort = 0)
+__parallel_stable_sort(__pstl::__internal::__openmp_backend tag, _ExecutionPolicy&& /*__exec*/,
+                       _RandomAccessIterator __xs, _RandomAccessIterator __xe, _Compare __comp, _LeafSort __leaf_sort,
+                       std::size_t __nsort = 0)
 {
     auto __count = static_cast<std::size_t>(__xe - __xs);
     if (__count <= __default_chunk_size || __nsort < __count)
     {
         __leaf_sort(__xs, __xe, __comp);
         return;
     }
 
     // TODO: the partial sort implementation should
     // be shared with the other backends.
 
     if (omp_in_parallel())
     {
         if (__count <= __nsort)
         {
             __pstl::__omp_backend::__parallel_stable_sort_body(__xs, __xe, __comp, __leaf_sort);
         }
         else
         {
-            __pstl::__omp_backend::__parallel_stable_partial_sort(__xs, __xe, __comp, __leaf_sort, __nsort);
+            __pstl::__omp_backend::__parallel_stable_partial_sort(tag, __xs, __xe, __comp, __leaf_sort, __nsort);
         }
     }
     else
     {
         _PSTL_PRAGMA(omp parallel)
         _PSTL_PRAGMA(omp single nowait)
         if (__count <= __nsort)
         {
             __pstl::__omp_backend::__parallel_stable_sort_body(__xs, __xe, __comp, __leaf_sort);
         }
         else
         {
-            __pstl::__omp_backend::__parallel_stable_partial_sort(__xs, __xe, __comp, __leaf_sort, __nsort);
+            __pstl::__omp_backend::__parallel_stable_partial_sort(tag, __xs, __xe, __comp, __leaf_sort, __nsort);
         }
     }
 }
 
 } // namespace __omp_backend
 } // namespace __pstl
 #endif // _PSTL_INTERNAL_OMP_PARALLEL_STABLE_SORT_H

pstl/include/pstl/internal/omp/parallel_transform_reduce.h

__transform_reduce_body(_RandomAccessIterator __first, _RandomAccessIterator __last, _UnaryOp __unary_op, _Value __init,
     }
 
     return __init;
 }
 
 template <class _ExecutionPolicy, class _RandomAccessIterator, class _UnaryOp, class _Value, class _Combiner,
           class _Reduction>
 _Value
-__parallel_transform_reduce(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last,
-                            _UnaryOp __unary_op, _Value __init, _Combiner __combiner, _Reduction __reduction)
+__parallel_transform_reduce(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _RandomAccessIterator __first,
+                            _RandomAccessIterator __last, _UnaryOp __unary_op, _Value __init, _Combiner __combiner,
+                            _Reduction __reduction)
 {
     _Value __result = __init;
     if (omp_in_parallel())
     {
         // We don't create a nested parallel region in an existing parallel
         // region: just create tasks
         __result = __pstl::__omp_backend::__transform_reduce_body(__first, __last, __unary_op, __init, __combiner,
                                                                   __reduction);

pstl/include/pstl/internal/omp/parallel_transform_scan.h

 
 namespace __pstl
 {
 namespace __omp_backend
 {
 
 template <class _ExecutionPolicy, class _Index, class _Up, class _Tp, class _Cp, class _Rp, class _Sp>
 _Tp
-__parallel_transform_scan(_ExecutionPolicy&&, _Index __n, _Up /* __u */, _Tp __init, _Cp /* __combine */,
-                          _Rp /* __brick_reduce */, _Sp __scan)
+__parallel_transform_scan(__pstl::__internal::__openmp_backend, _ExecutionPolicy&&, _Index __n, _Up /* __u */,
+                          _Tp __init, _Cp /* __combine */, _Rp /* __brick_reduce */, _Sp __scan)
 {
     // TODO: parallelize this function.
     return __scan(_Index(0), __n, __init);
 }
 
