[libc++] mdspan - implement mdspan class

[crtrott]

Summary

This adds the mdspan class and its tests.

[crtrott]

Rebase on top of main

[crtrott]

This implements about 80% of the necessary tests, but its already a lot of code so we can collect review feedback while we do the rest.

[philnik]

libcxx/include/__mdspan/mdspan.h

62

_Uglification!

116

131–132

libc++ doesn't work with MSVC anyways, so this workaround can be removed.

257

These should be _LIBCPP_NO_UNIQUE_ADDRESS.

275

276

279

282

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestAccessors.h

2

Missing license header and include guard

[crtrott]

Address some review comments, add conversion test.

[crtrott]

Fix module file and GCC build.

[crtrott]

Add assertion tests

[ldionne]

libcxx/include/__mdspan/mdspan.h

59–71

This can yield false positives if a type is both a mapping and a layout. It can actually lead to hard compiler errors if instantiating the nested types failed in a non SFINAE friendly way. This is a bit pedantic, but I'd be a lot more comfortable if we only restricted this to catching types that we know are *definitely* mappings, i.e. the mappings we provide. We could do that by defining the __libcxx_is_mapping nested type inside our mappings, and then checking:

template <class _Tp>
concept __is_definitely_mapping = requires { typename _Tp::__libcxx_is_mapping; };

Then we assert !__is_definitely_mapping<_LayoutPolicy> instead of !__is_possibly_mapping<_LayoutPolicy>. If this isn't harsh enough of a diagnostic cause it misses user-defined policies, I'd like to see this as an incremental change later, but not as part of the initial design of this class.

BTW we have a precedent for this with __is_join_view_iterator.

Edit:

Instead, I think we could do this too:

template <class _Layout, class _Extents>
concept __has_invalid_mapping = requires !requires { typename _Layout::template mapping<_Extents>; };

Then from within mdspan, you could do:

static_assert(!__has_invalid_mapping<_LayoutPolicy>,
    "mdspan: did you pass a valid LayoutPolicy to mdspan? A common mistake is passing a mapping instead of a layout policy");

WDYT? This keeps the guard rails for user-defined layouts and mappings as well. And there are no possibilities for false positives AFAICT since _Layout::template mapping<_Extents> *has* to be valid.

[crtrott]

Rebase on main, and adding swap test.

[crtrott]

Add a few more tests checking for explicitness of constructors, and size of mdspan.

[crtrott]

Fir formating.

[crtrott]

Split too long static_assert test, added some more non-constructibility checks

[ldionne]

libcxx/include/__mdspan/mdspan.h

66

Do you test that the default layout policy is layout_right and the default accessor policy is default_accessor<_ElementType>? You can do that with

static_assert(std::is_same_v<mdspan<T, E>, mdspan<T, E, layout_right, default_accessor<T>>>);

Should probably be in types.pass.cpp if not already there.

131–132

Not sure what this comment refers to, but the nvhpc comment should be removed.

137

This raises an interesting question. The spec has preconditions (https://eel.is/c++draft/views.multidim#mdspan.mdspan.cons-2), but we can't check them if we make this = default. The = default is not required by the standard but it's arguably a nice property to have. This allows making mdspan trivially default constructible if the data_handle_type, mapping_type and accessor_type are trivially default constructible, right?

Actually, considering this a bit more, per https://eel.is/c++draft/views.multidim#mdspan.mdspan.cons-3 the effects are supposed to be: Value-initializes ptr_, map_, and acc_.. This is not the case if we make this = default and we have a data_handle_type, mapping_type or accessor_type that's an aggregate (not all of those might be possible but you get the point). So I think this is non-conforming. For example, if you have an aggregate accessor_type with a dummy int member, that int will never be value-initialized (zeroed-out) with your current implementation, but it should be according to the Standard. This needs a test.

138–139

Do you have tests that those are trivial operations if the members are trivially copy/move constructible?

158

There's a few instances of that.

159

These requirements are wrong AFAICT. They should be using const& like the ones for array as described in https://eel.is/c++draft/views.multidim#mdspan.mdspan.cons-8.1.

I think what you want is is_convertible_v<const _OtherIndexType&, index_type> && is_nothrow_constructible_v<index_type, const _OtherIndexType&>.

Those need tests. If you have an index type where index_type(_OtherIndexType&&) works but index_type(_OtherIndexType const&) doesn't, then the requires before my comment will be satisfied but you will get a hard compiler error while trying to compile __map_(extents_type(__exts)) below (I think). Please also add the same test for the std::array constructor above: if I go and remove const& from the equivalent requires above, a test should fail.

164

If you forget to std::move here (or in any of the similar constructors), which test fails? If there's none, please make sure to add coverage.

167

This needs a test, which should be possible if you have an extents_type where mapping_type(extents_type&&) works but mapping_type(extents_type const&) doesn't. Then the requires before my comment would enable this mdspan constructor, but you'd get a hard compiler error while trying to compile __map_(__exts) below.

178–179

Needs tests.

185–186

191

193–194

205–206

Same question about testing for triviality.

209

These "stable" names seem to originate from a previous version of the standard. https://eel.is/c++draft/views.multidim#mdspan doesn't seem to have the same names.

214

Just for consistency with the spec.

216

We have an open patch to categorize those.

222

Needs a test. The span version has the same problem.

224

Is there any reason why we're doing this here instead of following the standard wording more closely? Is it because you want to avoid calling the variadic version in case you have a large number of dimensions?

I would inline all the uses of __idx_fold into their calling functions and use a lambda instead, that's easier to read. Otherwise the implementation of the function is basically 100+ lines above the function itself.

230

Note to self: I have not actually reviewed the implementation of these idx_fold operations yet.

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_array.pass.cpp

49–52

I don't think that works. The usual pattern we use here is

template <class T>
void accept(T);

template <class MDS, class Exts>
concept check_mdspan_ctor_implicit = requires(typename MDS::data_handle_type h, const Exts& exts) {
  accept<MDS>({h, exts});
};

You'll want to make all of your explicit tests consistent.

libcxx/test/std/containers/views/mdspan/mdspan/properties.pass.cpp

99

Those static functions should be assert(...)ed instead. Then we call those from a constexpr and a non-constexpr context.

Right now the following implementation passes your tests:

static constexpr rank_type rank() noexcept {
  if (std::is_constant_evaluated()) return extents_type::rank();
  else return 0;
}

I think all the "properties" are affected.

[crtrott]

Addressed all the comments, uploading patch now.

libcxx/include/__mdspan/mdspan.h

66

Added.

137

Doesn't the {} make it so its value initialized?

_LIBCPP_NO_UNIQUE_ADDRESS data_handle_type __ptr_{};
_LIBCPP_NO_UNIQUE_ADDRESS mapping_type __map_{};
_LIBCPP_NO_UNIQUE_ADDRESS accessor_type __acc_{};

I believe this makes it so that the three members are value initialized for the defaulted default constructor doesn't it?

Which means the only thing we are not doing is the uncheckable precondition.

138–139

Adding it.

159

index_type is integral so I don't think we can write a test for that using the move thing? I.e. the only thing you a move into int is an int. However, we could test that we have an OtherIndexType where the conversion operator to integral is non-const and/or missing no-except. Turns out that this was already wrong in extents itself, which I need to fix in order to test mdspan, because otherwise it can go through the constructor taking extents ...

164

Added coverage via move_counted_handle i.e. a handle type which increments a static variable during move.

167

Fixed and added test by modifying the layout_wrapped_integral depending on value of the _Wrap it now has a constructor from extents_type.&& Note that construction from extents and construction from array/span/integers has different requirements for the mapping. We are now testing both.

178–179

Added a whole bunch of testing, and made sure (i.e. tried) that any combination of missing const& both for the requires and the explicit will result in failure.

224

Yeah, we can't avoid having an expansion happening in this operator via idx_fold (or now changed via a lambda) but we don't have to instantiate another variadic function here. We have ongoing issues with inlining depth in our big applications, and so we make judgement calls regarding delegation.

This is the lambda version body of the function (a bit more than the fold expression but ok:

return __acc_.access(__ptr_, [&]<size_t ... _Idxs>(index_sequence<_Idxs...>) {
  return __map_(__indices[_Idxs]...);
}(make_index_sequence<rank()>()));

This is the forwarding version:

return [&, self = this]<size_t ... _Idxs>(index_sequence<_Idxs...>) -> reference {
   return (*self)[__indices[_Idxs]...];
}(make_index_sequence<rank()>());

I personally don't see that as better, we need to explicitly capture this, and need to define the return type (otherwise it returns a value not a reference.

230

Removed it.

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_array.pass.cpp

49–52

I don't think that works, because it's multi argument explicitlness. I.e. this doesn't even compile:

class Bar {
  constexpr Bar(int, int) {}
};

template<class T>
constexpr bool accept(T) { return true; }

static_assert(accept({1,1}));

My understanding is that for multi argument constructors the ONLY difference in whether you have an explicit or not is whether

T a = {....};

works.

It doesn't make any difference for function calls.

libcxx/test/std/containers/views/mdspan/mdspan/properties.pass.cpp

99

Yeah done.

[crtrott]

This addresses Louis review comments. Most notably fixing a lot of constraints regarding const& vs non-const arguments and adding testing for that.

[crtrott]

Fix CI

[crtrott]

Fix another warning from GCC

[crtrott]

Another CI fix and add missing static assert verification.

[ldionne]

libcxx/include/CMakeLists.txt

479

Let's add a release note and let's update the paper list.

libcxx/include/__mdspan/mdspan.h

137

You're right, sorry I missed that.

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestLayouts.h

39

50

Also no uglification in test code, since that makes the test code (pedantically) ill-formed for using reserved names.

52

Would it make sense to use layout_right::mapping under the hood to implement this? It would reduce the complexity of this test class quite a bit, no?

56–57

This shouldn't be using any libc++ internal utilities since this is test code.

66

We don't use these ABI macros in test code.

libcxx/test/std/containers/views/mdspan/mdspan/assert.conversion.pass.cpp

11

For the assertion tests, we now use // UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode, and you can drop ADDITIONAL_COMPILE_FLAGS.

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.cpp

110

This is a nitpick, but we usually just use test();, not (void)test();. Consider doing for consistency with the numerous other tests where we have that pattern.

libcxx/test/std/containers/views/mdspan/mdspan/conversion.pass.cpp

195

This is crazy. If there was a test thoroughness award, you'd get it. Thanks!

235

Is there a reason why you're not checking the conv_test_accessor_c versions at compile-time?

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_integers.pass.cpp

52

Whenever I see this kind of logic in tests, I feel like it would be too easy to make a mistake in the logic of the test and render the test useless. Instead, what would you think of passing whether you expect H, M and A to be constructible as bool template arguments to this function? Then this would be hardcoded at the point of call instead of figured out "automatically" by the test. This applies in a bunch of places.

libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp

43

Let's use TEST_CLANG_VER, it'll be easier to find in the future and remove the workaround. Also below (and maybe elsewhere, please check).

libcxx/test/std/containers/views/mdspan/mdspan/types.pass.cpp

65

Nice!

[crtrott]

Added release notes, rebased. Addressed Louis comments.

[crtrott]

Fix formatting, add more triviality testing

[crtrott]

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestLayouts.h

52

I looked at it, because we wanted a layout which is runtime conditionally non-unique and not-strides, almost all the functions are different. So nothing to be saved really.

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_integers.pass.cpp

52

OK done.

52

I did it for the ones where it seemed straight forward, and did not do it for conversion, where its kinda combinatorial what happens. That thing has more local checks to assure that we test what we believe we test.

[crtrott]

Fix warning on GCC

[crtrott]

Fix apple clang build ... TEST_CLANG_VER is not defined on apple clang ...

[crtrott]

libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp

43

TEST_CLANG_VER is not defined on apple clang so we would need to check for TEST_CLANG_VER and TEST_APPLECLANG_VER, clang-major works universally here.

[crtrott]

Fix 32bit builds and fix size assertion.

[crtrott]

Trigger all mdspan assertions in hardened mode: all of them are related in one form or another to bounds checking. Either directly, or in order to not screw up the bounds during construction/assignment/conversion etc. which would violate the invariants of mdspan, and thus could lead to bounds checks not triggering when they should.

[ldionne]

LGTM with passing CI. Thank you for all the effort you've put into this feature!

libcxx/test/std/containers/views/mdspan/extents/ctor_from_array.pass.cpp

86

There's a few instances of this typo.

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestLayouts.h

32

Please explain how the actual layout differs from layout_right and the other ones.

190

exposition-only is relevant in the standard, not in our tests. I would remove this.

322

Same here for exposition-only.

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.cpp

13

This synopsis is wrong, please go through them to make sure.

35–40

I think you can get rid of this by using a = default copy assignment operator. In all cases I don't understand why we're basically not testing anything in that case.

[ldionne]

libcxx/include/__mdspan/extents.h

178

I would keep this _LIBCPP_ASSERT_UNCATEGORIZED and then @var-const can categorize them appropriately.

[var-const]

libcxx/include/__mdspan/extents.h

210

Can you please double-check if this assertion is tested?

libcxx/test/std/containers/views/mdspan/layout_right/assert.conversion.pass.cpp

1 ↗

(On Diff #542988)

Nit: I think tests that check assertions usually are under libcxx/test/libcxx, not libcxx/test/std since they test libc++-specific behavior. This can be done in a follow-up.

[crtrott]

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.cpp

13

I think this is right?

35–40

part of the point of checked accessor is to NOT be trivially copy assignable, so that's why it has a user provided one.

[crtrott]

Address last comments: make all assertions UNCATEGORIZED

[crtrott]

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.cpp

13

sorry accidentally published that above comment: it was wrong (copy ctor instead of copy assign) and I fixed it

[crtrott]

libcxx/include/__mdspan/extents.h

210

Checked in libcxx/test/std/containers/views/mdspan/extents/assert.obs.pass.cpp

[crtrott]

Attempt to fix mingw failure.

[crtrott]

I investigated the failure on MinGW and tracked it down to a reproducer on godbolt, which indicates this is a bug in the compiler when using --target=x86_64-w64-windows-gnu
https://godbolt.org/z/Y6Mb7sMMc

Shortish version, you have something like this:

template<class T, class M, class A>
class mdspan {
    public:
  mdspan() = default;
  mdspan(const mdspan&) = default;
  mdspan(T* ptr_, M map_, A acc_):ptr(ptr_),map(map_),acc(acc_) {}
  template<class OT, class OM, class OA>
  mdspan(const mdspan<OT, OM, OA>& other):ptr(other.ptr), map(other.map), acc(other.acc) {}

  [[no_unique_address]] T* ptr;
  [[no_unique_address]] M map;
  [[no_unique_address]] A acc;
};

If you now do mdspan<const T, M2, A2>(mdspan<T, M1, A1>); where M1, A1 and A2 are empty, but M2 is not, and A1->A2 happens via a conversion operator in A1, not a converting constructor in A2 the last byte of ptr gets zeroed out.

The only standard conformant way to fix this is to not have [[no_unique_address]].
However this is a rather esoteric corner case, where you would need to have two accessors where A2 is constructible from A1, but doesn't have a constructor accepting const A1&.

So couple options:
a) I just don't test that combo on MinGW, and users could run into this behavior which we will report as a bug, if they do something weird.
b) we remove [[no_unique_address]] for now on mdspan for a MinGW build, and break ABI later if and when this bug gets fixed
c) I don't land mdspan today, and we hope the bug is fixed in MinGW with clang 17.

I personally would opt for a) since the chances users will run into this are virtually zero, and one can work around this bug.

[crtrott]

Work around MinGW issue by disabling test, not removing no-unique-address.

[crtrott]

Filed issue here: https://github.com/llvm/llvm-project/issues/64077

[Mordante]

libcxx/test/std/containers/views/mdspan/mdspan/conversion.pass.cpp

241

Please add a link to the bug you filed.

[crtrott]

Uploaded the wrong patch file last time around, where I guarded the wrong two tests on MinGW :-(

[Mordante]

This has landed as 488c3db245ce0fe3b70357d6798ee91c5baa82a2 in main and 9957225e7d6761d7e550561eac1df78e5e2c184b in LLVM-17 release branch.

libcxx/docs/ReleaseNotes/17.rst

 - P2711R1 - Making Multi-Param Constructors Of views explicit (``join_with_view`` is not done yet)
 - P2572R1 - ``std::format`` fill character allowances
 - P2510R3 - Formatting pointers
 - P2136R3 - ``invoke_r``
 - P2494R2 - Relaxing range adaptors to allow for move only types
 - P2585R0 - Improving default container formatting
 - P0408R7 - Efficient Access to ``basic_stringbuf``'s Buffer
 - P2474R2 - ``views::repeat``
+- P0009R18 - ``mdspan`` (``layout_stride`` is not done yet)
 
 Improvements and New Features
 -----------------------------
 - ``std::equal``, ``std::ranges::equal``, ``std::find``, and
   ``std::ranges::find``  are now forwarding to ``std::memcmp`` for trivially
   equality comparable types, which can lead up to 40x performance improvements.
 
 - ``std::string_view`` now provides iterators that check for out-of-bounds accesses when the safe

libcxx/include/CMakeLists.txt

set(files
   __iterator/reverse_access.h
   __iterator/reverse_iterator.h
   __iterator/segmented_iterator.h
   __iterator/size.h
   __iterator/sortable.h
   __iterator/unreachable_sentinel.h
   __iterator/wrap_iter.h
   __locale
   __locale_dir/locale_base_api/bsd_locale_defaults.h
   __locale_dir/locale_base_api/bsd_locale_fallbacks.h
   __locale_dir/locale_base_api/locale_guard.h
   __mbstate_t.h
   __mdspan/default_accessor.h
   __mdspan/extents.h
   __mdspan/layout_left.h
   __mdspan/layout_right.h
+  __mdspan/mdspan.h
   __memory/addressof.h
   __memory/align.h
   __memory/aligned_alloc.h
   __memory/allocate_at_least.h
   __memory/allocation_guard.h
   __memory/allocator.h
   __memory/allocator_arg_t.h
   __memory/allocator_destructor.h

libcxx/include/__mdspan/extents.h

_LIBCPP_HIDE_FROM_ABI constexpr __maybe_static_array(_DynVals... __vals) {
     for (size_t __i = 0; __i < __size_; __i++) {
       _TStatic __static_val = _StaticValues::__get(__i);
       if (__static_val == _DynTag) {
         __dyn_vals_[_DynamicIdxMap::__get(__i)] = __values[__i];
       }
       // Precondition check
       else
         _LIBCPP_ASSERT_UNCATEGORIZED(__values[__i] == static_cast<_TDynamic>(__static_val),
                                      "extents construction: mismatch of provided arguments with static extents.");
     }
   }
 
   template <class _Tp, size_t _Size>
     requires(_Size != __size_dynamic_)
   _LIBCPP_HIDE_FROM_ABI constexpr __maybe_static_array(const span<_Tp, _Size>& __vals) {
     static_assert((_Size == __size_) || (__size_ == dynamic_extent));
     for (size_t __i = 0; __i < __size_; __i++) {
public:
   }
 
   _LIBCPP_HIDE_FROM_ABI constexpr _TDynamic __value(size_t __i) const {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__i < __size_, "extents access: index must be less than rank");
     _TStatic __static_val = _StaticValues::__get(__i);
     return __static_val == _DynTag ? __dyn_vals_[_DynamicIdxMap::__get(__i)] : static_cast<_TDynamic>(__static_val);
   }
   _LIBCPP_HIDE_FROM_ABI constexpr _TDynamic operator[](size_t __i) const {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__i < __size_, "extents access: index must be less than rank");
     return __value(__i);
   }
 
