[libc++][ranges] Implement ranges::transform

[philnik]

• Add synopsis

[Mordante]

Thanks a lot for working on this!

libcxx/include/__algorithm/ranges_transform.h

39

For consistency I think using _Op like in the unary type would be better.

45

Is there a reason why the naming of the types and variables differs from the other ranges algorithms?
For example ranges_find.h uses

template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, class _Proj = identity>
  requires indirect_binary_predicate<ranges::equal_to, projected<_Ip, _Proj>, const _Tp*>
_LIBCPP_HIDE_FROM_ABI constexpr
_Ip operator()(_Ip __first, _Sp __last, const _Tp& __value, _Proj __proj = {}) const {
  auto __pred = [&](auto&& __e) { return std::forward<decltype(__e)>(__e) == __value; };
  return ranges::__find_if_impl(std::move(__first), std::move(__last), __pred, __proj);
}

I think it improves our codebase when we keep these names consistent.
I'm also not to fond of _InputIterator and _OutputIterator, they are used in the non-range algorithms and there they use the LegacyIterators. So I like that the range based algorithms use a different naming scheme,

libcxx/include/algorithm

99

unary_transform_result and binary_transform_result are also part of the synopsis.

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

112

I find this test with the extra whitespace more readable, could you adjust the other test cases in this file accordingly?

259

297

I would like a test where an unrelated type is written, for example using a unary lambda like
[](int i) { return '0' + i } which returns a char instead of an int.

302

I think it would be good to also test cpp17_input_iterator, same for test_iterators_in1_in2.

321

We should add a test using the cpp20_output_iterator as proposed in D122072. We want to make sure a non-copyable iterator works.

331

Should these test also use decltype(auto)?

417

[var-const]

libcxx/docs/Status/RangesAlgorithms.csv

47

Can you add a link to this patch here?

libcxx/include/__algorithm/ranges_transform.h

45

FWIW, I'm mildly in favor of using longer names (though I can see the argument for terser names given how verbose the template parameters are in these functions). I think range algorithms (both committed and in flight) aren't currently consistent in this regard.

libcxx/include/algorithm

104

Nit: s/first1/first/ (same for last1).

110

Nit: can you add blank lines at least between the unary and binary versions of transform, and perhaps between all of them? As it stands, I found it a little hard to find where one functions ends and another one starts due to how verbose the template arguments are.

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

60

To make the SFINAE test exhaustive, I think it also needs to check the weakly_incrementable, copy_constructible, indirectly_writeable, etc. constraints.

79–85

Optional: do you think it would be worthwhile to try reducing duplication here? Perhaps something like:

template <class In1, class Out>
struct Result {
  std::ranges::in_out_result<In1, Out> ret;
  Out* out = nullptr;
};

// ...

int a[] = {1, 2, 3, 4, 5};
int b1[5], b2[5];

std::same_as<std::ranges::in_out_result<In1, Out>> decltype(auto) ret_iter =
    std::ranges::transform(In1(a), Sent1(In1(a + 5)), Out(b1), [](int i) { return i * 2; });
auto range = std::ranges::subrange(In1(a), Sent1(In1(a + 5)));
std::same_as<std::ranges::in_out_result<In1, Out>> decltype(auto) ret =
    std::ranges::transform(range, Out(b2), [](int i) { return i * 2; });

for (const auto& result : {Result{ret_iter, b1}, Result{ret_range, b2}}) {
  assert((std::to_array(result.out) == std::array{2, 4, 6, 8, 10}));
  assert(base(result.ret.in) == a + 5);
  assert(base(result.ret.out) == b + 5);
}

112

+1.

182

Perhaps also check the case when both ranges are empty for binary?

210–213

Nit: one extra blank line.

332

Nit: I think continued lines should be indented 4 columns.

416

Please add return 0.

[Mordante]

libcxx/include/__algorithm/ranges_transform.h

45

I'm not against longer names in general, but when I read _InputIterator I expect a LegacyIterator in the <algorithm> header. I also would like consistent naming, which can be achieved in two ways. Use the old names in new code or change the old code to use new names.There's a thread on Discord so I think it's easier to discuss it there.