 } // namespace __omp_backend
 } // namespace __pstl
 #endif // _PSTL_INTERNAL_OMP_PARALLEL_TRANSFORM_SCAN_H

pstl/include/pstl/internal/parallel_backend_serial.h

 
 inline void
 __cancel_execution()
 {
 }
 
 template <class _ExecutionPolicy, class _Index, class _Fp>
 void
-__parallel_for(_ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
+__parallel_for(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
 {
     __f(__first, __last);
 }
 
 template <class _ExecutionPolicy, class _Value, class _Index, typename _RealBody, typename _Reduction>
 _Value
-__parallel_reduce(_ExecutionPolicy&&, _Index __first, _Index __last, const _Value& __identity,
-                  const _RealBody& __real_body, const _Reduction&)
+__parallel_reduce(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _Index __first, _Index __last,
+                  const _Value& __identity, const _RealBody& __real_body, const _Reduction&)
 {
     if (__first == __last)
     {
         return __identity;
     }
     else
     {
         return __real_body(__first, __last, __identity);
     }
 }
 
 template <class _ExecutionPolicy, class _Index, class _UnaryOp, class _Tp, class _BinaryOp, class _Reduce>
 _Tp
-__parallel_transform_reduce(_ExecutionPolicy&&, _Index __first, _Index __last, _UnaryOp, _Tp __init, _BinaryOp,
-                            _Reduce __reduce)
+__parallel_transform_reduce(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _Index __first, _Index __last,
+                            _UnaryOp, _Tp __init, _BinaryOp, _Reduce __reduce)
 {
     return __reduce(__first, __last, __init);
 }
 
 template <class _ExecutionPolicy, typename _Index, typename _Tp, typename _Rp, typename _Cp, typename _Sp, typename _Ap>
 void
-__parallel_strict_scan(_ExecutionPolicy&&, _Index __n, _Tp __initial, _Rp __reduce, _Cp __combine, _Sp __scan,
-                       _Ap __apex)
+__parallel_strict_scan(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _Index __n, _Tp __initial,
+                       _Rp __reduce, _Cp __combine, _Sp __scan, _Ap __apex)
 {
     _Tp __sum = __initial;
     if (__n)
         __sum = __combine(__sum, __reduce(_Index(0), __n));
     __apex(__sum);
     if (__n)
         __scan(_Index(0), __n, __initial);
 }
 
 template <class _ExecutionPolicy, class _Index, class _UnaryOp, class _Tp, class _BinaryOp, class _Reduce, class _Scan>
 _Tp
-__parallel_transform_scan(_ExecutionPolicy&&, _Index __n, _UnaryOp, _Tp __init, _BinaryOp, _Reduce, _Scan __scan)
+__parallel_transform_scan(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _Index __n, _UnaryOp, _Tp __init,
+                          _BinaryOp, _Reduce, _Scan __scan)
 {
     return __scan(_Index(0), __n, __init);
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _LeafSort>
 void
-__parallel_stable_sort(_ExecutionPolicy&&, _RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
-                       _LeafSort __leaf_sort, std::size_t = 0)
+__parallel_stable_sort(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _RandomAccessIterator __first,
+                       _RandomAccessIterator __last, _Compare __comp, _LeafSort __leaf_sort, std::size_t = 0)
 {
     __leaf_sort(__first, __last, __comp);
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator1, typename _RandomAccessIterator2,
           typename _RandomAccessIterator3, typename _Compare, typename _LeafMerge>
 void
-__parallel_merge(_ExecutionPolicy&&, _RandomAccessIterator1 __first1, _RandomAccessIterator1 __last1,
-                 _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2, _RandomAccessIterator3 __outit,
-                 _Compare __comp, _LeafMerge __leaf_merge)
+__parallel_merge(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _RandomAccessIterator1 __first1,
+                 _RandomAccessIterator1 __last1, _RandomAccessIterator2 __first2, _RandomAccessIterator2 __last2,
+                 _RandomAccessIterator3 __outit, _Compare __comp, _LeafMerge __leaf_merge)
 {
     __leaf_merge(__first1, __last1, __first2, __last2, __outit, __comp);
 }
 
 template <class _ExecutionPolicy, typename _F1, typename _F2>
 void
-__parallel_invoke(_ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
+__parallel_invoke(__pstl::__internal::__serial_backend, _ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
 {
     std::forward<_F1>(__f1)();
     std::forward<_F2>(__f2)();
 }
 
 } // namespace __serial_backend
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_PARALLEL_BACKEND_SERIAL_H */

pstl/include/pstl/internal/parallel_backend_tbb.h

class __parallel_for_body
   private:
     _RealBody _M_body;
 };
 
 //! Evaluation of brick f[i,j) for each subrange [i,j) of [first,last)
 // wrapper over tbb::parallel_for
 template <class _ExecutionPolicy, class _Index, class _Fp>
 void
-__parallel_for(_ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
+__parallel_for(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _Index __first, _Index __last, _Fp __f)
 {
     tbb::this_task_arena::isolate([=]() {
         tbb::parallel_for(tbb::blocked_range<_Index>(__first, __last), __parallel_for_body<_Index, _Fp>(__f));
     });
 }
 