   // observers
   _LIBCPP_HIDE_FROM_ABI static constexpr size_t __size() { return __size_; }
   _LIBCPP_HIDE_FROM_ABI static constexpr size_t __size_dynamic() { return __size_dynamic_; }
 };
 
requires((is_convertible_v<_OtherIndexTypes, index_type> && ...) &&
              (sizeof...(_OtherIndexTypes) == __rank_ || sizeof...(_OtherIndexTypes) == __rank_dynamic_))
   _LIBCPP_HIDE_FROM_ABI constexpr explicit extents(_OtherIndexTypes... __dynvals) noexcept
       : __vals_(static_cast<index_type>(__dynvals)...) {
     _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__are_representable_as<index_type>(__dynvals...),
                                  "extents ctor: arguments must be representable as index_type and nonnegative");
   }
 
   template <class _OtherIndexType, size_t _Size>
-    requires(is_convertible_v<_OtherIndexType, index_type> && is_nothrow_constructible_v<index_type, _OtherIndexType> &&
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&> &&
              (_Size == __rank_ || _Size == __rank_dynamic_))
   explicit(_Size != __rank_dynamic_)
       _LIBCPP_HIDE_FROM_ABI constexpr extents(const array<_OtherIndexType, _Size>& __exts) noexcept
       : __vals_(span(__exts)) {
     _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__are_representable_as<index_type>(span(__exts)),
                                  "extents ctor: arguments must be representable as index_type and nonnegative");
   }
 
   template <class _OtherIndexType, size_t _Size>
-    requires(is_convertible_v<_OtherIndexType, index_type> && is_nothrow_constructible_v<index_type, _OtherIndexType> &&
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&> &&
              (_Size == __rank_ || _Size == __rank_dynamic_))
   explicit(_Size != __rank_dynamic_)
       _LIBCPP_HIDE_FROM_ABI constexpr extents(const span<_OtherIndexType, _Size>& __exts) noexcept
       : __vals_(__exts) {
     _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__are_representable_as<index_type>(__exts),
                                  "extents ctor: arguments must be representable as index_type and nonnegative");
   }
 

libcxx/include/__mdspan/layout_left.h

private:
   static_assert((extents_type::rank_dynamic() > 0) || __required_span_size_is_representable(extents_type()),
                 "layout_left::mapping product of static extents must be representable as index_type.");
 
 public:
   // [mdspan.layout.left.cons], constructors
   _LIBCPP_HIDE_FROM_ABI constexpr mapping() noexcept               = default;
   _LIBCPP_HIDE_FROM_ABI constexpr mapping(const mapping&) noexcept = default;
   _LIBCPP_HIDE_FROM_ABI constexpr mapping(const extents_type& __ext) noexcept : __extents_(__ext) {
-    _LIBCPP_ASSERT(__required_span_size_is_representable(__ext),
-                   "layout_left::mapping extents ctor: product of extents must be representable as index_type.");
+    _LIBCPP_ASSERT_UNCATEGORIZED(
+        __required_span_size_is_representable(__ext),
   }
 
   template <class _OtherExtents>
     requires(is_constructible_v<extents_type, _OtherExtents>)
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
       mapping(const mapping<_OtherExtents>& __other) noexcept
       : __extents_(__other.extents()) {
-    _LIBCPP_ASSERT(
+    _LIBCPP_ASSERT_UNCATEGORIZED(
         __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
         "layout_left::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
 
   template <class _OtherExtents>
     requires(is_constructible_v<extents_type, _OtherExtents> && _OtherExtents::rank() <= 1)
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
       mapping(const layout_right::mapping<_OtherExtents>& __other) noexcept
       : __extents_(__other.extents()) {
-    _LIBCPP_ASSERT(
+    _LIBCPP_ASSERT_UNCATEGORIZED(
         __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
         "layout_left::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
 
 // FIXME: add when we add other layouts
 #  if 0
     template<class _OtherExtents>
       constexpr explicit(extents_type::rank() > 0)
for (size_t __r = 0; __r < extents_type::rank(); __r++)
       __size *= __extents_.extent(__r);
     return __size;
   }
 
   template <class... _Indices>
     requires((sizeof...(_Indices) == extents_type::rank()) && (is_convertible_v<_Indices, index_type> && ...) &&
              (is_nothrow_constructible_v<index_type, _Indices> && ...))
   _LIBCPP_HIDE_FROM_ABI constexpr index_type operator()(_Indices... __idx) const noexcept {
-    _LIBCPP_ASSERT(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
+    _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
                                  "layout_left::mapping: out of bounds indexing");
     array<index_type, extents_type::rank()> __idx_a{static_cast<index_type>(__idx)...};
     return [&]<size_t... _Pos>(index_sequence<_Pos...>) {
       index_type __res = 0;
       ((__res = __idx_a[extents_type::rank() - 1 - _Pos] + __extents_.extent(extents_type::rank() - 1 - _Pos) * __res),
        ...);
       return __res;
     }(make_index_sequence<sizeof...(_Indices)>());
   }
 
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept { return true; }
 
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_unique() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_exhaustive() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_strided() noexcept { return true; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const noexcept
     requires(extents_type::rank() > 0)
   {
-    _LIBCPP_ASSERT(__r < extents_type::rank(), "layout_left::mapping::stride(): invalid rank index");
+    _LIBCPP_ASSERT_UNCATEGORIZED(__r < extents_type::rank(), "layout_left::mapping::stride(): invalid rank index");
     index_type __s = 1;
     for (rank_type __i = extents_type::rank() - 1; __i > __r; __i--)
       __s *= __extents_.extent(__i);
     return __s;
   }
 
   template <class _OtherExtents>
     requires(_OtherExtents::rank() == extents_type::rank())
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool
   operator==(const mapping& __lhs, const mapping<_OtherExtents>& __rhs) noexcept {
     return __lhs.extents() == __rhs.extents();
   }
 
 private:
-  extents_type __extents_{}; // exposition only
+  _LIBCPP_NO_UNIQUE_ADDRESS extents_type __extents_{};
 };
 
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___MDSPAN_LAYOUT_LEFT_H

libcxx/include/__mdspan/layout_right.h

private:
   static_assert((extents_type::rank_dynamic() > 0) || __required_span_size_is_representable(extents_type()),
                 "layout_right::mapping product of static extents must be representable as index_type.");
 
 public:
   // [mdspan.layout.right.cons], constructors
   _LIBCPP_HIDE_FROM_ABI constexpr mapping() noexcept               = default;
   _LIBCPP_HIDE_FROM_ABI constexpr mapping(const mapping&) noexcept = default;
   _LIBCPP_HIDE_FROM_ABI constexpr mapping(const extents_type& __ext) noexcept : __extents_(__ext) {
-    _LIBCPP_ASSERT(__required_span_size_is_representable(__ext),
-                   "layout_right::mapping extents ctor: product of extents must be representable as index_type.");
+    _LIBCPP_ASSERT_UNCATEGORIZED(
+        __required_span_size_is_representable(__ext),
   }
 
   template <class _OtherExtents>
     requires(is_constructible_v<extents_type, _OtherExtents>)
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
       mapping(const mapping<_OtherExtents>& __other) noexcept
       : __extents_(__other.extents()) {
-    _LIBCPP_ASSERT(
+    _LIBCPP_ASSERT_UNCATEGORIZED(
         __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
         "layout_right::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
 
   template <class _OtherExtents>
     requires(is_constructible_v<extents_type, _OtherExtents> && _OtherExtents::rank() <= 1)
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
       mapping(const layout_left::mapping<_OtherExtents>& __other) noexcept
       : __extents_(__other.extents()) {
-    _LIBCPP_ASSERT(
+    _LIBCPP_ASSERT_UNCATEGORIZED(
         __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
         "layout_right::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
 
 // FIXME: add when we add other layouts
 #  if 0
     template<class _OtherExtents>
       constexpr explicit(extents_type::rank() > 0)
for (size_t __r = 0; __r < extents_type::rank(); __r++)
       __size *= __extents_.extent(__r);
     return __size;
   }
 
   template <class... _Indices>
     requires((sizeof...(_Indices) == extents_type::rank()) && (is_convertible_v<_Indices, index_type> && ...) &&
              (is_nothrow_constructible_v<index_type, _Indices> && ...))
   _LIBCPP_HIDE_FROM_ABI constexpr index_type operator()(_Indices... __idx) const noexcept {
-    _LIBCPP_ASSERT(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
+    _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
                                  "layout_right::mapping: out of bounds indexing");
     return [&]<size_t... _Pos>(index_sequence<_Pos...>) {
       index_type __res = 0;
       ((__res = static_cast<index_type>(__idx) + __extents_.extent(_Pos) * __res), ...);
       return __res;
     }(make_index_sequence<sizeof...(_Indices)>());
   }
 
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept { return true; }
 
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_unique() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_exhaustive() noexcept { return true; }
   _LIBCPP_HIDE_FROM_ABI static constexpr bool is_strided() noexcept { return true; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const noexcept
     requires(extents_type::rank() > 0)
   {
-    _LIBCPP_ASSERT(__r < extents_type::rank(), "layout_right::mapping::stride(): invalid rank index");
+    _LIBCPP_ASSERT_UNCATEGORIZED(__r < extents_type::rank(), "layout_right::mapping::stride(): invalid rank index");
     index_type __s = 1;
     for (rank_type __i = extents_type::rank() - 1; __i > __r; __i--)
       __s *= __extents_.extent(__i);
     return __s;
   }
 
   template <class _OtherExtents>
     requires(_OtherExtents::rank() == extents_type::rank())
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool
   operator==(const mapping& __lhs, const mapping<_OtherExtents>& __rhs) noexcept {
     return __lhs.extents() == __rhs.extents();
   }
 
 private:
-  extents_type __extents_{}; // exposition only
+  _LIBCPP_NO_UNIQUE_ADDRESS extents_type __extents_{};
 };
 
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___MDSPAN_LAYOUT_RIGHT_H

libcxx/include/__mdspan/mdspan.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MDSPAN_MDSPAN_H
+#define _LIBCPP___MDSPAN_MDSPAN_H
+
+#include <__assert>
+#include <__config>
+#include <__fwd/mdspan.h>
+#include <__mdspan/default_accessor.h>
+#include <__mdspan/extents.h>
+#include <__type_traits/extent.h>
+#include <__type_traits/is_abstract.h>
+#include <__type_traits/is_array.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/is_default_constructible.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_pointer.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/rank.h>
+#include <__type_traits/remove_all_extents.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/remove_pointer.h>
+#include <__type_traits/remove_reference.h>
+#include <__utility/integer_sequence.h>
+#include <array>
+#include <cinttypes>
+#include <cstddef>
+#include <limits>
+#include <span>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+// Helper for lightweight test checking that one did pass a layout policy as LayoutPolicy template argument
+namespace __mdspan_detail {
+template <class _Layout, class _Extents>
+concept __has_invalid_mapping = !requires { typename _Layout::template mapping<_Extents>; };
+} // namespace __mdspan_detail
+
+template <class _ElementType,
+          class _Extents,
+          class _LayoutPolicy   = layout_right,
+          class _AccessorPolicy = default_accessor<_ElementType> >
+class mdspan {
+private:
+  static_assert(__mdspan_detail::__is_extents_v<_Extents>,
+                "mdspan: Extents template parameter must be a specialization of extents.");
+  static_assert(!is_array_v<_ElementType>, "mdspan: ElementType template parameter may not be an array type");
+  static_assert(!is_abstract_v<_ElementType>, "mdspan: ElementType template parameter may not be an abstract class");
+  static_assert(is_same_v<_ElementType, typename _AccessorPolicy::element_type>,
+                "mdspan: ElementType template parameter must match AccessorPolicy::element_type");
+  static_assert(!__mdspan_detail::__has_invalid_mapping<_LayoutPolicy, _Extents>,
+                "mdspan: LayoutPolicy template parameter is invalid. A common mistake is to pass a layout mapping "
+                "instead of a layout policy");
+
+public:
+  using extents_type     = _Extents;
+  using layout_type      = _LayoutPolicy;
+  using accessor_type    = _AccessorPolicy;
+  using mapping_type     = typename layout_type::template mapping<extents_type>;
+  using element_type     = _ElementType;
+  using value_type       = remove_cv_t<element_type>;
+  using index_type       = typename extents_type::index_type;
+  using size_type        = typename extents_type::size_type;
+  using rank_type        = typename extents_type::rank_type;
+  using data_handle_type = typename accessor_type::data_handle_type;
+  using reference        = typename accessor_type::reference;
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank() noexcept { return extents_type::rank(); }
+  _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
+  _LIBCPP_HIDE_FROM_ABI static constexpr size_t static_extent(rank_type __r) noexcept {
+    return extents_type::static_extent(__r);
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr index_type extent(rank_type __r) const noexcept {
+    return __map_.extents().extent(__r);
+  };
+
+public:
+  //--------------------------------------------------------------------------------
+  // [mdspan.mdspan.cons], mdspan constructors, assignment, and destructor
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan()
+    requires((extents_type::rank_dynamic() > 0) && is_default_constructible_v<data_handle_type> &&
+             is_default_constructible_v<mapping_type> && is_default_constructible_v<accessor_type>)
+  = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan(const mdspan&) = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan(mdspan&&)      = default;
+
+  template <class... _OtherIndexTypes>
+    requires((is_convertible_v<_OtherIndexTypes, index_type> && ...) &&
+             (is_nothrow_constructible_v<index_type, _OtherIndexTypes> && ...) &&
+             ((sizeof...(_OtherIndexTypes) == rank()) || (sizeof...(_OtherIndexTypes) == rank_dynamic())) &&
+             is_constructible_v<mapping_type, extents_type> && is_default_constructible_v<accessor_type>)
+  _LIBCPP_HIDE_FROM_ABI explicit constexpr mdspan(data_handle_type __p, _OtherIndexTypes... __exts)
+      : __ptr_(std::move(__p)), __map_(extents_type(static_cast<index_type>(std::move(__exts))...)), __acc_{} {}
+
+  template <class _OtherIndexType, size_t _Size>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&> &&
+             ((_Size == rank()) || (_Size == rank_dynamic())) && is_constructible_v<mapping_type, extents_type> &&
+             is_default_constructible_v<accessor_type>)
+  explicit(_Size != rank_dynamic())
+      _LIBCPP_HIDE_FROM_ABI constexpr mdspan(data_handle_type __p, const array<_OtherIndexType, _Size>& __exts)
+      : __ptr_(std::move(__p)), __map_(extents_type(__exts)), __acc_{} {}
+
+  template <class _OtherIndexType, size_t _Size>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&> &&
+             ((_Size == rank()) || (_Size == rank_dynamic())) && is_constructible_v<mapping_type, extents_type> &&
+             is_default_constructible_v<accessor_type>)
+  explicit(_Size != rank_dynamic())
+      _LIBCPP_HIDE_FROM_ABI constexpr mdspan(data_handle_type __p, span<_OtherIndexType, _Size> __exts)
+      : __ptr_(std::move(__p)), __map_(extents_type(__exts)), __acc_{} {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan(data_handle_type __p, const extents_type& __exts)
+    requires(is_default_constructible_v<accessor_type> && is_constructible_v<mapping_type, const extents_type&>)
+      : __ptr_(std::move(__p)), __map_(__exts), __acc_{} {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan(data_handle_type __p, const mapping_type& __m)
+    requires(is_default_constructible_v<accessor_type>)
+      : __ptr_(std::move(__p)), __map_(__m), __acc_{} {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan(data_handle_type __p, const mapping_type& __m, const accessor_type& __a)
+      : __ptr_(std::move(__p)), __map_(__m), __acc_(__a) {}
+
+  template <class _OtherElementType, class _OtherExtents, class _OtherLayoutPolicy, class _OtherAccessor>
+    requires(is_constructible_v<mapping_type, const typename _OtherLayoutPolicy::template mapping<_OtherExtents>&> &&
+             is_constructible_v<accessor_type, const _OtherAccessor&>)
+  explicit(!is_convertible_v<const typename _OtherLayoutPolicy::template mapping<_OtherExtents>&, mapping_type> ||
+           !is_convertible_v<const _OtherAccessor&, accessor_type>)
+      _LIBCPP_HIDE_FROM_ABI constexpr mdspan(
+          const mdspan<_OtherElementType, _OtherExtents, _OtherLayoutPolicy, _OtherAccessor>& __other)
+      : __ptr_(__other.__ptr_), __map_(__other.__map_), __acc_(__other.__acc_) {
+    static_assert(is_constructible_v<data_handle_type, const typename _OtherAccessor::data_handle_type&>,
+                  "mdspan: incompatible data_handle_type for mdspan construction");
+    static_assert(
+        is_constructible_v<extents_type, _OtherExtents>, "mdspan: incompatible extents for mdspan construction");
+
+    // The following precondition is part of the standard, but is unlikely to be triggered.
+    // The extents constructor checks this and the mapping must be storing the extents, since
+    // its extents() function returns a const reference to extents_type.
+    // The only way this can be triggered is if the mapping conversion constructor would for example
+    // always construct its extents() only from the dynamic extents, instead of from the other extents.
+    if constexpr (rank() > 0) {
+      for (size_t __r = 0; __r < rank(); __r++) {
+        _LIBCPP_ASSERT_UNCATEGORIZED(
+            (static_extent(__r) == dynamic_extent) ||
+                (static_cast<index_type>(__other.extent(__r)) == static_cast<index_type>(static_extent(__r))),
+            "mdspan: conversion mismatch of source dynamic extents with static extents");
+      }
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan& operator=(const mdspan&) = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr mdspan& operator=(mdspan&&)      = default;
+
+  //--------------------------------------------------------------------------------
+  // [mdspan.mdspan.members], members
+
+  template <class... _OtherIndexTypes>
+    requires((is_convertible_v<_OtherIndexTypes, index_type> && ...) &&
+             (is_nothrow_constructible_v<index_type, _OtherIndexTypes> && ...) &&
+             (sizeof...(_OtherIndexTypes) == rank()))
+  _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](_OtherIndexTypes... __indices) const {
+    _LIBCPP_ASSERT_UNCATEGORIZED(__mdspan_detail::__is_multidimensional_index_in(extents(), __indices...),
+                                 "mdspan: operator[] out of bounds access");
+    return __acc_.access(__ptr_, __map_(static_cast<index_type>(std::move(__indices))...));
+  }
+
+  template <class _OtherIndexType>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](const array< _OtherIndexType, rank()>& __indices) const {
+    return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
+      return __map_(__indices[_Idxs]...);
+    }(make_index_sequence<rank()>()));
+  }
+
+  template <class _OtherIndexType>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](span<_OtherIndexType, rank()> __indices) const {
+    return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
+      return __map_(__indices[_Idxs]...);
+    }(make_index_sequence<rank()>()));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr size_type size() const noexcept {
+    _LIBCPP_ASSERT_UNCATEGORIZED(
+        false == ([&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
+          size_type __prod = 1;
+          return (__builtin_mul_overflow(__prod, extent(_Idxs), &__prod) || ... || false);
+        }(make_index_sequence<rank()>())),
+        "mdspan: size() is not representable as size_type");
+    return [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
+      return ((static_cast<size_type>(__map_.extents().extent(_Idxs))) * ... * size_type(1));
+    }(make_index_sequence<rank()>());
+  }
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool empty() const noexcept {
+    return [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
+      return (rank() > 0) && ((__map_.extents().extent(_Idxs) == index_type(0)) || ... || false);
+    }(make_index_sequence<rank()>());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(mdspan& __x, mdspan& __y) noexcept {
+    swap(__x.__ptr_, __y.__ptr_);
+    swap(__x.__map_, __y.__map_);
+    swap(__x.__acc_, __y.__acc_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr const extents_type& extents() const noexcept { return __map_.extents(); };
+  _LIBCPP_HIDE_FROM_ABI constexpr const data_handle_type& data_handle() const noexcept { return __ptr_; };
+  _LIBCPP_HIDE_FROM_ABI constexpr const mapping_type& mapping() const noexcept { return __map_; };
+  _LIBCPP_HIDE_FROM_ABI constexpr const accessor_type& accessor() const noexcept { return __acc_; };
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() { return mapping_type::is_always_unique(); };
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() { return mapping_type::is_always_exhaustive(); };
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() { return mapping_type::is_always_strided(); };
+
+  _LIBCPP_HIDE_FROM_ABI constexpr bool is_unique() const { return __map_.is_unique(); };
+  _LIBCPP_HIDE_FROM_ABI constexpr bool is_exhaustive() const { return __map_.is_exhaustive(); };
+  _LIBCPP_HIDE_FROM_ABI constexpr bool is_strided() const { return __map_.is_strided(); };
+  _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const { return __map_.stride(__r); };
+
+private:
+// TODO: fix when mingw build passes conversion test with no-unique-address enabled
+//       apparently there is a bug where the no-unique-address thing, makes the conversion constructor
+//       in the case where mapping and accessor the last byte of the data handle..
+#  ifndef _WIN32
+  _LIBCPP_NO_UNIQUE_ADDRESS data_handle_type __ptr_{};
+  _LIBCPP_NO_UNIQUE_ADDRESS mapping_type __map_{};
+  _LIBCPP_NO_UNIQUE_ADDRESS accessor_type __acc_{};
+#  else
+  data_handle_type __ptr_{};
+  mapping_type __map_{};
+  accessor_type __acc_{};
+#  endif
+
+  template <class, class, class, class>
+  friend class mdspan;
+};
+
+template <class _ElementType, class... _OtherIndexTypes>
+  requires((is_convertible_v<_OtherIndexTypes, size_t> && ...) && (sizeof...(_OtherIndexTypes) > 0))
+explicit mdspan(_ElementType*, _OtherIndexTypes...)
+    -> mdspan<_ElementType, dextents<size_t, sizeof...(_OtherIndexTypes)>>;
+
+template <class Pointer>
+  requires(is_pointer_v<remove_reference_t<Pointer>>)
+mdspan(Pointer&&) -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;
+
+template <class CArray>
+  requires(is_array_v<CArray> && (rank_v<CArray> == 1))
+mdspan(CArray&) -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;
+
+template <class _ElementType, class _OtherIndexType, size_t _Size>
+mdspan(_ElementType*, const array<_OtherIndexType, _Size>&) -> mdspan<_ElementType, dextents<size_t, _Size>>;
+
+template <class _ElementType, class _OtherIndexType, size_t _Size>
+mdspan(_ElementType*, span<_OtherIndexType, _Size>) -> mdspan<_ElementType, dextents<size_t, _Size>>;
+
+// This one is necessary because all the constructors take `data_handle_type`s, not
+// `_ElementType*`s, and `data_handle_type` is taken from `accessor_type::data_handle_type`, which
+// seems to throw off automatic deduction guides.
+template <class _ElementType, class _OtherIndexType, size_t... _ExtentsPack>
+mdspan(_ElementType*, const extents<_OtherIndexType, _ExtentsPack...>&)
+    -> mdspan<_ElementType, extents<_OtherIndexType, _ExtentsPack...>>;
+
+template <class _ElementType, class MappingType>
+mdspan(_ElementType*, const MappingType&)
+    -> mdspan<_ElementType, typename MappingType::extents_type, typename MappingType::layout_type>;
+
+template <class MappingType, class AccessorType>
+mdspan(const typename AccessorType::data_handle_type, const MappingType&, const AccessorType&)
+    -> mdspan<typename AccessorType::element_type,
+              typename MappingType::extents_type,
+              typename MappingType::layout_type,
+              AccessorType>;
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___MDSPAN_MDSPAN_H

libcxx/include/mdspan

struct default_accessor {
     constexpr default_accessor() noexcept = default;
     template<class OtherElementType>
       constexpr default_accessor(default_accessor<OtherElementType>) noexcept;
     constexpr reference access(data_handle_type p, size_t i) const noexcept;
     constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept;
   };
 }
 