[philnik]

• Address comments

libcxx/include/__algorithm/ranges_transform.h

39

I use _O1 for consistency with in_in_out_result here.

45

This has been discussed on discord.

libcxx/include/algorithm

104

This is consistent with the standard synopsis.

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

79–85

This looks way more complicated to make it a bit shorter to me.

210–213

I think it looks nicer without a blank line here.

331

The decltype(auto) was specifically because ranges::min and friends return const T&. But I can also add a decltype here if you want.

[ldionne]

I think this is good to go after my suggestions. I'll want to see this again just to double check, but there shouldn't be anything else to change. Thanks!

libcxx/include/__algorithm/ranges_transform.h

36–40

Can you please make sure you have tests that check that we define those two names in std::ranges specifically, i.e. not as part of testing ranges::transform() itself? This also applies to other reviews like minmax_element.

50

Can you make __unary and __binary private?

libcxx/include/algorithm

100–104

Those shouldn't use uglified names.

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

215

You could consider checking the return type only once for each relevant call, and then use auto elsewhere. That would simplify these tests quite a bit.

394–398

This shouldn't be commented out (ditto below).

409

Would it be possible to move these tests into the test_iterators function? LMK if compile-times explode in a bad way.

445

I don't think you mean "predicate" here, maybe just "function" is what you mean.

493

[ldionne]

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

51

You could fix the macOS CI issue (which is really an AppleClang-13 issue) by doing something like std::ranges::transform(r, out, std::identity()) instead.

[philnik]

• Address comments

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

409

The test takes 40s to compile and run on my system now. The tests that aren't in test_iterators can't be moved there easily, because they don't use int.

[var-const]

libcxx/include/algorithm

104

This is consistent with the standard synopsis.

Interesting. I presume it's a typo in the Standard, but sure, let's be consistent.

[var-const]

LGTM, but leaving the final approval for @ldionne who had some comments.

[philnik]

• Fix CI

[ldionne]

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.pass.cpp

455–461

Can you please add a comment like: static_asserting here to avoid hitting the constexpr evaluation step limit.

[bkramer]

clang/lib/ExtractAPI/Serialization/ranges_transform.module.verify.cpp

1–15

This file doesn't belong in clang/lib. Deleted it in 302fe7b3c40f7b949f3bebb74997bef9bf74d59f.

[philnik]

clang/lib/ExtractAPI/Serialization/ranges_transform.module.verify.cpp

1–15

Sorry, this must be a weird merge conflict. Thanks for removing it!

clang/lib/ExtractAPI/Serialization/ranges_transform.module.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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_transform.h'}}
+#include <__algorithm/ranges_transform.h>

libcxx/docs/Status/RangesAlgorithms.csv

 Write,copy,Not assigned,n/a,Not started
 Write,copy_if,Not assigned,n/a,Not started
 Write,copy_n,Not assigned,n/a,Not started
 Write,copy_backward,Not assigned,n/a,Not started
 Write,move,Not assigned,n/a,Not started
 Write,move_backward,Not assigned,n/a,Not started
 Write,fill,Not assigned,n/a,Not started
 Write,fill_n,Not assigned,n/a,Not started
-Write,transform,Not assigned,n/a,Not started
+Write,transform,Nikolas Klauser,`D122173 <https://llvm.org/D122173>`_,✅
 Write,generate,Not assigned,n/a,Not started
 Write,generate_n,Not assigned,n/a,Not started
 Write,remove_copy,Not assigned,n/a,Not started
 Write,remove_copy_if,Not assigned,n/a,Not started
 Write,replace,Not assigned,n/a,Not started
 Write,replace_if,Not assigned,n/a,Not started
 Write,replace_copy,Not assigned,n/a,Not started
 Write,replace_copy_if,Not assigned,n/a,Not started