 //! Evaluation of brick f[i,j) for each subrange [i,j) of [first,last)
 // wrapper over tbb::parallel_reduce
 template <class _ExecutionPolicy, class _Value, class _Index, typename _RealBody, typename _Reduction>
 _Value
-__parallel_reduce(_ExecutionPolicy&&, _Index __first, _Index __last, const _Value& __identity,
-                  const _RealBody& __real_body, const _Reduction& __reduction)
+__parallel_reduce(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _Index __first, _Index __last,
+                  const _Value& __identity, const _RealBody& __real_body, const _Reduction& __reduction)
 {
     return tbb::this_task_arena::isolate([__first, __last, &__identity, &__real_body, &__reduction]() -> _Value {
         return tbb::parallel_reduce(
             tbb::blocked_range<_Index>(__first, __last), __identity,
             [__real_body](const tbb::blocked_range<_Index>& __r, const _Value& __value) -> _Value {
                 return __real_body(__r.begin(), __r.end(), __value);
             },
             __reduction);
operator()(const tbb::blocked_range<_Index>& __range)
                 return;
         }
         sum() = _M_brick_reduce(__i, __j, sum());
     }
 };
 
 template <class _ExecutionPolicy, class _Index, class _Up, class _Tp, class _Cp, class _Rp>
 _Tp
-__parallel_transform_reduce(_ExecutionPolicy&&, _Index __first, _Index __last, _Up __u, _Tp __init, _Cp __combine,
-                            _Rp __brick_reduce)
+__parallel_transform_reduce(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _Index __first, _Index __last,
+                            _Up __u, _Tp __init, _Cp __combine, _Rp __brick_reduce)
 {
     __tbb_backend::__par_trans_red_body<_Index, _Up, _Tp, _Cp, _Rp> __body(__u, __init, __combine, __brick_reduce);
     // The grain size of 3 is used in order to provide mininum 2 elements for each body
     tbb::this_task_arena::isolate(
         [__first, __last, &__body]() { tbb::parallel_reduce(tbb::blocked_range<_Index>(__first, __last, 3), __body); });
     return __body.sum();
 }
 

 // reduce and scan are each called exactly once per subrange.
 // Thus callers can rely upon side effects in reduce.
 // combine must not throw an exception.
 // apex is called exactly once, after all calls to reduce and before all calls to scan.
 // For example, it's useful for allocating a __buffer used by scan but whose size is the sum of all reduction values.
 // T must have a trivial constructor and destructor.
 template <class _ExecutionPolicy, typename _Index, typename _Tp, typename _Rp, typename _Cp, typename _Sp, typename _Ap>
 void
-__parallel_strict_scan(_ExecutionPolicy&&, _Index __n, _Tp __initial, _Rp __reduce, _Cp __combine, _Sp __scan,
-                       _Ap __apex)
+__parallel_strict_scan(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _Index __n, _Tp __initial, _Rp __reduce,
+                       _Cp __combine, _Sp __scan, _Ap __apex)
 {
     tbb::this_task_arena::isolate([=, &__combine]() {
         if (__n > 1)
         {
             _Index __p = tbb::this_task_arena::max_concurrency();
             const _Index __slack = 4;
             _Index __tilesize = (__n - 1) / (__slack * __p) + 1;
             _Index __m = (__n - 1) / __tilesize;
tbb::this_task_arena::isolate([=, &__combine]() {
         __apex(__sum);
         if (__n)
             __scan(_Index(0), __n, __initial);
     });
 }
 
 template <class _ExecutionPolicy, class _Index, class _Up, class _Tp, class _Cp, class _Rp, class _Sp>
 _Tp
-__parallel_transform_scan(_ExecutionPolicy&&, _Index __n, _Up __u, _Tp __init, _Cp __combine, _Rp __brick_reduce,
-                          _Sp __scan)
+__parallel_transform_scan(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _Index __n, _Up __u, _Tp __init,
+                          _Cp __combine, _Rp __brick_reduce, _Sp __scan)
 {
     __trans_scan_body<_Index, _Up, _Tp, _Cp, _Rp, _Sp> __body(__u, __init, __combine, __brick_reduce, __scan);
     auto __range = tbb::blocked_range<_Index>(0, __n);
     tbb::this_task_arena::isolate([__range, &__body]() { tbb::parallel_scan(__range, __body); });
     return __body.sum();
 }
 