+// mdspan synopsis
+
+namespace std {
+  template<class ElementType, class Extents, class LayoutPolicy = layout_right,
+           class AccessorPolicy = default_accessor<ElementType>>
+  class mdspan {
+  public:
+    using extents_type = Extents;
+    using layout_type = LayoutPolicy;
+    using accessor_type = AccessorPolicy;
+    using mapping_type = typename layout_type::template mapping<extents_type>;
+    using element_type = ElementType;
+    using value_type = remove_cv_t<element_type>;
+    using index_type = typename extents_type::index_type;
+    using size_type = typename extents_type::size_type;
+    using rank_type = typename extents_type::rank_type;
+    using data_handle_type = typename accessor_type::data_handle_type;
+    using reference = typename accessor_type::reference;
+
+    static constexpr rank_type rank() noexcept { return extents_type::rank(); }
+    static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
+    static constexpr size_t static_extent(rank_type r) noexcept
+      { return extents_type::static_extent(r); }
+    constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); }
+
+    // [mdspan.mdspan.cons], constructors
+    constexpr mdspan();
+    constexpr mdspan(const mdspan& rhs) = default;
+    constexpr mdspan(mdspan&& rhs) = default;
+
+    template<class... OtherIndexTypes>
+      constexpr explicit mdspan(data_handle_type ptr, OtherIndexTypes... exts);
+    template<class OtherIndexType, size_t N>
+      constexpr explicit(N != rank_dynamic())
+        mdspan(data_handle_type p, span<OtherIndexType, N> exts);
+    template<class OtherIndexType, size_t N>
+      constexpr explicit(N != rank_dynamic())
+        mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);
+    constexpr mdspan(data_handle_type p, const extents_type& ext);
+    constexpr mdspan(data_handle_type p, const mapping_type& m);
+    constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);
+
+    template<class OtherElementType, class OtherExtents,
+             class OtherLayoutPolicy, class OtherAccessorPolicy>
+      constexpr explicit(see below)
+        mdspan(const mdspan<OtherElementType, OtherExtents,
+                            OtherLayoutPolicy, OtherAccessorPolicy>& other);
+
+    constexpr mdspan& operator=(const mdspan& rhs) = default;
+    constexpr mdspan& operator=(mdspan&& rhs) = default;
+
+    // [mdspan.mdspan.members], members
+    template<class... OtherIndexTypes>
+      constexpr reference operator[](OtherIndexTypes... indices) const;
+    template<class OtherIndexType>
+      constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
+    template<class OtherIndexType>
+      constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;
+
+    constexpr size_type size() const noexcept;
+    [[nodiscard]] constexpr bool empty() const noexcept;
+
+    friend constexpr void swap(mdspan& x, mdspan& y) noexcept;
+
+    constexpr const extents_type& extents() const noexcept { return map_.extents(); }
+    constexpr const data_handle_type& data_handle() const noexcept { return ptr_; }
+    constexpr const mapping_type& mapping() const noexcept { return map_; }
+    constexpr const accessor_type& accessor() const noexcept { return acc_; }
+
+    static constexpr bool is_always_unique()
+      { return mapping_type::is_always_unique(); }
+    static constexpr bool is_always_exhaustive()
+      { return mapping_type::is_always_exhaustive(); }
+    static constexpr bool is_always_strided()
+      { return mapping_type::is_always_strided(); }
+
+    constexpr bool is_unique() const
+      { return map_.is_unique(); }
+    constexpr bool is_exhaustive() const
+      { return map_.is_exhaustive(); }
+    constexpr bool is_strided() const
+      { return map_.is_strided(); }
+    constexpr index_type stride(rank_type r) const
+      { return map_.stride(r); }
+
+  private:
+    accessor_type acc_;         // exposition only
+    mapping_type map_;          // exposition only
+    data_handle_type ptr_;      // exposition only
+  };
+
+  template<class CArray>
+    requires(is_array_v<CArray> && rank_v<CArray> == 1)
+    mdspan(CArray&)
+      -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;
+
+  template<class Pointer>
+    requires(is_pointer_v<remove_reference_t<Pointer>>)
+    mdspan(Pointer&&)
+      -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;
+
+  template<class ElementType, class... Integrals>
+    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
+    explicit mdspan(ElementType*, Integrals...)
+      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
+
+  template<class ElementType, class OtherIndexType, size_t N>
+    mdspan(ElementType*, span<OtherIndexType, N>)
+      -> mdspan<ElementType, dextents<size_t, N>>;
+
+  template<class ElementType, class OtherIndexType, size_t N>
+    mdspan(ElementType*, const array<OtherIndexType, N>&)
+      -> mdspan<ElementType, dextents<size_t, N>>;
+
+  template<class ElementType, class IndexType, size_t... ExtentsPack>
+    mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&)
+      -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;
+
+  template<class ElementType, class MappingType>
+    mdspan(ElementType*, const MappingType&)
+      -> mdspan<ElementType, typename MappingType::extents_type,
+                typename MappingType::layout_type>;
+
+  template<class MappingType, class AccessorType>
+    mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
+           const AccessorType&)
+      -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
+                typename MappingType::layout_type, AccessorType>;
+}
 */
 
 #ifndef _LIBCPP_MDSPAN
 #define _LIBCPP_MDSPAN
 
 #include <__config>
 #include <__fwd/mdspan.h>
 #include <__mdspan/default_accessor.h>
 #include <__mdspan/extents.h>
 #include <__mdspan/layout_left.h>
 #include <__mdspan/layout_right.h>
+#include <__mdspan/mdspan.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #endif // _LIBCPP_MDSPAN

libcxx/include/module.modulemap.in

 
 module std_private_mdspan_default_accessor [system] { header "__mdspan/default_accessor.h" }
 module std_private_mdspan_extents          [system] {
   header "__mdspan/extents.h"
   export *
 }
 module std_private_mdspan_layout_left      [system] { header "__mdspan/layout_left.h" }
 module std_private_mdspan_layout_right     [system] { header "__mdspan/layout_right.h" }
+module std_private_mdspan_mdspan           [system] { header "__mdspan/mdspan.h" }
 module std_private_mdspan_mdspan_fwd       [system] { header "__fwd/mdspan.h" }
 
 module std_private_memory_addressof                       [system] { header "__memory/addressof.h" }
 module std_private_memory_align                           [system] { header "__memory/align.h" }
 module std_private_memory_aligned_alloc                   [system] { header "__memory/aligned_alloc.h" }
 module std_private_memory_allocate_at_least               [system] { header "__memory/allocate_at_least.h" }
 module std_private_memory_allocation_guard                [system] { header "__memory/allocation_guard.h" }
 module std_private_memory_allocator                       [system] { header "__memory/allocator.h" }

libcxx/modules/std/mdspan.cppm

export namespace std {
   using std::layout_left;
   using std::layout_right;
   // using std::layout_stride;
 
   // [mdspan.accessor.default], class template default_accessor
   using std::default_accessor;
 
   // [mdspan.mdspan], class template mdspan
-  // using std::mdspan;
+  using std::mdspan;
 } // namespace std

libcxx/test/std/containers/views/mdspan/ConvertibleToIntegral.h

struct IntType {
   constexpr IntType(int v) noexcept : val(v){};
 
   constexpr bool operator==(const IntType& rhs) const { return val == rhs.val; }
   constexpr operator int() const noexcept { return val; }
   constexpr operator unsigned char() const { return val; }
   constexpr operator char() const noexcept { return val; }
 };
 
+// only non-const convertible
+struct IntTypeNC {
+  int val;
+  constexpr IntTypeNC() = default;
+  constexpr IntTypeNC(int v) noexcept : val(v){};
+
+  constexpr bool operator==(const IntType& rhs) const { return val == rhs.val; }
+  constexpr operator int() noexcept { return val; }
+  constexpr operator unsigned() { return val; }
+  constexpr operator char() noexcept { return val; }
+};
+
+// weird configurability of convertibility to int
+template<bool conv_c, bool conv_nc, bool ctor_nt_c, bool ctor_nt_nc>
+struct IntConfig {
+  int val;
+  constexpr explicit IntConfig(int val_):val(val_){}
+  constexpr operator int() noexcept(ctor_nt_nc) requires(conv_nc) { return val; }
+  constexpr operator int() const noexcept(ctor_nt_c) requires(conv_c) { return val; }
+};
+
 #endif // TEST_STD_CONTAINERS_VIEWS_MDSPAN_CONVERTIBLE_TO_INTEGRAL_H

libcxx/test/std/containers/views/mdspan/extents/assert.obs.pass.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
 //
 //===----------------------------------------------------------------------===//
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // static constexpr size_t static_extent(rank_type i) noexcept;
 //
 //   Preconditions: i < rank() is true.
 //

libcxx/test/std/containers/views/mdspan/extents/ctor_from_array.pass.cpp

static_assert(std::is_constructible_v< std::extents<int, D, D, 5, D, D>, decltype(a5)>,
                 "extents unexpectectly not constructible");
   // check that implicit construction doesn't work
   assert((implicit_construction<std::extents<int, D, D, 5, D, D>>(a5).value == false));
 
   // test construction fails from types not convertible to index_type but convertible to other integer types
   static_assert(std::is_convertible_v<IntType, int>, "Test helper IntType unexpectedly not convertible to int");
   static_assert(!std::is_constructible_v< std::extents<unsigned long, D>, std::array<IntType, 1>>,
                 "extents constructible from illegal arguments");
+
+  // index_type is not nothrow constructible
+  static_assert(std::is_convertible_v<IntType, unsigned char>);
+  static_assert(std::is_convertible_v<const IntType&, unsigned char>);
+  static_assert(!std::is_nothrow_constructible_v<unsigned char, const IntType&>);
+  static_assert(!std::is_constructible_v<std::dextents<unsigned char, 2>, std::array<IntType, 2>>);
+
+  // convertible from non-const to index_type but not  from const
+  static_assert(std::is_convertible_v<IntTypeNC, int>);
+  static_assert(!std::is_convertible_v<const IntTypeNC&, int>);
+  static_assert(std::is_nothrow_constructible_v<int, IntTypeNC>);
+  static_assert(!std::is_constructible_v<std::dextents<int, 2>, std::array<IntTypeNC, 2>>);
   return 0;
 }

libcxx/test/std/containers/views/mdspan/extents/ctor_from_span.pass.cpp

static_assert(std::is_constructible_v< std::extents<int, D, D, 5, D, D>, decltype(s5)>,
                 "extents unexpectectly not constructible");
   // check that implicit construction doesn't work
   assert((implicit_construction<std::extents<int, D, D, 5, D, D>>(s5).value == false));
 
   // test construction fails from types not convertible to index_type but convertible to other integer types
   static_assert(std::is_convertible_v<IntType, int>, "Test helper IntType unexpectedly not convertible to int");
   static_assert(!std::is_constructible_v< std::extents<unsigned long, D>, std::span<IntType, 1>>,
                 "extents constructible from illegal arguments");
+
+  // index_type is not nothrow constructible
+  static_assert(std::is_convertible_v<IntType, unsigned char>);
+  static_assert(std::is_convertible_v<const IntType&, unsigned char>);
+  static_assert(!std::is_nothrow_constructible_v<unsigned char, const IntType&>);
+  static_assert(!std::is_constructible_v<std::dextents<unsigned char, 2>, std::span<IntType, 2>>);
+
+  // convertible from non-const to index_type but not  from const
+  static_assert(std::is_convertible_v<IntTypeNC, int>);
+  static_assert(!std::is_convertible_v<const IntTypeNC&, int>);
+  static_assert(std::is_nothrow_constructible_v<int, IntTypeNC>);
+  static_assert(!std::is_constructible_v<std::dextents<int, 2>, std::span<IntTypeNC, 2>>);
   return 0;
 }

libcxx/test/std/containers/views/mdspan/layout_left/assert.ctor.extents.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 
 // constexpr mapping(const extents_type& e) noexcept;
 //
 // Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).

libcxx/test/std/containers/views/mdspan/layout_left/assert.index_operator.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // template<class... Indices>
 //   constexpr index_type operator()(Indices... i) const noexcept;
 //
 // Constraints:

libcxx/test/std/containers/views/mdspan/layout_left/assert.stride.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // layout_left::mapping
 //
 // constexpr index_type stride(rank_type i) const noexcept;
 //

libcxx/test/std/containers/views/mdspan/layout_right/assert.ctor.extents.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // constexpr mapping(const extents_type& e) noexcept;
 //
 // Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).
 //

libcxx/test/std/containers/views/mdspan/layout_right/assert.index_operator.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // template<class... Indices>
 //   constexpr index_type operator()(Indices... i) const noexcept;
 //
 // Constraints:

libcxx/test/std/containers/views/mdspan/layout_right/assert.stride.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // REQUIRES: has-unix-headers
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
-// UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode
+// UNSUPPORTED: !libcpp-has-debug-mode
 // XFAIL: availability-verbose_abort-missing
 
 // <mdspan>
 
 // layout_right::mapping
 //
 // constexpr index_type stride(rank_type i) const noexcept;
 //

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestAccessors.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_ACCESSORS_H
+#define TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_ACCESSORS_H
+
+#include <mdspan>
+#include <type_traits>
+#include <cassert>
+
+// This contains a bunch of accessors and handles which have different properties
+// regarding constructibility and convertibility in order to test mdspan constraints
+
+// non default constructible data handle
+template <class T>
+struct no_default_ctor_handle {
+  T* ptr;
+  no_default_ctor_handle() = delete;
+  constexpr no_default_ctor_handle(T* ptr_) : ptr(ptr_) {}
+};
+
+// handle that can't convert from T to const T
+template <class T>
+struct not_const_convertible_handle {
+  T* ptr;
+  constexpr not_const_convertible_handle() : ptr(nullptr) {}
+  constexpr not_const_convertible_handle(T* ptr_) : ptr(ptr_) {}
+
+  constexpr T& operator[](size_t i) const { return ptr[i]; }
+};
+
+// handle where move has side effects
+template <class T>
+struct move_counted_handle {
+  T* ptr;
+  constexpr move_counted_handle()                           = default;
+  constexpr move_counted_handle(const move_counted_handle&) = default;
+  template <class OtherT>
+    requires(std::is_constructible_v<T*, OtherT*>)
+  constexpr move_counted_handle(const move_counted_handle<OtherT>& other) : ptr(other.ptr){};
+  constexpr move_counted_handle(move_counted_handle&& other) {
+    ptr = other.ptr;
+    if !consteval {
+      move_counter()++;
+    }
+  }
+  constexpr move_counted_handle(T* ptr_) : ptr(ptr_) {}
+
+  constexpr move_counted_handle& operator=(const move_counted_handle&) = default;
+
+  constexpr T& operator[](size_t i) const { return ptr[i]; }
+
+  static int& move_counter() {
+    static int c = 0;
+    return c;
+  }
+};
+
+// non-default constructible accessor with a bunch of different data handles
+template <class ElementType>
+struct checked_accessor {
+  size_t N;
+  using offset_policy    = std::default_accessor<ElementType>;
+  using element_type     = ElementType;
+  using reference        = ElementType&;
+  using data_handle_type = move_counted_handle<ElementType>;
+
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+  template <class OtherElementType>
+    requires(std::is_convertible_v<OtherElementType (*)[], element_type (*)[]>)
+  explicit constexpr checked_accessor(const checked_accessor<OtherElementType>& other) noexcept {
+    N = other.N;
+  }
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return data_handle_type(p.ptr + i);
+  }
+};
+
+static_assert(std::is_constructible_v<checked_accessor<const int>, const checked_accessor<int>&>);
+static_assert(!std::is_convertible_v<const checked_accessor<int>&, checked_accessor<const int>>);
+
+template <>
+struct checked_accessor<double> {
+  size_t N;
+  using offset_policy    = std::default_accessor<double>;
+  using element_type     = double;
+  using reference        = double&;
+  using data_handle_type = no_default_ctor_handle<double>;
+
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+
+  template <class OtherElementType>
+    requires(std::is_convertible_v<OtherElementType (*)[], element_type (*)[]>)
+  constexpr checked_accessor(checked_accessor<OtherElementType>&& other) noexcept {
+    N = other.N;
+  }
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p.ptr[i];
+  }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p.ptr + i;
+  }
+};
+
+template <>
+struct checked_accessor<unsigned> {
+  size_t N;
+  using offset_policy    = std::default_accessor<unsigned>;
+  using element_type     = unsigned;
+  using reference        = unsigned;
+  using data_handle_type = not_const_convertible_handle<unsigned>;
+
+  constexpr checked_accessor() : N(0) {}
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+  constexpr checked_accessor(const checked_accessor& acc) : N(acc.N) {}
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr auto offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p.ptr + i;
+  }
+};
+template <>
+struct checked_accessor<const unsigned> {
+  size_t N;
+  using offset_policy    = std::default_accessor<const unsigned>;
+  using element_type     = const unsigned;
+  using reference        = unsigned;
+  using data_handle_type = not_const_convertible_handle<const unsigned>;
+
+  constexpr checked_accessor() : N(0) {}
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+  constexpr checked_accessor(const checked_accessor& acc) : N(acc.N) {}
+
+  template <class OtherACC>
+  constexpr explicit(std::is_const_v<OtherACC>) checked_accessor(OtherACC&& acc) : N(acc.N) {}
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr auto offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p.ptr + i;
+  }
+};
+
+template <>
+struct checked_accessor<const float> {
+  size_t N;
+  using offset_policy    = std::default_accessor<const float>;
+  using element_type     = const float;
+  using reference        = const float&;
+  using data_handle_type = move_counted_handle<const float>;
+
+  constexpr checked_accessor() : N(0) {}
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+  constexpr checked_accessor(const checked_accessor& acc) : N(acc.N) {}
+
+  constexpr checked_accessor(checked_accessor<float>&& acc) : N(acc.N) {}
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return data_handle_type(p.ptr + i);
+  }
+};
+
+template <>
+struct checked_accessor<const double> {
+  size_t N;
+  using offset_policy    = std::default_accessor<const double>;
+  using element_type     = const double;
+  using reference        = const double&;
+  using data_handle_type = move_counted_handle<const double>;
+
+  constexpr checked_accessor() : N(0) {}
+  constexpr checked_accessor(size_t N_) : N(N_) {}
+  constexpr checked_accessor(const checked_accessor& acc) : N(acc.N) {}
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return data_handle_type(p.ptr + i);
+  }
+};
+
+// Data handle pair which has configurable conversion properties
+// bool template parameters are used to enable/disable ctors and assignment
+// the c is the one for const T the nc for non-const (so we can convert mdspan)
+// Note both take non-const T as template parameter though
+template <class T, bool, bool, bool, bool>
+struct conv_test_accessor_c;
+
+template <class T, bool conv_c, bool conv_nc>
+struct conv_test_accessor_nc {
+  using offset_policy    = std::default_accessor<T>;
+  using element_type     = T;
+  using reference        = T&;
+  using data_handle_type = T*;
+
+  constexpr conv_test_accessor_nc()                             = default;
+  constexpr conv_test_accessor_nc(const conv_test_accessor_nc&) = default;
+
+  template <bool b1, bool b2, bool b3, bool b4>
+  constexpr operator conv_test_accessor_c<T, b1, b2, b3, b4>()
+    requires(conv_nc)
+  {
+    return conv_test_accessor_c<T, b1, b2, b3, b4>{};
+  }
+  template <bool b1, bool b2, bool b3, bool b4>
+  constexpr operator conv_test_accessor_c<T, b1, b2, b3, b4>() const
+    requires(conv_c)
+  {
+    return conv_test_accessor_c<T, b1, b2, b3, b4>{};
+  }
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept { return p[i]; }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept { return p + i; }
+};
+
+template <class T, bool ctor_c, bool ctor_mv, bool assign_c, bool assign_mv>
+struct conv_test_accessor_c {
+  using offset_policy    = std::default_accessor<const T>;
+  using element_type     = const T;
+  using reference        = const T&;
+  using data_handle_type = const T*;
+
+  constexpr conv_test_accessor_c()                            = default;
+  constexpr conv_test_accessor_c(const conv_test_accessor_c&) = default;
+
+  template <bool b1, bool b2>
+  constexpr conv_test_accessor_c(const conv_test_accessor_nc<T, b1, b2>&)
+    requires(ctor_c)
+  {}
+  template <bool b1, bool b2>
+  constexpr conv_test_accessor_c(conv_test_accessor_nc<T, b1, b2>&&)
+    requires(ctor_mv)
+  {}
+  template <bool b1, bool b2>
+  constexpr conv_test_accessor_c& operator=(const conv_test_accessor_nc<T, b1, b2>&)
+    requires(assign_c)
+  {
+    return {};
+  }
+  template <bool b1, bool b2>
+  constexpr conv_test_accessor_c& operator=(conv_test_accessor_nc<T, b1, b2>&&)
+    requires(assign_mv)
+  {
+    return {};
+  }
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept { return p[i]; }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept { return p + i; }
+};
+
+template <class ElementType>
+struct convertible_accessor_but_not_handle {
+  size_t N;
+  using offset_policy    = std::default_accessor<ElementType>;
+  using element_type     = ElementType;
+  using reference        = ElementType&;
+  using data_handle_type = not_const_convertible_handle<element_type>;
+
+  constexpr convertible_accessor_but_not_handle() = default;
+  template <class OtherElementType>
+    requires(std::is_convertible_v<OtherElementType (*)[], element_type (*)[]>)
+  explicit constexpr convertible_accessor_but_not_handle(
+      const convertible_accessor_but_not_handle<OtherElementType>& other) noexcept {
+    N = other.N;
+  }
+
+  constexpr reference access(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return p[i];
+  }
+  constexpr data_handle_type offset(data_handle_type p, size_t i) const noexcept {
+    assert(i < N);
+    return data_handle_type(p.ptr + i);
+  }
+};
+
+#endif // TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_ACCESSORS_H