libcxx/include/CMakeLists.txt

set(files
   __algorithm/ranges_find_if.h
   __algorithm/ranges_find_if_not.h
   __algorithm/ranges_max.h
   __algorithm/ranges_max_element.h
   __algorithm/ranges_min.h
   __algorithm/ranges_min_element.h
   __algorithm/ranges_mismatch.h
   __algorithm/ranges_swap_ranges.h
+  __algorithm/ranges_transform.h
   __algorithm/remove.h
   __algorithm/remove_copy.h
   __algorithm/remove_copy_if.h
   __algorithm/remove_if.h
   __algorithm/replace.h
   __algorithm/replace_copy.h
   __algorithm/replace_copy_if.h
   __algorithm/replace_if.h

libcxx/include/__algorithm/ranges_transform.h

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_RANGES_TRANSFORM_H
+#define _LIBCPP___ALGORITHM_RANGES_TRANSFORM_H
+
+#include <__algorithm/in_in_out_result.h>
+#include <__algorithm/in_out_result.h>
+#include <__concepts/constructible.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__iterator/concepts.h>
+#include <__iterator/projected.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+
+template <class _Ip, class _Op>
+using unary_transform_result = in_out_result<_Ip, _Op>;
+
+template <class _I1, class _I2, class _O1>
+using binary_transform_result = in_in_out_result<_I1, _I2, _O1>;
+
+namespace __transform {
+struct __fn {
+private:
+  template <class _InIter, class _Sent,
+            class _OutIter,
+            class _Func,
+            class _Proj>
+  _LIBCPP_HIDE_FROM_ABI static constexpr
+  unary_transform_result<_InIter, _OutIter> __unary(_InIter __first, _Sent __last,
+                                                    _OutIter __result,
+                                                    _Func& __operation,
+                                                    _Proj& __projection) {
+    while (__first != __last) {
+      *__result = std::invoke(__operation, std::invoke(__projection, *__first));
+      ++__first;
+      ++__result;
+    }
+
+    return {std::move(__first), std::move(__result)};
+  }
+
+  template <class _InIter1, class _Sent1,
+            class _InIter2, class _Sent2,
+            class _OutIter,
+            class _Func,
+            class _Proj1,
+            class _Proj2>
+  _LIBCPP_HIDE_FROM_ABI static constexpr binary_transform_result<_InIter1, _InIter2, _OutIter>
+  __binary(_InIter1 __first1, _Sent1 __last1,
+           _InIter2 __first2, _Sent2 __last2,
+           _OutIter __result,
+           _Func& __binary_operation,
+           _Proj1& __projection1,
+           _Proj2& __projection2) {
+    while (__first1 != __last1 && __first2 != __last2) {
+      *__result = std::invoke(__binary_operation, std::invoke(__projection1, *__first1),
+                                                  std::invoke(__projection2, *__first2));
+      ++__first1;
+      ++__first2;
+      ++__result;
+    }
+    return {std::move(__first1), std::move(__first2), std::move(__result)};
+  }
+public:
+  template <input_iterator _InIter, sentinel_for<_InIter> _Sent,
+            weakly_incrementable _OutIter,
+            copy_constructible _Func,
+            class _Proj = identity>
+    requires indirectly_writable<_OutIter, indirect_result_t<_Func&, projected<_InIter, _Proj>>>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  unary_transform_result<_InIter, _OutIter> operator()(_InIter __first, _Sent __last,
+                                                       _OutIter __result,
+                                                       _Func __operation,
+                                                       _Proj __proj = {}) const {
+    return __unary(std::move(__first), std::move(__last), std::move(__result), __operation, __proj);