 //------------------------------------------------------------------------
__stable_sort_func<_RandomAccessIterator1, _RandomAccessIterator2, _Compare, _LeafSort>::operator()(__task* __self)
     _M_root = false;
     _M_xe = __xm;
 
     return __self;
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator, typename _Compare, typename _LeafSort>
 void
-__parallel_stable_sort(_ExecutionPolicy&&, _RandomAccessIterator __xs, _RandomAccessIterator __xe, _Compare __comp,
-                       _LeafSort __leaf_sort, std::size_t __nsort = 0)
+__parallel_stable_sort(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _RandomAccessIterator __xs,
+                       _RandomAccessIterator __xe, _Compare __comp, _LeafSort __leaf_sort, std::size_t __nsort = 0)
 {
     tbb::this_task_arena::isolate([=, &__nsort]() {
         //sorting based on task tree and parallel merge
         typedef typename std::iterator_traits<_RandomAccessIterator>::value_type _ValueType;
         typedef typename std::iterator_traits<_RandomAccessIterator>::difference_type _DifferenceType;
         const _DifferenceType __n = __xe - __xs;
         if (__nsort == __n)
             __nsort = 0; // 'partial_sort' becames 'sort'
operator()(__task* __self)
     _M_ye = __ym;
 
     return __self;
 }
 
 template <class _ExecutionPolicy, typename _RandomAccessIterator1, typename _RandomAccessIterator2,
           typename _RandomAccessIterator3, typename _Compare, typename _LeafMerge>
 void
-__parallel_merge(_ExecutionPolicy&&, _RandomAccessIterator1 __xs, _RandomAccessIterator1 __xe,
-                 _RandomAccessIterator2 __ys, _RandomAccessIterator2 __ye, _RandomAccessIterator3 __zs, _Compare __comp,
-                 _LeafMerge __leaf_merge)
+__parallel_merge(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _RandomAccessIterator1 __xs,
+                 _RandomAccessIterator1 __xe, _RandomAccessIterator2 __ys, _RandomAccessIterator2 __ye,
+                 _RandomAccessIterator3 __zs, _Compare __comp, _LeafMerge __leaf_merge)
 {
     typedef typename std::iterator_traits<_RandomAccessIterator1>::difference_type _DifferenceType1;
     typedef typename std::iterator_traits<_RandomAccessIterator2>::difference_type _DifferenceType2;
     typedef typename std::common_type<_DifferenceType1, _DifferenceType2>::type _SizeType;
     const _SizeType __n = (__xe - __xs) + (__ye - __ys);
     const _SizeType __merge_cut_off = _PSTL_MERGE_CUT_OFF;
     if (__n <= __merge_cut_off)
     {
__parallel_merge(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _RandomAccessIterator1 __xs,
     }
 }
 
 //------------------------------------------------------------------------
 // parallel_invoke
 //------------------------------------------------------------------------
 template <class _ExecutionPolicy, typename _F1, typename _F2>
 void
-__parallel_invoke(_ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
+__parallel_invoke(__pstl::__internal::__tbb_backend, _ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
 {
     //TODO: a version of tbb::this_task_arena::isolate with variadic arguments pack should be added in the future
     tbb::this_task_arena::isolate([&]() { tbb::parallel_invoke(std::forward<_F1>(__f1), std::forward<_F2>(__f2)); });
 }
 
 } // namespace __tbb_backend
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_PARALLEL_BACKEND_TBB_H */