libcxx/test/std/containers/views/mdspan/mdspan/CustomTestLayouts.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H
+#define TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <cinttypes>
+#include <concepts>
+#include <cstddef>
+#include <limits>
+#include <mdspan>
+#include <type_traits>
+#include <utility>
+
+// Layout that wraps indices to test some idiosyncratic behavior
+// - basically it is a layout_left where indicies are first wrapped i.e. i%Wrap
+// - only accepts integers as indices
+// - is_always_strided and is_always_unique are false
+// - is_strided and is_unique are true if all extents are smaller than Wrap
+// - not default constructible
+// - not extents constructible
+// - not trivally copyable
+// - does not check dynamic to static extent conversion in converting ctor
+// - check via side-effects that mdspan::swap calls mappings swap via ADL
+
+struct not_extents_constructible_tag {};
+
+template <size_t Wrap>
+class layout_wrapping_integral {
+public:
+  template <class Extents>
+  class mapping;
+};
+
+template <size_t WrapArg>
+template <class Extents>
+class layout_wrapping_integral<WrapArg>::mapping {
+  static constexpr typename Extents::index_type Wrap = static_cast<typename Extents::index_type>(WrapArg);
+
+public:
+  using extents_type = Extents;
+  using index_type   = typename extents_type::index_type;
+  using size_type    = typename extents_type::size_type;
+  using rank_type    = typename extents_type::rank_type;
+  using layout_type  = layout_wrapping_integral<Wrap>;
+
+private:
+  static constexpr bool required_span_size_is_representable(const extents_type& ext) {
+    if constexpr (extents_type::rank() == 0)
+      return true;
+
+    index_type prod = ext.extent(0);
+    for (rank_type r = 1; r < extents_type::rank(); r++) {
+      bool overflowed = __builtin_mul_overflow(prod, std::min(ext.extent(r), Wrap), &prod);
+      if (overflowed)
+        return false;
+    }
+    return true;
+  }
+
+public:
+  constexpr mapping() noexcept = delete;
+  constexpr mapping(const mapping& other) noexcept : extents_(other.extents()){};
+  constexpr mapping(extents_type&& ext) noexcept
+    requires(Wrap == 8)
+      : extents_(ext) {}
+  constexpr mapping(const extents_type& ext, not_extents_constructible_tag) noexcept : extents_(ext) {}
+
+  template <class OtherExtents>
+    requires(std::is_constructible_v<extents_type, OtherExtents> && (Wrap != 8))
+  constexpr explicit(!std::is_convertible_v<OtherExtents, extents_type>)
+      mapping(const mapping<OtherExtents>& other) noexcept {
+    std::array<index_type, extents_type::rank_dynamic()> dyn_extents;
+    rank_type count = 0;
+    for (rank_type r = 0; r < extents_type::rank(); r++) {
+      if (extents_type::static_extent(r) == std::dynamic_extent) {
+        dyn_extents[count++] = other.extents().extent(r);
+      }
+    }
+    extents_ = extents_type(dyn_extents);
+  }
+  template <class OtherExtents>
+    requires(std::is_constructible_v<extents_type, OtherExtents> && (Wrap == 8))
+  constexpr explicit(!std::is_convertible_v<OtherExtents, extents_type>)
+      mapping(mapping<OtherExtents>&& other) noexcept {
+    std::array<index_type, extents_type::rank_dynamic()> dyn_extents;
+    rank_type count = 0;
+    for (rank_type r = 0; r < extents_type::rank(); r++) {
+      if (extents_type::static_extent(r) == std::dynamic_extent) {
+        dyn_extents[count++] = other.extents().extent(r);
+      }
+    }
+    extents_ = extents_type(dyn_extents);
+  }
+
+  constexpr mapping& operator=(const mapping& other) noexcept {
+    extents_ = other.extents_;
+    return *this;
+  };
+
+  constexpr const extents_type& extents() const noexcept { return extents_; }
+
+  constexpr index_type required_span_size() const noexcept {
+    index_type size = 1;
+    for (size_t r = 0; r < extents_type::rank(); r++)
+      size *= extents_.extent(r) < Wrap ? extents_.extent(r) : Wrap;
+    return size;
+  }
+
+  template <std::integral... Indices>
+    requires((sizeof...(Indices) == extents_type::rank()) && (std::is_convertible_v<Indices, index_type> && ...) &&
+             (std::is_nothrow_constructible_v<index_type, Indices> && ...))
+  constexpr index_type operator()(Indices... idx) const noexcept {
+    std::array<index_type, extents_type::rank()> idx_a{static_cast<index_type>(static_cast<index_type>(idx) % Wrap)...};
+    return [&]<size_t... Pos>(std::index_sequence<Pos...>) {
+      index_type res = 0;
+      ((res = idx_a[extents_type::rank() - 1 - Pos] +
+              (extents_.extent(extents_type::rank() - 1 - Pos) < Wrap ? extents_.extent(extents_type::rank() - 1 - Pos)
+                                                                      : Wrap) *
+                  res),
+       ...);
+      return res;
+    }(std::make_index_sequence<sizeof...(Indices)>());
+  }
+
+  static constexpr bool is_always_unique() noexcept { return false; }
+  static constexpr bool is_always_exhaustive() noexcept { return true; }
+  static constexpr bool is_always_strided() noexcept { return false; }
+
+  constexpr bool is_unique() const noexcept {
+    for (rank_type r = 0; r < extents_type::rank(); r++) {
+      if (extents_.extent(r) > Wrap)
+        return false;
+    }
+    return true;
+  }
+  static constexpr bool is_exhaustive() noexcept { return true; }
+  constexpr bool is_strided() const noexcept {
+    for (rank_type r = 0; r < extents_type::rank(); r++) {
+      if (extents_.extent(r) > Wrap)
+        return false;
+    }
+    return true;
+  }
+
+  constexpr index_type stride(rank_type r) const noexcept
+    requires(extents_type::rank() > 0)
+  {
+    index_type s = 1;
+    for (rank_type i = extents_type::rank() - 1; i > r; i--)
+      s *= extents_.extent(i);
+    return s;
+  }
+
+  template <class OtherExtents>
+    requires(OtherExtents::rank() == extents_type::rank())
+  friend constexpr bool operator==(const mapping& lhs, const mapping<OtherExtents>& rhs) noexcept {
+    return lhs.extents() == rhs.extents();
+  }
+
+  friend constexpr void swap(mapping& x, mapping& y) noexcept {
+    swap(x.extents_, y.extents_);
+    if !consteval {
+      swap_counter()++;
+    }
+  }
+
+  static int& swap_counter() {
+    static int value = 0;
+    return value;
+  }
+
+private:
+  extents_type extents_{};
+};
+
+template <class Extents>
+constexpr auto construct_mapping(std::layout_left, Extents exts) {
+  return std::layout_left::mapping<Extents>(exts);
+}
+
+template <class Extents>
+constexpr auto construct_mapping(std::layout_right, Extents exts) {
+  return std::layout_right::mapping<Extents>(exts);
+}
+
+template <size_t Wraps, class Extents>
+constexpr auto construct_mapping(layout_wrapping_integral<Wraps>, Extents exts) {
+  return typename layout_wrapping_integral<Wraps>::template mapping<Extents>(exts, not_extents_constructible_tag{});
+}
+
+
+// This layout does not check convertibility of extents for its conversion ctor
+// Allows triggering mdspan's ctor static assertion on convertibility of extents
+class always_convertible_layout {
+public:
+  template <class Extents>
+  class mapping;
+};
+
+template <class Extents>
+class always_convertible_layout::mapping {
+public:
+  using extents_type = Extents;
+  using index_type   = typename extents_type::index_type;
+  using size_type    = typename extents_type::size_type;
+  using rank_type    = typename extents_type::rank_type;
+  using layout_type  = always_convertible_layout;
+
+private:
+  static constexpr bool required_span_size_is_representable(const extents_type& ext) {
+    if constexpr (extents_type::rank() == 0)
+      return true;
+
+    index_type prod = ext.extent(0);
+    for (rank_type r = 1; r < extents_type::rank(); r++) {
+      bool overflowed = __builtin_mul_overflow(prod, ext.extent(r), &prod);
+      if (overflowed)
+        return false;
+    }
+    return true;
+  }
+
+public:
+  constexpr mapping() noexcept = delete;
+  constexpr mapping(const mapping& other) noexcept : extents_(other.extents()){};
+  constexpr mapping(const extents_type& ext) noexcept : extents_(ext){};
+
+  template <class OtherExtents>
+  constexpr mapping(const mapping<OtherExtents>& other) noexcept {
+    if constexpr (extents_type::rank() == OtherExtents::rank()) {
+      std::array<index_type, extents_type::rank_dynamic()> dyn_extents;
+      rank_type count = 0;
+      for (rank_type r = 0; r < extents_type::rank(); r++) {
+        if (extents_type::static_extent(r) == std::dynamic_extent) {
+          dyn_extents[count++] = other.extents().extent(r);
+        }
+      }
+      extents_ = extents_type(dyn_extents);
+    } else {
+      extents_ = extents_type();
+    }
+  }
+
+  constexpr mapping& operator=(const mapping& other) noexcept {
+    extents_ = other.extents_;
+    return *this;
+  };
+
+  constexpr const extents_type& extents() const noexcept { return extents_; }
+
+  constexpr index_type required_span_size() const noexcept {
+    index_type size = 1;
+    for (size_t r = 0; r < extents_type::rank(); r++)
+      size *= extents_.extent(r);
+    return size;
+  }
+
+  template <std::integral... Indices>
+    requires((sizeof...(Indices) == extents_type::rank()) && (std::is_convertible_v<Indices, index_type> && ...) &&
+             (std::is_nothrow_constructible_v<index_type, Indices> && ...))
+  constexpr index_type operator()(Indices... idx) const noexcept {
+    std::array<index_type, extents_type::rank()> idx_a{static_cast<index_type>(static_cast<index_type>(idx))...};
+    return [&]<size_t... Pos>(std::index_sequence<Pos...>) {
+      index_type res = 0;
+      ((res = idx_a[extents_type::rank() - 1 - Pos] + extents_.extent(extents_type::rank() - 1 - Pos) * res), ...);
+      return res;
+    }(std::make_index_sequence<sizeof...(Indices)>());
+  }
+
+  static constexpr bool is_always_unique() noexcept { return false; }
+  static constexpr bool is_always_exhaustive() noexcept { return true; }
+  static constexpr bool is_always_strided() noexcept { return false; }
+
+  static constexpr bool is_unique() noexcept { return true; }
+  static constexpr bool is_exhaustive() noexcept { return true; }
+  static constexpr bool is_strided() noexcept { return true; }
+
+  constexpr index_type stride(rank_type r) const noexcept
+    requires(extents_type::rank() > 0)
+  {
+    index_type s = 1;
+    for (rank_type i = extents_type::rank() - 1; i > r; i--)
+      s *= extents_.extent(i);
+    return s;
+  }
+
+  template <class OtherExtents>
+    requires(OtherExtents::rank() == extents_type::rank())
+  friend constexpr bool operator==(const mapping& lhs, const mapping<OtherExtents>& rhs) noexcept {
+    return lhs.extents() == rhs.extents();
+  }
+
+  friend constexpr void swap(mapping& x, mapping& y) noexcept {
+    swap(x.extents_, y.extents_);
+    if !consteval {
+      swap_counter()++;
+    }
+  }
+
+  static int& swap_counter() {
+    static int value = 0;
+    return value;
+  }
+
+private:
+  extents_type extents_{};
+};
+#endif                     // TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H