+  }
+
+  template <input_range _Range,
+            weakly_incrementable _OutIter,
+            copy_constructible _Func,
+            class _Proj = identity>
+    requires indirectly_writable<_OutIter, indirect_result_t<_Func, projected<iterator_t<_Range>, _Proj>>>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  unary_transform_result<borrowed_iterator_t<_Range>, _OutIter> operator()(_Range&& __range,
+                                                                           _OutIter __result,
+                                                                           _Func __operation,
+                                                                           _Proj __projection = {}) const {
+    return __unary(ranges::begin(__range), ranges::end(__range), std::move(__result), __operation, __projection);
+  }
+
+  template <input_iterator _InIter1, sentinel_for<_InIter1> _Sent1,
+            input_iterator _InIter2, sentinel_for<_InIter2> _Sent2,
+            weakly_incrementable _OutIter,
+            copy_constructible _Func,
+            class _Proj1 = identity,
+            class _Proj2 = identity>
+    requires indirectly_writable<_OutIter, indirect_result_t<_Func&, projected<_InIter1, _Proj1>,
+                                                                     projected<_InIter2, _Proj2>>>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  binary_transform_result<_InIter1, _InIter2, _OutIter> operator()(_InIter1 __first1, _Sent1 __last1,
+                                                                   _InIter2 __first2, _Sent2 __last2,
+                                                                   _OutIter __result,
+                                                                   _Func __binary_operation,
+                                                                   _Proj1 __projection1 = {},
+                                                                   _Proj2 __projection2 = {}) const {
+    return __binary(std::move(__first1), std::move(__last1),
+                    std::move(__first2), std::move(__last2),
+                    std::move(__result),
+                    __binary_operation,
+                    __projection1,
+                    __projection2);
+  }
+
+  template <input_range _Range1,
+            input_range _Range2,
+            weakly_incrementable _OutIter,
+            copy_constructible _Func,
+            class _Proj1 = identity,
+            class _Proj2 = identity>
+    requires indirectly_writable<_OutIter, indirect_result_t<_Func&, projected<iterator_t<_Range1>, _Proj1>,
+                                                                     projected<iterator_t<_Range2>, _Proj2>>>
+  _LIBCPP_HIDE_FROM_ABI constexpr
+  binary_transform_result<borrowed_iterator_t<_Range1>, borrowed_iterator_t<_Range2>, _OutIter>
+  operator()(_Range1&& __range1,
+             _Range2&& __range2,
+             _OutIter __result,
+             _Func __binary_operation,
+             _Proj1 __projection1 = {},
+             _Proj2 __projection2 = {}) const {
+    return __binary(ranges::begin(__range1), ranges::end(__range1),
+                    ranges::begin(__range2), ranges::end(__range2),
+                    std::move(__result),
+                    __binary_operation,
+                    __projection1,
+                    __projection2);
+  }
+
+};
+} // namespace __transform
+
+inline namespace __cpo {
+  inline constexpr auto transform = __transform::__fn{};
+} // namespace __cpo
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+#endif // _LIBCPP___ALGORITHM_RANGES_TRANSFORM_H