pstl/include/pstl/internal/parallel_impl.h

 namespace __internal
 {
 
 //------------------------------------------------------------------------
 // parallel_find
 //-----------------------------------------------------------------------
 /** Return extremum value returned by brick f[i,j) for subranges [i,j) of [first,last)
 Each f[i,j) must return a value in [i,j). */
-template <class _ExecutionPolicy, class _Index, class _Brick, class _Compare>
+template <class _BackendTag, class _ExecutionPolicy, class _Index, class _Brick, class _Compare>
 _Index
-__parallel_find(_ExecutionPolicy&& __exec, _Index __first, _Index __last, _Brick __f, _Compare __comp, bool __b_first)
+__parallel_find(_BackendTag __tag, _ExecutionPolicy&& __exec, _Index __first, _Index __last, _Brick __f,
+                _Compare __comp, bool __b_first)
 {
     typedef typename std::iterator_traits<_Index>::difference_type _DifferenceType;
     const _DifferenceType __n = __last - __first;
     _DifferenceType __initial_dist = __b_first ? __n : -1;
     std::atomic<_DifferenceType> __extremum(__initial_dist);
     // TODO: find out what is better here: parallel_for or parallel_reduce
-    __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __first, __last,
+    __par_backend::__parallel_for(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
                                   [__comp, __f, __first, &__extremum](_Index __i, _Index __j) {
                                       // See "Reducing Contention Through Priority Updates", PPoPP '13, for discussion of
                                       // why using a shared variable scales fairly well in this situation.
                                       if (__comp(__i - __first, __extremum))
                                       {
                                           _Index __res = __f(__i, __j);
                                           // If not '__last' returned then we found what we want so put this to extremum
                                           if (__res != __j)
                                           {
                                               const _DifferenceType __k = __res - __first;
                                               for (_DifferenceType __old = __extremum; __comp(__k, __old);
                                                    __old = __extremum)
                                               {
                                                   __extremum.compare_exchange_weak(__old, __k);
                                               }
                                           }
                                       }
                                   });
     return __extremum != __initial_dist ? __first + __extremum : __last;
 }
 
 //------------------------------------------------------------------------
 // parallel_or
 //------------------------------------------------------------------------
 //! Return true if brick f[i,j) returns true for some subrange [i,j) of [first,last)
-template <class _ExecutionPolicy, class _Index, class _Brick>
+template <class _BackendTag, class _ExecutionPolicy, class _Index, class _Brick>
 bool
-__parallel_or(_ExecutionPolicy&& __exec, _Index __first, _Index __last, _Brick __f)
+__parallel_or(_BackendTag __tag, _ExecutionPolicy&& __exec, _Index __first, _Index __last, _Brick __f)
 {
     std::atomic<bool> __found(false);
-    __par_backend::__parallel_for(std::forward<_ExecutionPolicy>(__exec), __first, __last,
+    __par_backend::__parallel_for(__tag, std::forward<_ExecutionPolicy>(__exec), __first, __last,
                                   [__f, &__found](_Index __i, _Index __j) {
                                       if (!__found.load(std::memory_order_relaxed) && __f(__i, __j))
                                       {
                                           __found.store(true, std::memory_order_relaxed);
                                           __par_backend::__cancel_execution();
                                       }
                                   });
     return __found;
 }
 
 } // namespace __internal
 } // namespace __pstl
 
 _PSTL_HIDE_FROM_ABI_POP
 
 #endif /* _PSTL_PARALLEL_IMPL_H */

pstl/test/std/numerics/numeric.ops/scan.fail.cpp

 // -*- C++ -*-
 //===-- scan.fail.cpp -----------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14
 
 #include <execution>
 #include <numeric>
 
 struct CustomPolicy
 {
-    constexpr std::false_type
-    __allow_vector()
-    {
-        return std::false_type{};
-    }
-    constexpr std::false_type
-    __allow_parallel()
-    {
-        return std::false_type{};
-    }
 } policy;
 
 int32_t
 main()
 {
     int *first = nullptr, *last = nullptr, *result = nullptr;
 
     std::exclusive_scan(policy, first, last, result, 0); // expected-error {{no matching function for call to 'exclusive_scan'}}
     std::exclusive_scan(policy, first, last, result, 0, std::plus<int>()); // expected-error {{no matching function for call to 'exclusive_scan'}}
 
     return 0;
 }

pstl/test/support/utils.h

test_algo_basic_double(F&& f)
     invoke_on_all_policies(f, in.begin(), out.begin());
 }
 
 template <typename Policy, typename F>
 static void
 invoke_if(Policy&&, F f)
 {
 #if defined(_PSTL_ICC_16_VC14_TEST_SIMD_LAMBDA_DEBUG_32_BROKEN) || defined(_PSTL_ICC_17_VC141_TEST_SIMD_LAMBDA_DEBUG_32_BROKEN)
-    __pstl::__internal::invoke_if_not(__pstl::__internal::allow_unsequenced<Policy>(), f);
+    using decay_policy = typename std::decay<Policy>::type;
+    using allow_unsequenced =
+        std::integral_constant<bool, (std::is_same<decay_policy, std::execution::unsequenced_policy>::value ||
+                                      std::is_same<decay_policy, std::execution::parallel_unsequenced_policy>::value)>;
+    __pstl::__internal::__invoke_if_not(allow_unsequenced{}, f);
 #else
     f();
 #endif
 }
 
 } /* namespace TestUtils */