libcxx/test/std/containers/views/mdspan/mdspan/assert.conversion.pass.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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: !libcpp-has-debug-mode
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class OtherElementType, class OtherExtents,
+//         class OtherLayoutPolicy, class OtherAccessor>
+//  constexpr explicit(see below)
+//    mdspan(const mdspan<OtherElementType, OtherExtents,
+//                        OtherLayoutPolicy, OtherAccessor>& other);
+//
+// Constraints:
+//   - is_constructible_v<mapping_type, const OtherLayoutPolicy::template mapping<OtherExtents>&> is true, and
+//   - is_constructible_v<accessor_type, const OtherAccessor&> is true.
+// Mandates:
+//   - is_constructible_v<data_handle_type, const OtherAccessor::data_handle_type&> is
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions:
+//   - For each rank index r of extents_type, static_extent(r) == dynamic_extent || static_extent(r) == other.extent(r) is true.
+//   - [0, map_.required_span_size()) is an accessible range of ptr_ and acc_ for values of ptr_, map_, and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with other.ptr_,
+//   - direct-non-list-initializes map_ with other.map_, and
+//   - direct-non-list-initializes acc_ with other.acc_.
+//
+// Remarks: The expression inside explicit is equivalent to:
+//   !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type>
+//   || !is_convertible_v<const OtherAccessor&, accessor_type>
+
+#include <array>
+#include <cassert>
+#include <mdspan>
+
+#include "check_assertion.h"
+#include "CustomTestLayouts.h"
+
+// We use a funky mapping in this test that doesn't check the dynamic/static extents mismatch itself
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  std::array<float, 10> data;
+  typename layout_wrapping_integral<4>::template mapping<std::dextents<int, 2>> src_map(
+      std::dextents<int, 2>(5, 2), not_extents_constructible_tag());
+  std::mdspan<float, std::dextents<int, 2>, layout_wrapping_integral<4>> arg(data.data(), src_map);
+
+  // working case
+  {
+    [[maybe_unused]] std::mdspan<float, std::extents<size_t, D, 2>, layout_wrapping_integral<4>> m(arg); // should work
+  }
+  // mismatch of static extent
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::mdspan<float, std::extents<size_t, D, 3>, layout_wrapping_integral<4>> m(arg); }()),
+        "mdspan: conversion mismatch of source dynamic extents with static extents");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/assert.index_operator.pass.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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: !libcpp-has-debug-mode
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class... OtherIndexTypes>
+//   constexpr reference operator[](OtherIndexTypes... indices) const;
+// Constraints:
+//   - (is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
+//   - (is_nothrow_constructible_v<index_type, OtherIndexTypes> && ...) is true, and
+//   - sizeof...(OtherIndexTypes) == rank() is true.
+//
+// Let I be extents_type::index-cast(std::move(indices)).
+//
+// Preconditions: I is a multidimensional index in extents().
+//   Note 1: This implies that map_(I) < map_.required_span_size() is true.
+//
+// Effects: Equivalent to:
+//   return acc_.access(ptr_, map_(static_cast<index_type>(std::move(indices))...));
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  float data[1024];
+  // value out of range
+  {
+    std::mdspan m(data, std::extents<unsigned char, 5>());
+    TEST_LIBCPP_ASSERT_FAILURE(m[-1], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[-130], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[5], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[1000], "mdspan: operator[] out of bounds access");
+  }
+  {
+    std::mdspan m(data, std::extents<signed char, 5>());
+    TEST_LIBCPP_ASSERT_FAILURE(m[-1], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[-130], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[5], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[1000], "mdspan: operator[] out of bounds access");
+  }
+  {
+    std::mdspan m(data, std::dextents<unsigned char, 1>(5));
+    TEST_LIBCPP_ASSERT_FAILURE(m[-1], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[-130], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[5], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[1000], "mdspan: operator[] out of bounds access");
+  }
+  {
+    std::mdspan m(data, std::dextents<signed char, 1>(5));
+    TEST_LIBCPP_ASSERT_FAILURE(m[-1], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[-130], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[5], "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE(m[1000], "mdspan: operator[] out of bounds access");
+  }
+  {
+    std::mdspan m(data, std::dextents<int, 3>(5, 7, 9));
+    TEST_LIBCPP_ASSERT_FAILURE((m[-1, -1, -1]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[-1, 0, 0]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[0, -1, 0]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[0, 0, -1]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[5, 3, 3]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[3, 7, 3]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[3, 3, 9]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[5, 7, 9]), "mdspan: operator[] out of bounds access");
+  }
+  {
+    std::mdspan m(data, std::dextents<unsigned, 3>(5, 7, 9));
+    TEST_LIBCPP_ASSERT_FAILURE((m[-1, -1, -1]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[-1, 0, 0]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[0, -1, 0]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[0, 0, -1]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[5, 3, 3]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[3, 7, 3]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[3, 3, 9]), "mdspan: operator[] out of bounds access");
+    TEST_LIBCPP_ASSERT_FAILURE((m[5, 7, 9]), "mdspan: operator[] out of bounds access");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/assert.size.pass.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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: !libcpp-has-debug-mode
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// constexpr size_type size() const noexcept;
+//
+// Preconditions: The size of the multidimensional index space extents() is representable as a value of type size_type ([basic.fundamental]).
+//
+// Returns: extents().fwd-prod-of-extents(rank()).
+
+#include <array>
+#include <cassert>
+#include <mdspan>
+
+#include "check_assertion.h"
+#include "CustomTestLayouts.h"
+
+// We use a funky mapping in this test where required_span_size is much smaller than the size of the index space
+int main(int, char**) {
+  std::array<float, 10> data;
+  // make sure we are not failing because of using index_type instead of size_type
+  {
+    typename layout_wrapping_integral<4>::template mapping<std::dextents<char, 2>> map(
+      std::dextents<char, 2>(100, 2), not_extents_constructible_tag());
+    std::mdspan<float, std::dextents<char, 2>, layout_wrapping_integral<4>> mds(data.data(), map);
+    assert(map.required_span_size() == char(8));
+    assert((static_cast<unsigned char>(200) == mds.size()));
+  }
+  {
+    typename layout_wrapping_integral<4>::template mapping<std::dextents<char, 2>> map(
+      std::dextents<char, 2>(100, 3), not_extents_constructible_tag());
+    std::mdspan<float, std::dextents<char, 2>, layout_wrapping_integral<4>> mds(data.data(), map);
+    // sanity check
+    assert(map.required_span_size() == char(12));
+    // 100 x 3 exceeds 256
+    {
+      TEST_LIBCPP_ASSERT_FAILURE(([=] { mds.size(); }()), "mdspan: size() is not representable as size_type");
+    }
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan& operator=(const mdspan& rhs) = default;
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  MDS m_org(handle, map, acc);
+  MDS m(handle, map, acc);
+
+  // The defaulted assignment operator seems to be deprecated because:
+  //   error: definition of implicit copy assignment operator for 'checked_accessor<const double>' is deprecated
+  //   because it has a user-provided copy constructor [-Werror,-Wdeprecated-copy-with-user-provided-copy]
+  if constexpr (!std::is_same_v<A, checked_accessor<const double>>)
+    m = m_org;
+  // even though the following checks out:
+  static_assert(std::copyable<checked_accessor<const double>>);
+  static_assert(std::is_assignable_v<checked_accessor<const double>, checked_accessor<const double>>);
+
+  static_assert(noexcept(m = m_org));
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == acc);
+
+  static_assert(std::is_trivially_assignable_v<MDS, const MDS&> ==
+                ((!std::is_class_v<H> ||
+                  std::is_trivially_assignable_v<H, const H&>)&&std::is_trivially_assignable_v<M, const M&> &&
+                 std::is_trivially_assignable_v<A, const A&>));
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  // make sure we test a trivially assignable mapping
+  static_assert(std::is_trivially_assignable_v<typename std::layout_left::template mapping<std::extents<int>>,
+                                               const typename std::layout_left::template mapping<std::extents<int>>&>);
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  // make sure we test a not trivially assignable mapping
+  static_assert(!std::is_trivially_assignable_v<
+                typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                const typename layout_wrapping_integral<4>::template mapping<std::extents<int>>&>);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  // make sure we test trivially constructible accessor and data_handle
+  static_assert(std::is_trivially_copyable_v<std::default_accessor<T>>);
+  static_assert(std::is_trivially_copyable_v<typename std::default_accessor<T>::data_handle_type>);
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is noexcept copy constructible except for const double
+  checked_accessor<T> acc(1024);
+  static_assert(noexcept(checked_accessor<T>(acc)) != std::is_same_v<T, const double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), acc);
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/conversion.pass.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, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherElementType, class OtherExtents,
+//         class OtherLayoutPolicy, class OtherAccessor>
+//  constexpr explicit(see below)
+//    mdspan(const mdspan<OtherElementType, OtherExtents,
+//                        OtherLayoutPolicy, OtherAccessor>& other);
+//
+// Constraints:
+//   - is_constructible_v<mapping_type, const OtherLayoutPolicy::template mapping<OtherExtents>&> is true, and
+//   - is_constructible_v<accessor_type, const OtherAccessor&> is true.
+// Mandates:
+//   - is_constructible_v<data_handle_type, const OtherAccessor::data_handle_type&> is
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions:
+//   - For each rank index r of extents_type, static_extent(r) == dynamic_extent || static_extent(r) == other.extent(r) is true.
+//   - [0, map_.required_span_size()) is an accessible range of ptr_ and acc_ for values of ptr_, map_, and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with other.ptr_,
+//   - direct-non-list-initializes map_ with other.map_, and
+//   - direct-non-list-initializes acc_ with other.acc_.
+//
+// Remarks: The expression inside explicit is equivalent to:
+//   !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type>
+//   || !is_convertible_v<const OtherAccessor&, accessor_type>
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class ToMDS, class FromMDS>
+constexpr void test_implicit_conversion(ToMDS to_mds, FromMDS from_mds) {
+  assert(to_mds.extents() == from_mds.extents());
+  if constexpr (std::equality_comparable_with<typename ToMDS::data_handle_type, typename FromMDS::data_handle_type>)
+    assert(to_mds.data_handle() == from_mds.data_handle());
+  if constexpr (std::equality_comparable_with<typename ToMDS::mapping_type, typename FromMDS::mapping_type>)
+    assert(to_mds.mapping() == from_mds.mapping());
+  if constexpr (std::equality_comparable_with<typename ToMDS::accessor_type, typename FromMDS::accessor_type>)
+    assert(to_mds.accessor() == from_mds.accessor());
+}
+
+template <class M>
+concept mapping_requirements = requires() {
+  requires(std::copyable<M> && std::equality_comparable<M>) && std::is_nothrow_move_constructible_v<M> &&
+              std::is_nothrow_move_assignable_v<M> && std::is_nothrow_swappable_v<M>;
+};
+
+template <class ToMDS, class FromMDS>
+constexpr void test_conversion(FromMDS from_mds) {
+  // check some requirements, to see we didn't screw up our test layouts/accessors
+  static_assert(mapping_requirements<typename ToMDS::mapping_type>);
+  static_assert(mapping_requirements<typename FromMDS::mapping_type>);
+
+  constexpr bool constructible =
+      std::is_constructible_v<typename ToMDS::mapping_type, const typename FromMDS::mapping_type&> &&
+      std::is_constructible_v<typename ToMDS::accessor_type, const typename FromMDS::accessor_type&>;
+  constexpr bool convertible =
+      std::is_convertible_v<const typename FromMDS::mapping_type&, typename ToMDS::mapping_type> &&
+      std::is_convertible_v<const typename FromMDS::accessor_type&, typename ToMDS::accessor_type>;
+  constexpr bool passes_mandates =
+      std::is_constructible_v<typename ToMDS::data_handle_type, const typename FromMDS::data_handle_type&> &&
+      std::is_constructible_v<typename ToMDS::extents_type, typename FromMDS::extents_type>;
+
+  if constexpr (constructible) {
+    if constexpr (passes_mandates) {
+      ToMDS to_mds(from_mds);
+      assert(to_mds.extents() == from_mds.extents());
+      if constexpr (std::equality_comparable_with<typename ToMDS::data_handle_type, typename FromMDS::data_handle_type>)
+        assert(to_mds.data_handle() == from_mds.data_handle());
+      if constexpr (std::equality_comparable_with<typename ToMDS::mapping_type, typename FromMDS::mapping_type>)
+        assert(to_mds.mapping() == from_mds.mapping());
+      if constexpr (std::equality_comparable_with<typename ToMDS::accessor_type, typename FromMDS::accessor_type>)
+        assert(to_mds.accessor() == from_mds.accessor());
+      if constexpr (convertible) {
+        test_implicit_conversion(from_mds, from_mds);
+      } else {
+        static_assert(!std::is_convertible_v<FromMDS, ToMDS>);
+      }
+    }
+  } else {
+    static_assert(!std::is_constructible_v<ToMDS, FromMDS>);
+  }
+}
+
+template <class ToL, class ToE, class ToA, class FromH, class FromL, class FromE, class FromA>
+constexpr void construct_from_mds(const FromH& handle, const FromL& layout, const FromE& exts, const FromA& acc) {
+  using ToMDS   = std::mdspan<typename ToA::element_type, ToE, ToL, ToA>;
+  using FromMDS = std::mdspan<typename FromA::element_type, FromE, FromL, FromA>;
+  test_conversion<ToMDS>(FromMDS(handle, construct_mapping(layout, exts), acc));
+}
+
+template <class ToL, class ToA, class FromH, class FromL, class FromA>
+constexpr void mixin_extents(const FromH& handle, const FromL& layout, const FromA& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  // constructible and convertible
+  construct_from_mds<ToL, std::dextents<int, 0>, ToA>(handle, layout, std::dextents<int, 0>(), acc);
+  construct_from_mds<ToL, std::dextents<int, 1>, ToA>(handle, layout, std::dextents<int, 1>(4), acc);
+  construct_from_mds<ToL, std::dextents<int, 1>, ToA>(handle, layout, std::extents<int, 4>(), acc);
+  construct_from_mds<ToL, std::dextents<int, 2>, ToA>(handle, layout, std::dextents<int, 2>(4, 5), acc);
+  construct_from_mds<ToL, std::dextents<unsigned, 2>, ToA>(handle, layout, std::dextents<int, 2>(4, 5), acc);
+  construct_from_mds<ToL, std::dextents<unsigned, 2>, ToA>(handle, layout, std::extents<int, D, 5>(4), acc);
+  construct_from_mds<ToL, std::extents<int, D, 5>, ToA>(handle, layout, std::extents<int, D, 5>(4), acc);
+  construct_from_mds<ToL, std::extents<int, D, 5>, ToA>(handle, layout, std::extents<int, D, 5>(4), acc);
+  construct_from_mds<ToL, std::extents<int, D, 5, D, 7>, ToA>(handle, layout, std::extents<int, D, 5, D, 7>(4, 6), acc);
+
+  // not convertible
+  construct_from_mds<ToL, std::dextents<int, 1>, ToA>(handle, layout, std::dextents<unsigned, 1>(4), acc);
+  construct_from_mds<ToL, std::extents<int, D, 5, D, 7>, ToA>(
+      handle, layout, std::extents<int, D, 5, D, D>(4, 6, 7), acc);
+
+  // not constructible
+  construct_from_mds<ToL, std::dextents<int, 1>, ToA>(handle, layout, std::dextents<int, 2>(4, 5), acc);
+  construct_from_mds<ToL, std::extents<int, D, 5, D, 8>, ToA>(handle, layout, std::extents<int, D, 5, D, 7>(4, 6), acc);
+}
+
+template <class ToA, class FromH, class FromA>
+constexpr void mixin_layout(const FromH& handle, const FromA& acc) {
+  mixin_extents<std::layout_left, ToA>(handle, std::layout_left(), acc);
+  mixin_extents<std::layout_right, ToA>(handle, std::layout_right(), acc);
+  // Check layout policy conversion
+  // different layout policies, but constructible and convertible
+  static_assert(std::is_constructible_v<std::layout_left::mapping<std::dextents<int, 1>>,
+                                        const std::layout_right::mapping<std::dextents<int, 1>>&>);
+  static_assert(std::is_convertible_v<const std::layout_right::mapping<std::dextents<int, 1>>&,
+                                      std::layout_left::mapping<std::dextents<int, 1>>>);
+  // different layout policies, not constructible
+  static_assert(!std::is_constructible_v<std::layout_left::mapping<std::dextents<int, 2>>,
+                                         const std::layout_right::mapping<std::dextents<int, 2>>&>);
+  // different layout policies, constructible and not convertible
+  static_assert(std::is_constructible_v<std::layout_left::mapping<std::dextents<int, 1>>,
+                                        const std::layout_right::mapping<std::dextents<size_t, 1>>&>);
+  static_assert(!std::is_convertible_v<const std::layout_right::mapping<std::dextents<size_t, 1>>&,
+                                       std::layout_left::mapping<std::dextents<int, 1>>>);
+
+  mixin_extents<std::layout_left, ToA>(handle, std::layout_right(), acc);
+  mixin_extents<layout_wrapping_integral<4>, ToA>(handle, layout_wrapping_integral<4>(), acc);
+  // different layout policies, constructible and not convertible
+  static_assert(!std::is_constructible_v<layout_wrapping_integral<8>::mapping<std::dextents<unsigned, 2>>,
+                                         const layout_wrapping_integral<8>::mapping<std::dextents<int, 2>>&>);
+  static_assert(std::is_constructible_v<layout_wrapping_integral<8>::mapping<std::dextents<unsigned, 2>>,
+                                        layout_wrapping_integral<8>::mapping<std::dextents<int, 2>>>);
+  mixin_extents<layout_wrapping_integral<8>, ToA>(handle, layout_wrapping_integral<8>(), acc);
+}
+
+// check that we cover all corners with respect to constructibility and convertibility
+template <bool constructible_constref_acc,
+          bool convertible_constref_acc,
+          bool constructible_nonconst_acc,
+          bool convertible_nonconst_acc,
+          bool constructible_constref_handle,
+          bool convertible_constref_handle,
+          bool constructible_nonconst_handle,
+          bool convertible_nonconst_handle,
+          class ToA,
+          class FromA>
+constexpr bool test(FromA from_acc) {
+  static_assert(std::copyable<ToA>);
+  static_assert(std::copyable<FromA>);
+  static_assert(std::is_constructible_v<ToA, const FromA&> == constructible_constref_acc);
+  static_assert(std::is_constructible_v<ToA, FromA> == constructible_nonconst_acc);
+  static_assert(std::is_constructible_v<typename ToA::data_handle_type, const typename FromA::data_handle_type&> ==
+                constructible_constref_handle);
+  static_assert(std::is_constructible_v<typename ToA::data_handle_type, typename FromA::data_handle_type> ==
+                constructible_nonconst_handle);
+  static_assert(std::is_convertible_v<const FromA&, ToA> == convertible_constref_acc);
+  static_assert(std::is_convertible_v<FromA, ToA> == convertible_nonconst_acc);
+  static_assert(std::is_convertible_v<const typename FromA::data_handle_type&, typename ToA::data_handle_type> ==
+                convertible_constref_handle);
+  static_assert(std::is_convertible_v<typename FromA::data_handle_type, typename ToA::data_handle_type> ==
+                convertible_nonconst_handle);
+
+  ElementPool<typename FromA::element_type, 1024> elements;
+  mixin_layout<ToA>(typename FromA::data_handle_type(elements.get_ptr()), from_acc);
+  return true;
+}
+
+int main(int, char**) {
+  // using shorthands here: t and o for better visual distinguishability
+  constexpr bool t = true;
+  constexpr bool o = false;
+
+  // possibility matrix for constructibility and convertibility https://godbolt.org/z/98KGo8Wbc
+  // you can't have convertibility without constructibility
+  // and if you take const T& then you also can take T
+  // this leaves 7 combinations
+  // const_ref_ctor, const_ref_conv, nonconst_ctor, nonconst_conv, tested
+  // o o o o X
+  // o o t o X
+  // o o t t X
+  // t o t o X
+  // t o t t X
+  // t t t o X
+  // t t t t X
+
+  // checked_accessor has various weird data handles and some weird conversion properties
+  // conv_test_accessor_c/nc is an accessor pair which has configurable conversion properties, but plain ptr as data handle
+  // accessor constructible
+  test<t, t, t, t, t, t, t, t, std::default_accessor<float>>(std::default_accessor<float>());
+  test<t, t, t, t, t, t, t, t, std::default_accessor<const float>>(std::default_accessor<float>());
+  test<t, t, t, t, t, t, t, t, std::default_accessor<MinimalElementType>>(std::default_accessor<MinimalElementType>());
+  test<t, t, t, t, t, t, t, t, std::default_accessor<const MinimalElementType>>(
+      std::default_accessor<MinimalElementType>());
+  test<t, t, t, t, t, t, t, t, checked_accessor<int>>(checked_accessor<int>(1024));
+  test<t, o, t, o, t, t, t, t, checked_accessor<const int>>(checked_accessor<int>(1024));
+  test<t, t, t, t, o, o, o, o, checked_accessor<const unsigned>>(checked_accessor<unsigned>(1024));
+  test<t, t, t, t, t, t, t, t, checked_accessor<float>>(checked_accessor<float>(1024));
+  test<t, t, t, t, t, t, t, t, checked_accessor<double>>(checked_accessor<double>(1024));
+  test<t, t, t, t, t, t, t, t, checked_accessor<MinimalElementType>>(checked_accessor<MinimalElementType>(1024));
+  test<t, o, t, o, t, t, t, t, checked_accessor<const MinimalElementType>>(checked_accessor<MinimalElementType>(1024));
+  test<t, o, t, o, t, t, t, t, conv_test_accessor_c<int, t, t, t, t>>(conv_test_accessor_nc<int, t, t>());
+  test<t, o, t, t, t, t, t, t, conv_test_accessor_c<int, t, t, o, o>>(conv_test_accessor_nc<int, t, o>());
+  test<t, t, t, o, t, t, t, t, conv_test_accessor_c<int, o, t, t, t>>(conv_test_accessor_nc<int, t, t>());
+  test<t, t, t, t, t, t, t, t, conv_test_accessor_c<int, o, o, o, o>>(conv_test_accessor_nc<int, t, o>());
+
+  // ElementType convertible, but accessor not constructible
+  test<o, o, o, o, o, o, o, o, std::default_accessor<float>>(std::default_accessor<int>());
+  test<o, o, o, o, o, o, o, o, checked_accessor<const double>>(checked_accessor<double>(1024));
+  test<o, o, t, t, t, t, t, t, checked_accessor<const float>>(checked_accessor<float>(1024));
+  test<o, o, o, o, t, t, t, t, conv_test_accessor_c<int, o, o, t, t>>(conv_test_accessor_nc<int, o, o>());
+  test<o, o, t, o, t, t, t, t, conv_test_accessor_c<int, o, t, o, o>>(conv_test_accessor_nc<int, o, t>());
+  test<o, o, t, t, t, t, t, t, conv_test_accessor_c<int, o, o, t, t>>(conv_test_accessor_nc<int, o, t>());
+
+  // Ran into trouble with doing it all in one static_assert: exceeding step limit for consteval
+  static_assert(test<t, t, t, t, t, t, t, t, std::default_accessor<float>>(std::default_accessor<float>()));
+  static_assert(test<t, t, t, t, t, t, t, t, std::default_accessor<const float>>(std::default_accessor<float>()));
+  static_assert(test<t, t, t, t, t, t, t, t, std::default_accessor<MinimalElementType>>(
+      std::default_accessor<MinimalElementType>()));
+  static_assert(test<t, t, t, t, t, t, t, t, std::default_accessor<const MinimalElementType>>(
+      std::default_accessor<MinimalElementType>()));
+  static_assert(test<t, t, t, t, t, t, t, t, checked_accessor<int>>(checked_accessor<int>(1024)));
+  static_assert(test<t, o, t, o, t, t, t, t, checked_accessor<const int>>(checked_accessor<int>(1024)));
+  static_assert(test<t, t, t, t, o, o, o, o, checked_accessor<const unsigned>>(checked_accessor<unsigned>(1024)));
+  static_assert(test<t, t, t, t, t, t, t, t, checked_accessor<float>>(checked_accessor<float>(1024)));
+  static_assert(test<t, t, t, t, t, t, t, t, checked_accessor<double>>(checked_accessor<double>(1024)));
+  static_assert(
+      test<t, t, t, t, t, t, t, t, checked_accessor<MinimalElementType>>(checked_accessor<MinimalElementType>(1024)));
+  static_assert(test<t, o, t, o, t, t, t, t, checked_accessor<const MinimalElementType>>(
+      checked_accessor<MinimalElementType>(1024)));
+  static_assert(
+      test<t, o, t, o, t, t, t, t, conv_test_accessor_c<int, t, t, t, t>>(conv_test_accessor_nc<int, t, t>()));
+  static_assert(
+      test<t, o, t, t, t, t, t, t, conv_test_accessor_c<int, t, t, o, o>>(conv_test_accessor_nc<int, t, o>()));
+  static_assert(
+      test<t, t, t, o, t, t, t, t, conv_test_accessor_c<int, o, t, t, t>>(conv_test_accessor_nc<int, t, t>()));
+  static_assert(
+      test<t, t, t, t, t, t, t, t, conv_test_accessor_c<int, o, o, o, o>>(conv_test_accessor_nc<int, t, o>()));
+  static_assert(test<o, o, o, o, o, o, o, o, std::default_accessor<float>>(std::default_accessor<int>()));
+  static_assert(test<o, o, o, o, o, o, o, o, checked_accessor<const double>>(checked_accessor<double>(1024)));
+  static_assert(test<o, o, t, t, t, t, t, t, checked_accessor<const float>>(checked_accessor<float>(1024)));
+  static_assert(
+      test<o, o, o, o, t, t, t, t, conv_test_accessor_c<int, o, o, t, t>>(conv_test_accessor_nc<int, o, o>()));
+  static_assert(
+      test<o, o, t, o, t, t, t, t, conv_test_accessor_c<int, o, t, o, o>>(conv_test_accessor_nc<int, o, t>()));
+  static_assert(
+      test<o, o, t, t, t, t, t, t, conv_test_accessor_c<int, o, o, t, t>>(conv_test_accessor_nc<int, o, t>()));
+
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/conversion.verify.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, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherElementType, class OtherExtents,
+//         class OtherLayoutPolicy, class OtherAccessor>
+//  constexpr explicit(see below)
+//    mdspan(const mdspan<OtherElementType, OtherExtents,
+//                        OtherLayoutPolicy, OtherAccessor>& other);
+//
+// Constraints:
+//   - is_constructible_v<mapping_type, const OtherLayoutPolicy::template mapping<OtherExtents>&> is true, and
+//   - is_constructible_v<accessor_type, const OtherAccessor&> is true.
+// Mandates:
+//   - is_constructible_v<data_handle_type, const OtherAccessor::data_handle_type&> is
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions:
+//   - For each rank index r of extents_type, static_extent(r) == dynamic_extent || static_extent(r) == other.extent(r) is true.
+//   - [0, map_.required_span_size()) is an accessible range of ptr_ and acc_ for values of ptr_, map_, and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with other.ptr_,
+//   - direct-non-list-initializes map_ with other.map_, and
+//   - direct-non-list-initializes acc_ with other.acc_.
+//
+// Remarks: The expression inside explicit is equivalent to:
+//   !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type>
+//   || !is_convertible_v<const OtherAccessor&, accessor_type>
+
+#include <mdspan>
+#include "CustomTestAccessors.h"
+#include "CustomTestLayouts.h"
+
+void cant_construct_data_handle_type() {
+  int data;
+  std::mdspan<int, std::extents<int>, std::layout_right, convertible_accessor_but_not_handle<int>> m_nc(&data);
+  // expected-error-re@*:* {{{{.*}}no matching constructor for initialization of {{.*}} (aka 'not_const_convertible_handle<const int>')}}
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: incompatible data_handle_type for mdspan construction}}
+  [[maybe_unused]] std::
+      mdspan<const int, std::extents<int>, std::layout_right, convertible_accessor_but_not_handle<const int>>
+          m_c(m_nc);
+}
+
+void mapping_constructible_despite_extents_compatibility() {
+  int data;
+  std::mdspan<int, std::extents<int>, always_convertible_layout> m(&data);
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: incompatible extents for mdspan construction}}
+  [[maybe_unused]] std::mdspan<int, std::extents<int, 5>, always_convertible_layout> m2(m);
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.copy.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan(const mdspan&) = default;
+//
+// A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_type are trivially copyable types.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  MDS m_org(handle, map, acc);
+  MDS m(m_org);
+  static_assert(noexcept(MDS(m_org)) == (noexcept(H(handle))&& noexcept(M(map))&& noexcept(A(acc))));
+  static_assert(
+      std::is_trivially_copyable_v<MDS> ==
+      (std::is_trivially_copyable_v<H> && std::is_trivially_copyable_v<M> && std::is_trivially_copyable_v<A>));
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == acc);
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  // make sure we test a trivially copyable mapping
+  static_assert(std::is_trivially_copyable_v<typename std::layout_left::template mapping<std::extents<int>>>);
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  // make sure we test a not trivially copyable mapping
+  static_assert(
+      !std::is_trivially_copyable_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>>);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  // make sure we test trivially constructible accessor and data_handle
+  static_assert(std::is_trivially_copyable_v<std::default_accessor<T>>);
+  static_assert(std::is_trivially_copyable_v<typename std::default_accessor<T>::data_handle_type>);
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is noexcept copy constructible except for const double
+  checked_accessor<T> acc(1024);
+  static_assert(noexcept(checked_accessor<T>(acc)) != std::is_same_v<T, const double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), acc);
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.default.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan();
+// Constraints:
+//   - rank_dynamic() > 0 is true.
+//   - is_default_constructible_v<data_handle_type> is true.
+//   - is_default_constructible_v<mapping_type> is true.
+//   - is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, map_.required_span_size()) is an accessible range of ptr_
+//               and acc_ for the values of map_ and acc_ after the invocation of this constructor.
+//
+// Effects: Value-initializes ptr_, map_, and acc_.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <bool hc, bool mc, bool ac, class H, class M, class A>
+constexpr void test_mdspan_types(const H&, const M&, const A&) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  static_assert(hc == std::is_default_constructible_v<H>);
+  static_assert(mc == std::is_default_constructible_v<M>);
+  static_assert(ac == std::is_default_constructible_v<A>);
+
+  if constexpr (MDS::rank_dynamic() > 0 && hc && mc && ac) {
+    MDS m;
+    static_assert(noexcept(MDS()) == (noexcept(H())&& noexcept(M())&& noexcept(A())));
+    assert(m.extents() == typename MDS::extents_type());
+    if constexpr (std::equality_comparable<H>)
+      assert(m.data_handle() == H());
+    if constexpr (std::equality_comparable<M>)
+      assert(m.mapping() == M());
+    if constexpr (std::equality_comparable<A>)
+      assert(m.accessor() == A());
+  } else {
+    static_assert(!std::is_default_constructible_v<MDS>);
+  }
+}
+
+template <bool hc, bool mc, bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types<hc, mc, ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types<hc, mc, ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types<hc, mc, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types<hc, mc, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types<hc, mc, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types<hc, mc, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <bool hc, bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<hc, true, ac>(handle, std::layout_left(), acc);
+  mixin_extents<hc, true, ac>(handle, std::layout_right(), acc);
+
+  // Use weird layout, make sure it has the properties we want to test
+  constexpr size_t D = std::dynamic_extent;
+  static_assert(
+      !std::is_default_constructible_v< typename layout_wrapping_integral<4>::template mapping<std::extents<char, D>>>);
+  mixin_extents<hc, false, ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true, true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  // checked_accessor's data handle type is not default constructible for double