libcxx/include/algorithm

template<copyable T, class Proj = identity,
            indirect_strict_weak_order<projected<const T*, Proj>> Comp = ranges::less>
     constexpr T min(initializer_list<T> r, Comp comp = {}, Proj proj = {});         // since C++20
 
  template<input_range R, class Proj = identity,
           indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
    requires indirectly_copyable_storable<iterator_t<R>, range_value_t<R>*>
    constexpr range_value_t<R>
      min(R&& r, Comp comp = {}, Proj proj = {});                                    // since C++20
 
   template<class T, class Proj = identity,
            indirect_strict_weak_order<projected<const T*, Proj>> Comp = ranges::less>
     constexpr const T& max(const T& a, const T& b, Comp comp = {}, Proj proj = {}); // since C++20
 
   template<copyable T, class Proj = identity,
            indirect_strict_weak_order<projected<const T*, Proj>> Comp = ranges::less>
     constexpr T max(initializer_list<T> r, Comp comp = {}, Proj proj = {});         // since C++20
 
   template<input_range R, class Proj = identity,
            indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
     requires indirectly_copyable_storable<iterator_t<R>, range_value_t<R>*>
     constexpr range_value_t<R>
       max(R&& r, Comp comp = {}, Proj proj = {});                                   // since C++20
+
+  template<class I, class O>
+    using unary_transform_result = in_out_result<I, O>;                             // since C++20
+
+  template<class I1, class I2, class O>
+    using binary_transform_result = in_in_out_result<I1, I2, O>;                    // since C++20
+
+  template<input_iterator I, sentinel_for<I> S, weakly_incrementable O,
+           copy_constructible F, class Proj = identity>
+    requires indirectly_writable<O, indirect_result_t<F&, projected<I, Proj>>>
+    constexpr ranges::unary_transform_result<I, O>
+      transform(I first1, S last1, O result, F op, Proj proj = {});                 // since C++20
+
+  template<input_range R, weakly_incrementable O, copy_constructible F,
+           class Proj = identity>
+    requires indirectly_writable<O, indirect_result_t<F&, projected<iterator_t<R>, Proj>>>
+    constexpr ranges::unary_transform_result<borrowed_iterator_t<R>, O>
+      transform(R&& r, O result, F op, Proj proj = {});                             // since C++20
+
+  template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+           weakly_incrementable O, copy_constructible F, class Proj1 = identity,
+           class Proj2 = identity>
+    requires indirectly_writable<O, indirect_result_t<F&, projected<I1, Proj1>,
+                                           projected<I2, Proj2>>>
+    constexpr ranges::binary_transform_result<I1, I2, O>
+      transform(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+                        F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {});           // since C++20
+
+  template<input_range R1, input_range R2, weakly_incrementable O,
+           copy_constructible F, class Proj1 = identity, class Proj2 = identity>
+    requires indirectly_writable<O, indirect_result_t<F&, projected<iterator_t<R1>, Proj1>,
+                                           projected<iterator_t<R2>, Proj2>>>
+    constexpr ranges::binary_transform_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
+      transform(R1&& r1, R2&& r2, O result,
+                        F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {});           // since C++20
 }
 
     constexpr bool     // constexpr in C++20
     all_of(InputIterator first, InputIterator last, Predicate pred);
 
 template <class InputIterator, class Predicate>
     constexpr bool     // constexpr in C++20
     any_of(InputIterator first, InputIterator last, Predicate pred);

 #include <__algorithm/ranges_find_if.h>
 #include <__algorithm/ranges_find_if_not.h>
 #include <__algorithm/ranges_max.h>
 #include <__algorithm/ranges_max_element.h>
 #include <__algorithm/ranges_min.h>
 #include <__algorithm/ranges_min_element.h>
 #include <__algorithm/ranges_mismatch.h>
 #include <__algorithm/ranges_swap_ranges.h>
+#include <__algorithm/ranges_transform.h>
 #include <__algorithm/remove.h>
 #include <__algorithm/remove_copy.h>
 #include <__algorithm/remove_copy_if.h>
 #include <__algorithm/remove_if.h>
 #include <__algorithm/replace.h>
 #include <__algorithm/replace_copy.h>
 #include <__algorithm/replace_copy_if.h>
 #include <__algorithm/replace_if.h>

libcxx/include/module.modulemap

module __algorithm {
       module ranges_find_if           { private header "__algorithm/ranges_find_if.h" }
       module ranges_find_if_not       { private header "__algorithm/ranges_find_if_not.h" }
       module ranges_max               { private header "__algorithm/ranges_max.h" }
       module ranges_max_element       { private header "__algorithm/ranges_max_element.h" }
       module ranges_min               { private header "__algorithm/ranges_min.h" }
       module ranges_min_element       { private header "__algorithm/ranges_min_element.h" }
       module ranges_mismatch          { private header "__algorithm/ranges_mismatch.h" }
       module ranges_swap_ranges       { private header "__algorithm/ranges_swap_ranges.h" }
+      module ranges_transform         { private header "__algorithm/ranges_transform.h" }
       module remove                   { private header "__algorithm/remove.h" }
       module remove_copy              { private header "__algorithm/remove_copy.h" }
       module remove_copy_if           { private header "__algorithm/remove_copy_if.h" }
       module remove_if                { private header "__algorithm/remove_if.h" }
       module replace                  { private header "__algorithm/replace.h" }
       module replace_copy             { private header "__algorithm/replace_copy.h" }
       module replace_copy_if          { private header "__algorithm/replace_copy_if.h" }
       module replace_if               { private header "__algorithm/replace_if.h" }