+  static_assert(
+      std::is_default_constructible_v<typename checked_accessor<T>::data_handle_type> != std::is_same_v<T, double>);
+  mixin_layout<!std::is_same_v<T, double>, std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_array.pass.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, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherIndexType, size_t N>
+//   constexpr explicit(N != rank_dynamic())
+//     mdspan(data_handle_type p, const array<OtherIndexType, N>& exts);
+//
+// Constraints:
+//   - is_convertible_v<const OtherIndexType&, index_type> is true,
+//   - (is_nothrow_constructible<index_type, const OtherIndexType&> && ...) is true,
+//   - N == rank() || N == rank_dynamic() is true,
+//   - is_constructible_v<mapping_type, extents_type> is true, and
+//   - is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_
+//                for the values of map_ and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with extents_type(exts), and
+//   - value-initializes acc_.
+
+#include <array>
+#include <concepts>
+#include <cassert>
+#include <mdspan>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#include "../ConvertibleToIntegral.h"
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class Extents, size_t... Idxs>
+constexpr auto array_from_extents(const Extents& exts, std::index_sequence<Idxs...>) {
+  return std::array<typename Extents::index_type, Extents::rank()>{exts.extent(Idxs)...};
+}
+
+template <class MDS, class Exts>
+concept check_mdspan_ctor_implicit = requires(MDS m, typename MDS::data_handle_type h, const Exts& exts) {
+  m = {h, exts};
+};
+
+template <class H, class M, class A, size_t N>
+constexpr void
+test_mdspan_ctor_array(const H& handle, const M& map, const A&, std::array<typename M::index_type, N> exts) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+  if !consteval {
+    move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+  }
+  MDS m(handle, exts);
+  if !consteval {
+    if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+      assert((H::move_counter() == 1));
+    }
+  }
+
+  static_assert(!noexcept(MDS(handle, exts)));
+
+  static_assert(check_mdspan_ctor_implicit<MDS, decltype(exts)> == (N == MDS::rank_dynamic()));
+
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == A());
+}
+
+template <bool mec, bool ac, class H, class M, class A>
+constexpr void test_mdspan_ctor(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+  static_assert(mec == std::is_constructible_v<M, typename M::extents_type>);
+  static_assert(ac == std::is_default_constructible_v<A>);
+  if constexpr (mec && ac) {
+    // test from all extents
+    auto exts = array_from_extents(map.extents(), std::make_index_sequence<MDS::rank()>());
+    test_mdspan_ctor_array(handle, map, acc, exts);
+
+    // test from dynamic extents
+    std::array<typename MDS::index_type, MDS::rank_dynamic()> exts_dynamic{};
+    size_t r_dyn = 0;
+    for (size_t r = 0; r < MDS::rank(); r++) {
+      if (MDS::static_extent(r) == std::dynamic_extent)
+        exts_dynamic[r_dyn++] = exts[r];
+    }
+    test_mdspan_ctor_array(handle, map, acc, exts_dynamic);
+  } else {
+    static_assert(!std::is_constructible_v<MDS, const H&, const std::array<typename MDS::index_type, MDS::rank()>&>);
+  }
+}
+
+template <bool mec, bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_ctor<mec, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<true, ac>(handle, std::layout_left(), acc);
+  mixin_extents<true, ac>(handle, std::layout_right(), acc);
+
+  // Sanity check that this layouts mapping is constructible from extents (via its move constructor)
+  static_assert(std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                        std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<true, ac>(handle, layout_wrapping_integral<8>(), acc);
+  // Sanity check that this layouts mapping is not constructible from extents
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<false, ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  mixin_layout<std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // test non-constructibility from wrong array type
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<unsigned, 3, D, D>>;
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, std::array<int, 3>>);
+  static_assert(std::is_constructible_v<mds_t, float*, std::array<int, 2>>);
+  // wrong size
+  static_assert(!std::is_constructible_v<mds_t, float*, std::array<int, 1>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::array<int, 4>>);
+  // not convertible to index_type
+  static_assert(std::is_convertible_v<const IntType&, int>);
+  static_assert(!std::is_convertible_v<const IntType&, unsigned>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::array<IntType, 2>>);
+
+  // index_type is not nothrow constructible
+  using mds_uchar_t = std::mdspan<float, std::extents<unsigned char, 3, D, D>>;
+  static_assert(std::is_convertible_v<IntType, unsigned char>);
+  static_assert(std::is_convertible_v<const IntType&, unsigned char>);
+  static_assert(!std::is_nothrow_constructible_v<unsigned char, const IntType&>);
+  static_assert(!std::is_constructible_v<mds_uchar_t, float*, std::array<IntType, 2>>);
+
+  // convertible from non-const to index_type but not  from const
+  using mds_int_t = std::mdspan<float, std::extents<int, 3, D, D>>;
+  static_assert(std::is_convertible_v<IntTypeNC, int>);
+  static_assert(!std::is_convertible_v<const IntTypeNC&, int>);
+  static_assert(std::is_nothrow_constructible_v<int, IntTypeNC>);
+  static_assert(!std::is_constructible_v<mds_int_t, float*, std::array<IntTypeNC, 2>>);
+
+  // can't test a combo where std::is_nothrow_constructible_v<int, const IntTypeNC&> is true,
+  // but std::is_convertible_v<const IntType&, int> is false
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, std::array<int, 2>>);
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_extents.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan(data_handle_type p, const extents_type& ext);
+//
+// Constraints:
+//   - is_constructible_v<mapping_type, const extents_type&> is true, and
+//   - is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_
+//                for the values of map_ and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with ext, and
+//   - value-initializes acc_.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <bool mec, bool ac, class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A&) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  static_assert(mec == std::is_constructible_v<M, const typename M::extents_type&>);
+  static_assert(ac == std::is_default_constructible_v<A>);
+  if constexpr (mec && ac) {
+    if !consteval {
+      move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+    }
+    // use formulation of constructor which tests that its not explicit
+    MDS m = {handle, map.extents()};
+    if !consteval {
+      if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+        assert((H::move_counter() == 1));
+      }
+    }
+    static_assert(!noexcept(MDS(handle, map.extents())));
+    assert(m.extents() == map.extents());
+    if constexpr (std::equality_comparable<H>)
+      assert(m.data_handle() == handle);
+    if constexpr (std::equality_comparable<M>)
+      assert(m.mapping() == map);
+    if constexpr (std::equality_comparable<A>)
+      assert(m.accessor() == A());
+  } else {
+    static_assert(!std::is_constructible_v<MDS, const H&, const typename M::extents_type&>);
+  }
+}
+
+template <bool mec, bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types<mec, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<true, ac>(handle, std::layout_left(), acc);
+  mixin_extents<true, ac>(handle, std::layout_right(), acc);
+
+  // Use weird layout, make sure it has the properties we want to test
+  // Sanity check that this layouts mapping is constructible from extents (via its move constructor)
+  static_assert(std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                        std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<false, ac>(handle, layout_wrapping_integral<8>(), acc);
+  // Sanity check that this layouts mapping is not constructible from extents
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<false, ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  mixin_layout<std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // test non-constructibility from wrong extents type
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<int, 3, D, D>>;
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, std::extents<int, 3, D, D>>);
+  // wrong size
+  static_assert(!std::is_constructible_v<mds_t, float*, std::extents<int, D, D>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::extents<int, D, D, D, D>>);
+  // wrong type in general: note the extents constructor does NOT convert, since it takes by const&
+  static_assert(!std::is_constructible_v<mds_t, float*, std::extents<int, D, D, D>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::extents<unsigned, 3, D, D>>);
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, std::extents<int, 3, D, D>>);
+
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_integers.pass.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, c++17, c++20
+
+// <mdspan>
+
+// template<class... OtherIndexTypes>
+//   constexpr explicit mdspan(data_handle_type p, OtherIndexTypes... exts);
+//
+// Let N be sizeof...(OtherIndexTypes).
+//
+// Constraints:
+//   - (is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
+//   - (is_nothrow_constructible<index_type, OtherIndexTypes> && ...) is true,
+//   - N == rank() || N == rank_dynamic() is true,
+//   - is_constructible_v<mapping_type, extents_type> is true, and
+//   - is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_
+//                for the values of map_ and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with extents_type(static_cast<index_type>(std::move(exts))...), and
+//   - value-initializes acc_.
+
+#include <array>
+#include <concepts>
+#include <cassert>
+#include <mdspan>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class MDS, class... Args>
+concept check_mdspan_ctor_implicit = requires(MDS m, Args... args) { m = {args...}; };
+
+template <bool mec, bool ac, class H, class M, class A, class... Idxs>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A&, Idxs... idxs) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  static_assert(mec == std::is_constructible_v<M, typename M::extents_type>);
+  static_assert(ac == std::is_default_constructible_v<A>);
+
+  if constexpr (mec && ac) {
+    if !consteval {
+      move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+    }
+    MDS m(handle, idxs...);
+    if !consteval {
+      if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+        assert((H::move_counter() == 1));
+      }
+    }
+
+    // sanity check that concept works
+    static_assert(check_mdspan_ctor_implicit<MDS, H, std::array<typename MDS::index_type, MDS::rank_dynamic()>>);
+    // check that the constructor from integral is explicit
+    static_assert(!check_mdspan_ctor_implicit<MDS, H, Idxs...>);
+
+    assert(m.extents() == map.extents());
+    if constexpr (std::equality_comparable<H>)
+      assert(m.data_handle() == handle);
+    if constexpr (std::equality_comparable<M>)
+      assert(m.mapping() == map);
+    if constexpr (std::equality_comparable<A>)
+      assert(m.accessor() == A());
+  } else {
+    static_assert(!std::is_constructible_v<MDS, const H&, Idxs... >);
+  }
+}
+
+template <bool mec, bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  // construct from just dynamic extents
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc, 7);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc, 2, 3);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc, 0, 3);
+  test_mdspan_types<mec, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc, 1, 2, 3, 2);
+
+  // construct from all extents
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc, 7);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc, 2, 4, 3);
+  test_mdspan_types<mec, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc, 0, 7, 3);
+  test_mdspan_types<mec, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc, 1, 7, 2, 4, 3, 2);
+}
+
+template <bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<true, ac>(handle, std::layout_left(), acc);
+  mixin_extents<true, ac>(handle, std::layout_right(), acc);
+
+  // Use weird layout, make sure it has the properties we want to test
+  // Sanity check that this layouts mapping is constructible from extents (via its move constructor)
+  static_assert(std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                        std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<true, ac>(handle, layout_wrapping_integral<8>(), acc);
+  // Sanity check that this layouts mapping is not constructible from extents
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<false, ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  mixin_layout<std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // test non-constructibility from wrong integer types
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<int, 3, D, D>>;
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, int, int, int>);
+  static_assert(std::is_constructible_v<mds_t, float*, int, int>);
+  // wrong number of arguments
+  static_assert(!std::is_constructible_v<mds_t, float*, int>);
+  static_assert(!std::is_constructible_v<mds_t, float*, int, int, int, int>);
+  // not convertible to int
+  static_assert(!std::is_constructible_v<mds_t, float*, int, int, std::dextents<int, 1>>);
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, int, int>);
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_map.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan(data_handle_type p, const mapping_type& m);
+//
+// Constraints: is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, m.required_span_size()) is an accessible range of p and acc_
+//                for the value of acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with m, and
+//   - value-initializes acc_.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <bool ac, class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A&) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  static_assert(ac == std::is_default_constructible_v<A>);
+  if constexpr (ac) {
+    if !consteval {
+      move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+    }
+    // use formulation of constructor which tests that it is not explicit
+    MDS m = {handle, map};
+    if !consteval {
+      if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+        assert((H::move_counter() == 1));
+      }
+    }
+    static_assert(!noexcept(MDS(handle, map)));
+    assert(m.extents() == map.extents());
+    if constexpr (std::equality_comparable<H>)
+      assert(m.data_handle() == handle);
+    if constexpr (std::equality_comparable<M>)
+      assert(m.mapping() == map);
+    if constexpr (std::equality_comparable<A>)
+      assert(m.accessor() == A());
+  } else {
+    static_assert(!std::is_constructible_v<MDS, const H&, const M&>);
+  }
+}
+
+template <bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types<ac>(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<ac>(handle, std::layout_left(), acc);
+  mixin_extents<ac>(handle, std::layout_right(), acc);
+  mixin_extents<ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  mixin_layout<std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+template <class E>
+using mapping_t = typename std::layout_right::template mapping<E>;
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<int, 3, D, D>>;
+
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, 3, D, D>>>);
+
+  // test non-constructibility from wrong mapping type
+  // wrong rank
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D>>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D, D, D>>>);
+  // wrong type in general: note the map constructor does NOT convert, since it takes by const&
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D, D>>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<unsigned, 3, D, D>>>);
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, mapping_t<std::extents<int, 3, D, D>>>);
+
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_map_acc.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan(data_handle_type p, const mapping_type& m, const accessor_type& a);
+//
+// Preconditions: [0, m.required_span_size()) is an accessible range of p and a.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with m, and
+//   - direct-non-list-initializes acc_ with a.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  if !consteval {
+    move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+  }
+  // use formulation of constructor which tests that it is not explicit
+  MDS m = {handle, map, acc};
+  if !consteval {
+    if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+      assert((H::move_counter() == 1));
+    }
+  }
+  static_assert(!noexcept(MDS(handle, map, acc)));
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == acc);
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  // checked_accessor's data handle type is not default constructible for double
+  static_assert(
+      std::is_default_constructible_v<typename checked_accessor<T>::data_handle_type> != std::is_same_v<T, double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+template <class E>
+using mapping_t = typename std::layout_right::template mapping<E>;
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // test non-constructibility from wrong args
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<int, 3, D, D>>;
+  using acc_t        = std::default_accessor<float>;
+
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, 3, D, D>>, acc_t>);
+
+  // test non-constructibility from wrong accessor
+  static_assert(
+      !std::
+          is_constructible_v<mds_t, float*, mapping_t<std::extents<int, 3, D, D>>, std::default_accessor<const float>>);
+
+  // test non-constructibility from wrong mapping type
+  // wrong rank
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D>>, acc_t>);
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D, D, D>>, acc_t>);
+  // wrong type in general: note the map constructor does NOT convert, since it takes by const&
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<int, D, D, D>>, acc_t>);
+  static_assert(!std::is_constructible_v<mds_t, float*, mapping_t<std::extents<unsigned, 3, D, D>>, acc_t>);
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, mapping_t<std::extents<int, 3, D, D>>, acc_t>);
+
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_span.pass.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, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherIndexType, size_t N>
+//   constexpr explicit(N != rank_dynamic())
+//     mdspan(data_handle_type p, span<OtherIndexType, N> exts);
+//
+// Constraints:
+//   - is_convertible_v<const OtherIndexType&, index_type> is true,
+//   - (is_nothrow_constructible<index_type, const OtherIndexType&> && ...) is true,
+//   - N == rank() || N == rank_dynamic() is true,
+//   - is_constructible_v<mapping_type, extents_type> is true, and
+//   - is_default_constructible_v<accessor_type> is true.
+//
+// Preconditions: [0, map_.required_span_size()) is an accessible range of p and acc_
+//                for the values of map_ and acc_ after the invocation of this constructor.
+//
+// Effects:
+//   - Direct-non-list-initializes ptr_ with std::move(p),
+//   - direct-non-list-initializes map_ with extents_type(exts), and
+//   - value-initializes acc_.
+
+#include <array>
+#include <concepts>
+#include <cassert>
+#include <mdspan>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#include "../ConvertibleToIntegral.h"
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class Extents, size_t... Idxs>
+constexpr auto array_from_extents(const Extents& exts, std::index_sequence<Idxs...>) {
+  return std::array<typename Extents::index_type, Extents::rank()>{exts.extent(Idxs)...};
+}
+
+template <class MDS, class Exts>
+concept check_mdspan_ctor_implicit = requires(MDS m, typename MDS::data_handle_type h, const Exts& exts) {
+  m = {h, exts};
+};
+
+template <class H, class M, class A, size_t N>
+constexpr void
+test_mdspan_ctor_span(const H& handle, const M& map, const A&, std::span<typename M::index_type, N> exts) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+  if !consteval {
+    move_counted_handle<typename MDS::element_type>::move_counter() = 0;
+  }
+  MDS m(handle, exts);
+  if !consteval {
+    if constexpr (std::is_same_v<H, move_counted_handle<typename MDS::element_type>>) {
+      assert((H::move_counter() == 1));
+    }
+  }
+
+  static_assert(!noexcept(MDS(handle, exts)));
+
+  static_assert(check_mdspan_ctor_implicit<MDS, decltype(exts)> == (N == MDS::rank_dynamic()));
+
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == A());
+}
+
+template <bool mec, bool ac, class H, class M, class A>
+constexpr void test_mdspan_ctor(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+  static_assert(mec == std::is_constructible_v<M, typename M::extents_type>);
+  static_assert(ac == std::is_default_constructible_v<A>);
+  if constexpr (mec && ac) {
+    // test from all extents
+    auto exts = array_from_extents(map.extents(), std::make_index_sequence<MDS::rank()>());
+    test_mdspan_ctor_span(handle, map, acc, std::span(exts));
+
+    // test from dynamic extents
+    std::array<typename MDS::index_type, MDS::rank_dynamic()> exts_dynamic{};
+    size_t r_dyn = 0;
+    for (size_t r = 0; r < MDS::rank(); r++) {
+      if (MDS::static_extent(r) == std::dynamic_extent)
+        exts_dynamic[r_dyn++] = exts[r];
+    }
+    test_mdspan_ctor_span(handle, map, acc, std::span(exts_dynamic));
+  } else {
+    static_assert(!std::is_constructible_v<MDS, const H&, std::span<typename MDS::index_type, MDS::rank()>>);
+  }
+}
+
+template <bool mec, bool ac, class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_ctor<mec, ac>(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_ctor<mec, ac>(
+      handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <bool ac, class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents<true, ac>(handle, std::layout_left(), acc);
+  mixin_extents<true, ac>(handle, std::layout_right(), acc);
+
+  // Sanity check that this layouts mapping is constructible from extents (via its move constructor)
+  static_assert(std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                        std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<8>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<true, ac>(handle, layout_wrapping_integral<8>(), acc);
+  // Sanity check that this layouts mapping is not constructible from extents
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         std::extents<int>>);
+  static_assert(!std::is_constructible_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>,
+                                         const std::extents<int>&>);
+  mixin_extents<false, ac>(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout<true>(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is not default constructible except for const double, where it is not noexcept
+  static_assert(std::is_default_constructible_v<checked_accessor<T>> == std::is_same_v<T, const double>);
+  mixin_layout<std::is_same_v<T, const double>>(
+      typename checked_accessor<T>::data_handle_type(elements.get_ptr()), checked_accessor<T>(1024));
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // test non-constructibility from wrong span type
+  constexpr size_t D = std::dynamic_extent;
+  using mds_t        = std::mdspan<float, std::extents<unsigned, 3, D, D>>;
+  // sanity check
+  static_assert(std::is_constructible_v<mds_t, float*, std::span<int, 3>>);
+  static_assert(std::is_constructible_v<mds_t, float*, std::span<int, 2>>);
+  // wrong size
+  static_assert(!std::is_constructible_v<mds_t, float*, std::span<int, 1>>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::span<int, 4>>);
+  // not convertible to index_type
+  static_assert(std::is_convertible_v<const IntType&, int>);
+  static_assert(!std::is_convertible_v<const IntType&, unsigned>);
+  static_assert(!std::is_constructible_v<mds_t, float*, std::span<IntType, 2>>);
+
+  // index_type is not nothrow constructible
+  using mds_uchar_t = std::mdspan<float, std::extents<unsigned char, 3, D, D>>;
+  static_assert(std::is_convertible_v<IntType, unsigned char>);
+  static_assert(std::is_convertible_v<const IntType&, unsigned char>);
+  static_assert(!std::is_nothrow_constructible_v<unsigned char, const IntType&>);
+  static_assert(!std::is_constructible_v<mds_uchar_t, float*, std::span<IntType, 2>>);
+
+  // convertible from non-const to index_type but not  from const
+  using mds_int_t = std::mdspan<float, std::extents<int, 3, D, D>>;
+  static_assert(std::is_convertible_v<IntTypeNC, int>);
+  static_assert(!std::is_convertible_v<const IntTypeNC&, int>);
+  static_assert(std::is_nothrow_constructible_v<int, IntTypeNC>);
+  static_assert(!std::is_constructible_v<mds_int_t, float*, std::span<IntTypeNC, 2>>);
+
+  // can't test a combo where std::is_nothrow_constructible_v<int, const IntTypeNC&> is true,
+  // but std::is_convertible_v<const IntType&, int> is false
+
+  // test non-constructibility from wrong handle_type
+  static_assert(!std::is_constructible_v<mds_t, const float*, std::span<int, 2>>);
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/ctor.move.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan(mdspan&&) = default;
+//
+// A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_type are trivially copyable types.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  MDS m_org(handle, map, acc);
+  MDS m(std::move(m_org));
+  static_assert(std::is_trivially_move_constructible_v<MDS> ==
+                (std::is_trivially_move_constructible_v<H> && std::is_trivially_move_constructible_v<M> &&
+                 std::is_trivially_move_constructible_v<A>));
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == acc);
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  // make sure we test a trivially copyable mapping
+  static_assert(std::is_trivially_move_constructible_v<typename std::layout_left::template mapping<std::extents<int>>>);
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  // make sure we test a not trivially copyable mapping
+  static_assert(!std::is_trivially_move_constructible_v<
+                typename layout_wrapping_integral<4>::template mapping<std::extents<int>>>);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  // make sure we test trivially constructible accessor and data_handle
+  static_assert(std::is_trivially_move_constructible_v<std::default_accessor<T>>);
+  static_assert(std::is_trivially_move_constructible_v<typename std::default_accessor<T>::data_handle_type>);
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is noexcept copy constructible except for const double
+  checked_accessor<T> acc(1024);
+  static_assert(std::is_trivially_move_constructible_v<typename checked_accessor<T>::data_handle_type> ==
+                std::is_same_v<T, double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), acc);
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.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, c++17, c++20
+
+// <mdspan>
+
+//  template<class CArray>
+//    requires(is_array_v<CArray> && rank_v<CArray> == 1)
+//    mdspan(CArray&)
+//      -> mdspan<remove_all_extents_t<CArray>, extents<size_t, extent_v<CArray, 0>>>;
+//
+//  template<class Pointer>
+//    requires(is_pointer_v<remove_reference_t<Pointer>>)
+//    mdspan(Pointer&&)
+//      -> mdspan<remove_pointer_t<remove_reference_t<Pointer>>, extents<size_t>>;
+//
+//  template<class ElementType, class... Integrals>
+//    requires((is_convertible_v<Integrals, size_t> && ...) && sizeof...(Integrals) > 0)
+//    explicit mdspan(ElementType*, Integrals...)
+//      -> mdspan<ElementType, dextents<size_t, sizeof...(Integrals)>>;
+//
+//  template<class ElementType, class OtherIndexType, size_t N>
+//    mdspan(ElementType*, span<OtherIndexType, N>)
+//      -> mdspan<ElementType, dextents<size_t, N>>;
+//
+//  template<class ElementType, class OtherIndexType, size_t N>
+//    mdspan(ElementType*, const array<OtherIndexType, N>&)
+//      -> mdspan<ElementType, dextents<size_t, N>>;
+//
+//  template<class ElementType, class IndexType, size_t... ExtentsPack>
+//    mdspan(ElementType*, const extents<IndexType, ExtentsPack...>&)
+//      -> mdspan<ElementType, extents<IndexType, ExtentsPack...>>;
+//
+//  template<class ElementType, class MappingType>
+//    mdspan(ElementType*, const MappingType&)
+//      -> mdspan<ElementType, typename MappingType::extents_type,
+//                typename MappingType::layout_type>;
+//
+//  template<class MappingType, class AccessorType>
+//    mdspan(const typename AccessorType::data_handle_type&, const MappingType&,
+//           const AccessorType&)
+//      -> mdspan<typename AccessorType::element_type, typename MappingType::extents_type,
+//                typename MappingType::layout_type, AccessorType>;
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  // deduction from data_handle_type (including non-pointer), mapping and accessor
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle, map, acc)), MDS);
+
+  if constexpr (std::is_same_v<A, std::default_accessor<typename A::element_type>>) {
+    // deduction from pointer and mapping
+    // non-pointer data-handle-types have other accessor
+    ASSERT_SAME_TYPE(decltype(std::mdspan(handle, map)), MDS);
+    if constexpr (std::is_same_v<typename M::layout_type, std::layout_right>) {
+      // deduction from pointer and extents
+      ASSERT_SAME_TYPE(decltype(std::mdspan(handle, map.extents())), MDS);
+    }
+  }
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+struct SizeTIntType {
+  size_t val;
+  constexpr SizeTIntType(size_t val_) : val(val_) {}
+  constexpr operator size_t() const noexcept { return size_t(val); }
+};
+
+template <class H, class A>
+  requires(sizeof(decltype(std::mdspan(std::declval<H>(), 10))) > 0)
+constexpr bool test_no_layout_deduction_guides(const H& handle, const A&) {
+  using T = typename A::element_type;
+  // deduction from pointer alone
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle)), std::mdspan<T, std::extents<size_t>>);
+  // deduction from pointer and integral like
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle, 5, SizeTIntType(6))), std::mdspan<T, std::dextents<size_t, 2>>);
+
+  std::array<char, 3> exts;
+  // deduction from pointer and array
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle, exts)), std::mdspan<T, std::dextents<size_t, 3>>);
+  // deduction from pointer and span
+  ASSERT_SAME_TYPE(decltype(std::mdspan(handle, std::span(exts))), std::mdspan<T, std::dextents<size_t, 3>>);
+  return true;
+}
+
+template <class H, class A>
+constexpr bool test_no_layout_deduction_guides(const H&, const A&) {
+  return false;
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+
+  // checking that there is no deduction happen for non-pointer handle type
+  assert((test_no_layout_deduction_guides(handle, acc) == std::is_same_v<H, typename A::element_type*>));
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is noexcept copy constructible except for const double
+  checked_accessor<T> acc(1024);
+  static_assert(noexcept(checked_accessor<T>(acc)) != std::is_same_v<T, const double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), acc);
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // deduction from array alone
+  float a[12];
+  ASSERT_SAME_TYPE(decltype(std::mdspan(a)), std::mdspan<float, std::extents<size_t, 12>>);
+
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/element_type.verify.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, c++17, c++20
+
+// <mdspan>
+
+// template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = default_accessor>
+// class mdspan;
+//
+// Mandates:
+//   - ElementType is a complete object type that is neither an abstract class type nor an array type.
+//   - is_same_v<ElementType, typename AccessorPolicy::element_type> is true.
+
+#include <mdspan>
+
+class AbstractClass {
+public:
+  virtual void method() = 0;
+};
+
+void not_abstract_class() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: ElementType template parameter may not be an abstract class}}
+  [[maybe_unused]] std::mdspan<AbstractClass, std::extents<int>> m;
+}
+
+void not_array_type() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: ElementType template parameter may not be an array type}}
+  [[maybe_unused]] std::mdspan<int[5], std::extents<int>> m;
+}
+
+void element_type_mismatch() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: ElementType template parameter must match AccessorPolicy::element_type}}
+  [[maybe_unused]] std::mdspan<int, std::extents<int>, std::layout_right, std::default_accessor<const int>> m;
+}

libcxx/test/std/containers/views/mdspan/mdspan/extents.verify.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, c++17, c++20
+
+// <mdspan>
+
+// template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = default_accessor>
+// class mdspan;
+//
+// Mandates:
+//  - Extents is a specialization of extents
+
+#include <mdspan>
+
+void not_extents() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: Extents template parameter must be a specialization of extents.}}
+  [[maybe_unused]] std::mdspan<int, int> m;
+}

libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.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, c++17, c++20
+
+// <mdspan>
+
+// Test default iteration:
+//
+// template<class... Indices>
+//   constexpr reference operator[](Indices...) const noexcept;
+//
+// Constraints:
+//   * sizeof...(Indices) == extents_type::rank() is true,
+//   * (is_convertible_v<Indices, index_type> && ...) is true, and
+//   * (is_nothrow_constructible_v<index_type, Indices> && ...) is true.
+//
+// Preconditions:
+//   * extents_type::index-cast(i) is a multidimensional index in extents_.
+
+// GCC warns about comma operator changing its meaning inside [] in C++23
+#if defined(__GNUC__) && !defined(__clang_major__)
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wcomma-subscript"
+#endif
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+#include "../ConvertibleToIntegral.h"
+#include "CustomTestLayouts.h"
+
+// Clang 15 and 16 do not support argument packs as input to operator []
+#if defined(__clang_major__) && __clang_major__ < 17
+template <class MDS>
+constexpr auto& access(MDS mds) {
+  return mds[];
+}
+template <class MDS>
+constexpr auto& access(MDS mds, int64_t i0) {
+  return mds[i0];
+}
+template <class MDS>
+constexpr auto& access(MDS mds, int64_t i0, int64_t i1) {
+  return mds[i0, i1];
+}
+template <class MDS>
+constexpr auto& access(MDS mds, int64_t i0, int64_t i1, int64_t i2) {
+  return mds[i0, i1, i2];
+}
+template <class MDS>
+constexpr auto& access(MDS mds, int64_t i0, int64_t i1, int64_t i2, int64_t i3) {
+  return mds[i0, i1, i2, i3];
+}
+#endif
+
+template <class MDS, class... Indices>
+concept operator_constraints = requires(MDS m, Indices... idxs) {
+  { std::is_same_v<decltype(m[idxs...]), typename MDS::reference> };
+};
+
+template <class MDS, class... Indices>
+  requires(operator_constraints<MDS, Indices...>)
+constexpr bool check_operator_constraints(MDS m, Indices... idxs) {
+  (void)m[idxs...];
+  return true;
+}
+
+template <class MDS, class... Indices>
+constexpr bool check_operator_constraints(MDS, Indices...) {
+  return false;
+}
+
+template <class MDS, class... Args>
+constexpr void iterate(MDS mds, Args... args) {
+  constexpr int r = static_cast<int>(MDS::extents_type::rank()) - 1 - static_cast<int>(sizeof...(Args));
+  if constexpr (-1 == r) {
+#if defined(__clang_major__) && __clang_major__ < 17
+    int* ptr1 = &access(mds, args...);
+#else
+    int* ptr1 = &mds[args...];
+#endif
+    int* ptr2 = &(mds.accessor().access(mds.data_handle(), mds.mapping()(args...)));
+    assert(ptr1 == ptr2);
+
+    std::array<typename MDS::index_type, MDS::rank()> args_arr{static_cast<typename MDS::index_type>(args)...};
+    int* ptr3 = &mds[args_arr];
+    assert(ptr3 == ptr2);
+    int* ptr4 = &mds[std::span(args_arr)];
+    assert(ptr4 == ptr2);
+  } else {
+    for (typename MDS::index_type i = 0; i < mds.extents().extent(r); i++) {
+      iterate(mds, i, args...);
+    }
+  }
+}
+
+template <class Mapping>
+constexpr void test_iteration(Mapping m) {
+  std::array<int, 1024> data;
+  using MDS = std::mdspan<int, typename Mapping::extents_type, typename Mapping::layout_type>;
+  MDS mds(data.data(), m);
+
+  iterate(mds);
+}
+
+template <class Layout>
+constexpr void test_layout() {
+  constexpr size_t D = std::dynamic_extent;
+  test_iteration(construct_mapping(Layout(), std::extents<int>()));
+  test_iteration(construct_mapping(Layout(), std::extents<unsigned, D>(1)));
+  test_iteration(construct_mapping(Layout(), std::extents<unsigned, D>(7)));
+  test_iteration(construct_mapping(Layout(), std::extents<unsigned, 7>()));
+  test_iteration(construct_mapping(Layout(), std::extents<unsigned, 7, 8>()));
+  test_iteration(construct_mapping(Layout(), std::extents<char, D, D, D, D>(1, 1, 1, 1)));
+
+// TODO enable for GCC 13, when the CI pipeline is switched, doesn't work with GCC 12
+#if defined(__clang_major__) && __clang_major__ >= 17
+  int data[1];
+  // Check operator constraint for number of arguments
+  static_assert(check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), 0));
+  static_assert(
+      !check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), 0, 0));
+
+  // Check operator constraint for convertibility of arguments to index_type
+  static_assert(
+      check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), IntType(0)));
+  static_assert(!check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<unsigned, D>(1))), IntType(0)));
+
+  // Check operator constraint for no-throw-constructibility of index_type from arguments
+  static_assert(!check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<unsigned char, D>(1))), IntType(0)));
+
+  // Check that mixed integrals work: note the second one tests that mdspan casts: layout_wrapping_integral does not accept IntType
+  static_assert(check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<unsigned char, D, D>(1, 1))), int(0), size_t(0)));
+  static_assert(check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))), unsigned(0), IntType(0)));
+
+  constexpr bool t = true;
+  constexpr bool o = false;
+  static_assert(!check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))),
+      unsigned(0),
+      IntConfig<o, o, t, t>(0)));
+  static_assert(check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))),
+      unsigned(0),
+      IntConfig<o, t, t, t>(0)));
+  static_assert(check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))),
+      unsigned(0),
+      IntConfig<o, t, o, t>(0)));
+  static_assert(!check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))),
+      unsigned(0),
+      IntConfig<t, o, o, t>(0)));
+  static_assert(check_operator_constraints(
+      std::mdspan(data, construct_mapping(Layout(), std::extents<int, D, D>(1, 1))),
+      unsigned(0),
+      IntConfig<t, o, t, o>(0)));
+
+  // layout_wrapped wouldn't quite work here the way we wrote the check
+  // IntConfig has configurable conversion properties: convert from const&, convert from non-const, no-throw-ctor from const&, no-throw-ctor from non-const
+  if constexpr (std::is_same_v<Layout, std::layout_left>) {
+    static_assert(!check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<o, o, t, t>(0)}));
+    static_assert(!check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<o, t, t, t>(0)}));
+    static_assert(!check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<t, o, o, t>(0)}));
+    static_assert(!check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<t, t, o, t>(0)}));
+    static_assert(check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<t, o, t, o>(0)}));
+    static_assert(check_operator_constraints(
+        std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), std::array{IntConfig<t, t, t, t>(0)}));
+
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+    {
+      std::array idx{IntConfig<o, o, t, t>(0)};
+      std::span s(idx);
+      assert(!check_operator_constraints(std::mdspan(data, construct_mapping(Layout(), std::extents<int, D>(1))), s));
+    }
+  }
+#endif
+}
+
+template <class Layout>
+constexpr void test_layout_large() {
+  constexpr size_t D = std::dynamic_extent;
+  test_iteration(construct_mapping(Layout(), std::extents<int64_t, D, 4, D, D>(3, 5, 6)));
+  test_iteration(construct_mapping(Layout(), std::extents<int64_t, D, 4, 1, D>(3, 6)));
+}
+
+// mdspan::operator[] casts to index_type before calling mapping
+// mapping requirements only require the index operator to mixed integer types not anything convertible to index_type
+constexpr void test_index_cast_happens() {}
+
+constexpr bool test() {
+  test_layout<std::layout_left>();
+  test_layout<std::layout_right>();
+  test_layout<layout_wrapping_integral<4>>();
+  return true;
+}
+
+constexpr bool test_large() {
+  test_layout_large<std::layout_left>();
+  test_layout_large<std::layout_right>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  // The large test iterates over ~10k loop indices.
+  // With assertions enabled this triggered the maximum default limit
+  // for steps in consteval expressions. Assertions roughly double the
+  // total number of instructions, so this was already close to the maximum.
+  test_large();
+  return 0;
+}
+#if defined(__GNUC__) && !defined(__clang_major__)
+#  pragma GCC diagnostic pop
+#endif

libcxx/test/std/containers/views/mdspan/mdspan/mapping.verify.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, c++17, c++20
+
+// <mdspan>
+
+// template<class ElementType, class Extents, class LayoutPolicy = layout_right, class AccessorPolicy = default_accessor>
+// class mdspan;
+//
+// Mandates:
+//  - LayoutPolicy shall meet the layout mapping policy requirements ([mdspan.layout.policy.reqmts])
+
+#include <mdspan>
+
+void not_layout_policy() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: LayoutPolicy template parameter is invalid. A common mistake is to pass a layout mapping instead of a layout policy}}
+  [[maybe_unused]] std::mdspan<int, std::extents<int>, std::layout_left::template mapping<std::extents<int>>> m;
+}

libcxx/test/std/containers/views/mdspan/mdspan/move.pass.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, c++17, c++20
+
+// <mdspan>
+
+// constexpr mdspan& operator=(mdspan&&) = default;
+//
+// A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_type are trivially copyable types.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+#include "CustomTestAccessors.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+
+  MDS m_org(handle, map, acc);
+  MDS m_copy(m_org);
+  MDS m(std::move(m_copy));
+
+  assert(m.extents() == map.extents());
+  if constexpr (std::equality_comparable<H>)
+    assert(m.data_handle() == handle);
+  if constexpr (std::equality_comparable<M>)
+    assert(m.mapping() == map);
+  if constexpr (std::equality_comparable<A>)
+    assert(m.accessor() == acc);
+
+  static_assert(std::is_trivially_move_assignable_v<MDS> ==
+                (std::is_trivially_move_assignable_v<H> && std::is_trivially_move_assignable_v<M> &&
+                 std::is_trivially_move_assignable_v<A>));
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  // make sure we test a trivially copyable mapping
+  static_assert(std::is_trivially_move_assignable_v<typename std::layout_left::template mapping<std::extents<int>>>);
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  // make sure we test a not trivially copyable mapping
+  static_assert(
+      !std::is_trivially_move_assignable_v<typename layout_wrapping_integral<4>::template mapping<std::extents<int>>>);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  // make sure we test trivially constructible accessor and data_handle
+  static_assert(std::is_trivially_copyable_v<std::default_accessor<T>>);
+  static_assert(std::is_trivially_copyable_v<typename std::default_accessor<T>::data_handle_type>);
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+
+  // Using weird accessor/data_handle
+  // Make sure they actually got the properties we want to test
+  // checked_accessor is noexcept copy constructible except for const double
+  checked_accessor<T> acc(1024);
+  static_assert(noexcept(checked_accessor<T>(acc)) != std::is_same_v<T, const double>);
+  mixin_layout(typename checked_accessor<T>::data_handle_type(elements.get_ptr()), acc);
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/properties.pass.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, c++17, c++20
+
+// <mdspan>
+
+// template<class ElementType, class Extents, class LayoutPolicy = layout_right,
+//            class AccessorPolicy = default_accessor<ElementType>>
+//   class mdspan {
+//   public:
+//     static constexpr rank_type rank() noexcept { return extents_type::rank(); }
+//     static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
+//     static constexpr size_t static_extent(rank_type r) noexcept
+//       { return extents_type::static_extent(r); }
+//     constexpr index_type extent(rank_type r) const noexcept { return extents().extent(r); }
+//
+//     constexpr size_type size() const noexcept;
+//     [[nodiscard]] constexpr bool empty() const noexcept;
+//
+//
+//     constexpr const extents_type& extents() const noexcept { return map_.extents(); }
+//     constexpr const data_handle_type& data_handle() const noexcept { return ptr_; }
+//     constexpr const mapping_type& mapping() const noexcept { return map_; }
+//     constexpr const accessor_type& accessor() const noexcept { return acc_; }
+//     static constexpr bool is_always_unique()
+//       { return mapping_type::is_always_unique(); }
+//     static constexpr bool is_always_exhaustive()
+//       { return mapping_type::is_always_exhaustive(); }
+//     static constexpr bool is_always_strided()
+//       { return mapping_type::is_always_strided(); }
+//
+//     constexpr bool is_unique() const
+//       { return map_.is_unique(); }
+//     constexpr bool is_exhaustive() const
+//       { return map_.is_exhaustive(); }
+//     constexpr bool is_strided() const
+//       { return map_.is_strided(); }
+//     constexpr index_type stride(rank_type r) const
+//       { return map_.stride(r); }
+//   };
+//
+// Each specialization MDS of mdspan models copyable and
+//    - is_nothrow_move_constructible_v<MDS> is true,
+//    - is_nothrow_move_assignable_v<MDS> is true, and
+//    - is_nothrow_swappable_v<MDS> is true.
+// A specialization of mdspan is a trivially copyable type if its accessor_type, mapping_type, and data_handle_type are trivially copyable types.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+
+template <class H, class M, class A>
+constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
+  using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
+  MDS m(handle, map, acc);
+
+  // =====================================
+  // Traits for every mdspan
+  // =====================================
+  static_assert(std::copyable<MDS>);
+  static_assert(std::is_nothrow_move_constructible_v<MDS>);
+  static_assert(std::is_nothrow_move_assignable_v<MDS>);
+  static_assert(std::is_nothrow_swappable_v<MDS>);
+
+  // =====================================
+  // Invariants coming from data handle
+  // =====================================
+  // data_handle()
+  ASSERT_SAME_TYPE(decltype(m.data_handle()), const H&);
+  ASSERT_NOEXCEPT(m.data_handle());
+  if constexpr (std::equality_comparable<H>) {
+    assert(m.data_handle() == handle);
+  }
+
+  // =====================================
+  // Invariants coming from extents
+  // =====================================
+
+  // extents()
+  ASSERT_SAME_TYPE(decltype(m.extents()), const typename MDS::extents_type&);
+  ASSERT_NOEXCEPT(m.extents());
+  assert(m.extents() == map.extents());
+
+  // rank()
+  ASSERT_SAME_TYPE(decltype(m.rank()), typename MDS::rank_type);
+  ASSERT_NOEXCEPT(m.rank());
+  assert(MDS::rank() == MDS::extents_type::rank());
+
+  // rank_dynamic()
+  ASSERT_SAME_TYPE(decltype(m.rank_dynamic()), typename MDS::rank_type);
+  ASSERT_NOEXCEPT(m.rank_dynamic());
+  assert(MDS::rank_dynamic() == MDS::extents_type::rank_dynamic());
+
+  // extent(r), static_extent(r), size()
+  if constexpr (MDS::rank() > 0) {
+    typename MDS::size_type size = 1;
+    for (typename MDS::rank_type r = 0; r < MDS::rank(); r++) {
+      ASSERT_SAME_TYPE(decltype(MDS::static_extent(r)), size_t);
+      ASSERT_NOEXCEPT(MDS::static_extent(r));
+      assert(MDS::static_extent(r) == MDS::extents_type::static_extent(r));
+      ASSERT_SAME_TYPE(decltype(m.extent(r)), typename MDS::index_type);
+      ASSERT_NOEXCEPT(m.extent(r));
+      assert(m.extent(r) == m.extents().extent(r));
+      size *= m.extent(r);
+    }
+    assert(m.size() == size);
+  } else {
+    assert(m.size() == 1);
+  }
+  ASSERT_SAME_TYPE(decltype(m.size()), typename MDS::size_type);
+  ASSERT_NOEXCEPT(m.size());
+
+  // empty()
+  ASSERT_SAME_TYPE(decltype(m.empty()), bool);
+  ASSERT_NOEXCEPT(m.empty());
+  assert(m.empty() == (m.size() == 0));
+
+  // =====================================
+  // Invariants coming from mapping
+  // =====================================
+
+  // mapping()
+  ASSERT_SAME_TYPE(decltype(m.mapping()), const M&);
+  ASSERT_NOEXCEPT(m.mapping());
+
+  // is_[always_]unique/exhaustive/strided()
+  ASSERT_SAME_TYPE(decltype(MDS::is_always_unique()), bool);
+  ASSERT_SAME_TYPE(decltype(MDS::is_always_exhaustive()), bool);
+  ASSERT_SAME_TYPE(decltype(MDS::is_always_strided()), bool);
+  ASSERT_SAME_TYPE(decltype(m.is_unique()), bool);
+  ASSERT_SAME_TYPE(decltype(m.is_exhaustive()), bool);
+  ASSERT_SAME_TYPE(decltype(m.is_strided()), bool);
+  assert(!noexcept(MDS::is_always_unique()));
+  assert(!noexcept(MDS::is_always_exhaustive()));
+  assert(!noexcept(MDS::is_always_strided()));
+  assert(!noexcept(m.is_unique()));
+  assert(!noexcept(m.is_exhaustive()));
+  assert(!noexcept(m.is_strided()));
+  assert(MDS::is_always_unique() == M::is_always_unique());
+  assert(MDS::is_always_exhaustive() == M::is_always_exhaustive());
+  assert(MDS::is_always_strided() == M::is_always_strided());
+  assert(m.is_unique() == map.is_unique());
+  assert(m.is_exhaustive() == map.is_exhaustive());
+  assert(m.is_strided() == map.is_strided());
+
+  // stride(r)
+  if constexpr (MDS::rank() > 0) {
+    if (m.is_strided()) {
+      for (typename MDS::rank_type r = 0; r < MDS::rank(); r++) {
+        ASSERT_SAME_TYPE(decltype(m.stride(r)), typename MDS::index_type);
+        assert(!noexcept(m.stride(r)));
+        assert(m.stride(r) == map.stride(r));
+      }
+    }
+  }
+
+  // =====================================
+  // Invariants coming from accessor
+  // =====================================
+
+  // accessor()
+  ASSERT_SAME_TYPE(decltype(m.accessor()), const A&);
+  ASSERT_NOEXCEPT(m.accessor());
+}
+
+template <class H, class L, class A>
+constexpr void mixin_extents(const H& handle, const L& layout, const A& acc) {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D>(7)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<unsigned, 7>()), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<size_t, D, 4, D>(2, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<char, D, 7, D>(0, 3)), acc);
+  test_mdspan_types(handle, construct_mapping(layout, std::extents<int64_t, D, 7, D, 4, D, D>(1, 2, 3, 2)), acc);
+}
+
+template <class H, class A>
+constexpr void mixin_layout(const H& handle, const A& acc) {
+  mixin_extents(handle, std::layout_left(), acc);
+  mixin_extents(handle, std::layout_right(), acc);
+  mixin_extents(handle, layout_wrapping_integral<4>(), acc);
+}
+
+template <class T>
+constexpr void mixin_accessor() {
+  ElementPool<T, 1024> elements;
+  mixin_layout(elements.get_ptr(), std::default_accessor<T>());
+}
+
+constexpr bool test() {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+  return true;
+}
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/swap.pass.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, c++17, c++20
+
+// <mdspan>
+//
+// friend constexpr void swap(mdspan& x, mdspan& y) noexcept;
+//
+// Effects: Equivalent to:
+//   swap(x.ptr_, y.ptr_);
+//   swap(x.map_, y.map_);
+//   swap(x.acc_, y.acc_);
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestLayouts.h"
+
+template <class MDS>
+constexpr void test_swap(MDS a, MDS b) {
+  auto org_a = a;
+  auto org_b = b;
+  swap(a, b);
+  assert(a.extents() == org_b.extents());
+  assert(b.extents() == org_a.extents());
+  if constexpr (std::equality_comparable<typename MDS::mapping_type>) {
+    assert(a.mapping() == org_b.mapping());
+    assert(b.mapping() == org_a.mapping());
+  }
+  if constexpr (std::equality_comparable<typename MDS::data_handle_type>) {
+    assert(a.data_handle() == org_b.data_handle());
+    assert(b.data_handle() == org_a.data_handle());
+  }
+  // This check uses a side effect of layout_wrapping_integral::swap to make sure
+  // mdspan calls the underlying components' swap via ADL
+  if !consteval {
+    if constexpr (std::is_same_v<typename MDS::layout_type, layout_wrapping_integral<4>>) {
+      assert(MDS::mapping_type::swap_counter() > 0);
+    }
+  }
+}
+
+constexpr bool test() {
+  using extents_t = std::extents<int, 4, std::dynamic_extent>;
+  float data_a[1024];
+  float data_b[1024];
+  {
+    std::mdspan a(data_a, extents_t(12));
+    std::mdspan b(data_b, extents_t(5));
+    test_swap(a, b);
+  }
+  {
+    layout_wrapping_integral<4>::template mapping<extents_t> map_a(extents_t(12), not_extents_constructible_tag()),
+        map_b(extents_t(5), not_extents_constructible_tag());
+    std::mdspan a(data_a, map_a);
+    std::mdspan b(data_b, map_b);
+    test_swap(a, b);
+  }
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/types.pass.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, c++17, c++20
+
+// <mdspan>
+//
+//  template<class ElementType, class Extents, class LayoutPolicy = layout_right,
+//           class AccessorPolicy = default_accessor<ElementType>>
+//  class mdspan {
+//  public:
+//    using extents_type = Extents;
+//    using layout_type = LayoutPolicy;
+//    using accessor_type = AccessorPolicy;
+//    using mapping_type = typename layout_type::template mapping<extents_type>;
+//    using element_type = ElementType;
+//    using value_type = remove_cv_t<element_type>;
+//    using index_type = typename extents_type::index_type;
+//    using size_type = typename extents_type::size_type;
+//    using rank_type = typename extents_type::rank_type;
+//    using data_handle_type = typename accessor_type::data_handle_type;
+//    using reference = typename accessor_type::reference;
+//    ...
+//  };
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../MinimalElementType.h"
+#include "CustomTestAccessors.h"
+#include "CustomTestLayouts.h"
+
+// Calculated expected size of an mdspan
+// Note this expectes that only default_accessor is empty
+template<class MDS>
+constexpr size_t expected_size() {
+  size_t sizeof_dht = sizeof(typename MDS::data_handle_type);
+  size_t result = sizeof_dht;
+  if(MDS::rank_dynamic() > 0) {
+    size_t alignof_idx = alignof(typename MDS::index_type);
+    size_t sizeof_idx = sizeof(typename MDS::index_type);
+    // add alignment if necessary
+    result += sizeof_dht%alignof_idx == 0?0:alignof_idx - (sizeof_dht%alignof_idx);
+    // add sizeof stored extents
+    result += MDS::rank_dynamic() * sizeof_idx;
+  }
+  using A = typename MDS::accessor_type;
+  if(!std::is_same_v<A, std::default_accessor<typename MDS::element_type>>) {
+    size_t alignof_acc = alignof(A);
+    size_t sizeof_acc = sizeof(A);
+    // add alignment if necessary
+    result += result%alignof_acc == 0?0:alignof_acc - (result%alignof_acc);
+    // add sizeof stored accessor
+    result += sizeof_acc;
+  }
+  // add alignment of the mdspan itself
+  result += result%alignof(MDS) == 0?0:alignof(MDS) - (result%alignof(MDS));
+  return result;
+}
+
+// check triviality
+template <class T>
+constexpr bool trv_df_ctor = std::is_trivially_default_constructible_v<T>;
+template <class T>
+constexpr bool trv_cp_ctor = std::is_trivially_copy_constructible_v<T>;
+template <class T>
+constexpr bool trv_mv_ctor = std::is_trivially_move_constructible_v<T>;
+template <class T>
+constexpr bool trv_dstruct = std::is_trivially_destructible_v<T>;
+template <class T>
+constexpr bool trv_cp_asgn = std::is_trivially_copy_assignable_v<T>;
+template <class T>
+constexpr bool trv_mv_asgn = std::is_trivially_move_assignable_v<T>;
+
+template <class MDS, bool default_ctor, bool copy_ctor, bool move_ctor, bool destr, bool copy_assign, bool move_assign>
+void check_triviality() {
+  static_assert(trv_df_ctor<MDS> == default_ctor);
+  static_assert(trv_cp_ctor<MDS> == copy_ctor);
+  static_assert(trv_mv_ctor<MDS> == move_ctor);
+  static_assert(trv_dstruct<MDS> == destr);
+  static_assert(trv_cp_asgn<MDS> == copy_assign);
+  static_assert(trv_mv_asgn<MDS> == move_assign);
+}
+
+template <class T, class E, class L, class A>
+void test_mdspan_types() {
+  using MDS = std::mdspan<T, E, L, A>;
+
+  ASSERT_SAME_TYPE(typename MDS::extents_type, E);
+  ASSERT_SAME_TYPE(typename MDS::layout_type, L);
+  ASSERT_SAME_TYPE(typename MDS::accessor_type, A);
+  ASSERT_SAME_TYPE(typename MDS::mapping_type, typename L::template mapping<E>);
+  ASSERT_SAME_TYPE(typename MDS::element_type, T);
+  ASSERT_SAME_TYPE(typename MDS::value_type, std::remove_cv_t<T>);
+  ASSERT_SAME_TYPE(typename MDS::index_type, typename E::index_type);
+  ASSERT_SAME_TYPE(typename MDS::size_type, typename E::size_type);
+  ASSERT_SAME_TYPE(typename MDS::rank_type, typename E::rank_type);
+  ASSERT_SAME_TYPE(typename MDS::data_handle_type, typename A::data_handle_type);
+  ASSERT_SAME_TYPE(typename MDS::reference, typename A::reference);
+
+// This miserably failed with clang-cl - likely because it doesn't honor/enable
+// no-unique-address fully by default
+#ifndef _WIN32
+  // check the size of mdspan
+  if constexpr (std::is_same_v<L, std::layout_left> || std::is_same_v<L, std::layout_right>) {
+    LIBCPP_STATIC_ASSERT(sizeof(MDS) == expected_size<MDS>());
+  }
+#endif
+
+  // check default template parameters:
+  ASSERT_SAME_TYPE(std::mdspan<T, E>, std::mdspan<T, E, std::layout_right, std::default_accessor<T>>);
+  ASSERT_SAME_TYPE(std::mdspan<T, E, L>, std::mdspan<T, E, L, std::default_accessor<T>>);
+
+  // check triviality
+  using DH = typename MDS::data_handle_type;
+  using MP = typename MDS::mapping_type;
+
+  check_triviality<MDS,
+                   false, // mdspan is never trivially constructible right now
+                   trv_cp_ctor<DH> && trv_cp_ctor<MP> && trv_cp_ctor<A>,
+                   trv_mv_ctor<DH> && trv_mv_ctor<MP> && trv_mv_ctor<A>,
+                   trv_dstruct<DH> && trv_dstruct<MP> && trv_dstruct<A>,
+                   trv_cp_asgn<DH> && trv_cp_asgn<MP> && trv_cp_asgn<A>,
+                   trv_mv_asgn<DH> && trv_mv_asgn<MP> && trv_mv_asgn<A>>();
+}
+
+template <class T, class L, class A>
+void mixin_extents() {
+  constexpr size_t D = std::dynamic_extent;
+  test_mdspan_types<T, std::extents<int>, L, A>();
+  test_mdspan_types<T, std::extents<char, D>, L, A>();
+  test_mdspan_types<T, std::dextents<char, 7>, L, A>();
+  test_mdspan_types<T, std::dextents<char, 9>, L, A>();
+  test_mdspan_types<T, std::extents<unsigned, 7>, L, A>();
+  test_mdspan_types<T, std::extents<unsigned, D, D, D>, L, A>();
+  test_mdspan_types<T, std::extents<size_t, D, 7, D>, L, A>();
+  test_mdspan_types<T, std::extents<int64_t, D, 7, D, 4, D, D>, L, A>();
+}
+
+template <class T, class A>
+void mixin_layout() {
+  mixin_extents<T, std::layout_left, A>();
+  mixin_extents<T, std::layout_right, A>();
+  mixin_extents<T, layout_wrapping_integral<4>, A>();
+}
+
+template <class T>
+void mixin_accessor() {
+  mixin_layout<T, std::default_accessor<T>>();
+  mixin_layout<T, checked_accessor<T>>();
+}
+
+int main(int, char**) {
+  mixin_accessor<int>();
+  mixin_accessor<const int>();
+  mixin_accessor<double>();
+  mixin_accessor<const double>();
+  mixin_accessor<MinimalElementType>();
+  mixin_accessor<const MinimalElementType>();
+
+  // sanity checks for triviality
+  check_triviality<std::mdspan<int, std::extents<int>>, false, true, true, true, true, true>();
+  check_triviality<std::mdspan<int, std::dextents<int, 1>>, false, true, true, true, true, true>();
+  check_triviality<std::mdspan<int, std::dextents<int, 1>, std::layout_right, checked_accessor<int>>,
+                   false,
+                   true,
+                   false,
+                   true,
+                   true,
+                   true>();
+  return 0;
+}