libcxx/test/libcxx/private_headers.verify.cpp

 #include <__algorithm/ranges_find_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_find_if.h'}}
 #include <__algorithm/ranges_find_if_not.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_find_if_not.h'}}
 #include <__algorithm/ranges_max.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max.h'}}
 #include <__algorithm/ranges_max_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_max_element.h'}}
 #include <__algorithm/ranges_min.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_min.h'}}
 #include <__algorithm/ranges_min_element.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_min_element.h'}}
 #include <__algorithm/ranges_mismatch.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_mismatch.h'}}
 #include <__algorithm/ranges_swap_ranges.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_swap_ranges.h'}}
+#include <__algorithm/ranges_transform.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/ranges_transform.h'}}
 #include <__algorithm/remove.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove.h'}}
 #include <__algorithm/remove_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove_copy.h'}}
 #include <__algorithm/remove_copy_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove_copy_if.h'}}
 #include <__algorithm/remove_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/remove_if.h'}}
 #include <__algorithm/replace.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/replace.h'}}
 #include <__algorithm/replace_copy.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/replace_copy.h'}}
 #include <__algorithm/replace_copy_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/replace_copy_if.h'}}
 #include <__algorithm/replace_if.h> // expected-error@*:* {{use of private header from outside its module: '__algorithm/replace_if.h'}}

libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/ranges.transform.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
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// <algorithm>
+
+// template<input_iterator I, sentinel_for<I> S, weakly_incrementable O,
+//          copy_constructible F, class Proj = identity>
+//   requires indirectly_writable<O, indirect_result_t<F&, projected<I, Proj>>>
+//   constexpr ranges::unary_transform_result<I, O>
+//     ranges::transform(I first1, S last1, O result, F op, Proj proj = {});
+// template<input_range R, weakly_incrementable O, copy_constructible F,
+//          class Proj = identity>
+//   requires indirectly_writable<O, indirect_result_t<F&, projected<iterator_t<R>, Proj>>>
+//   constexpr ranges::unary_transform_result<borrowed_iterator_t<R>, O>
+//     ranges::transform(R&& r, O result, F op, Proj proj = {});
+// template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2,
+//          weakly_incrementable O, copy_constructible F, class Proj1 = identity,
+//          class Proj2 = identity>
+//   requires indirectly_writable<O, indirect_result_t<F&, projected<I1, Proj1>,
+//                                          projected<I2, Proj2>>>
+//   constexpr ranges::binary_transform_result<I1, I2, O>
+//     ranges::transform(I1 first1, S1 last1, I2 first2, S2 last2, O result,
+//                       F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {});
+// template<input_range R1, input_range R2, weakly_incrementable O,
+//          copy_constructible F, class Proj1 = identity, class Proj2 = identity>
+//   requires indirectly_writable<O, indirect_result_t<F&, projected<iterator_t<R1>, Proj1>,
+//                                          projected<iterator_t<R2>, Proj2>>>
+//   constexpr ranges::binary_transform_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
+//     ranges::transform(R1&& r1, R2&& r2, O result,
+//                       F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {});
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <ranges>
+
+#include "test_iterators.h"
+#include "almost_satisfies_types.h"
+
+struct BinaryFunc {
+  int operator()(int, int);
+};
+
+template <class Range>
+concept HasTranformR = requires(Range r, int* out) {
+  std::ranges::transform(r, out, std::identity{});
+  std::ranges::transform(r, r, out, BinaryFunc{});
+};
+static_assert(HasTranformR<std::array<int, 1>>);
+static_assert(!HasTranformR<int>);
+static_assert(!HasTranformR<InputRangeNotDerivedFrom>);
+static_assert(!HasTranformR<InputRangeNotIndirectlyReadable>);
+static_assert(!HasTranformR<InputRangeNotInputOrOutputIterator>);
+static_assert(!HasTranformR<InputRangeNotSentinelSemiregular>);
+static_assert(!HasTranformR<InputRangeNotSentinelEqualityComparableWith>);
+
+template <class It, class Sent = It>
+concept HasTransformIt = requires(It it, Sent sent, int* out) {
+  std::ranges::transform(it, sent, out, std::identity{});
+  std::ranges::transform(it, sent, it, sent, out, BinaryFunc{});
+};
+static_assert(HasTransformIt<int*>);
+static_assert(!HasTransformIt<InputIteratorNotDerivedFrom>);
+static_assert(!HasTransformIt<InputIteratorNotIndirectlyReadable>);
+static_assert(!HasTransformIt<InputIteratorNotInputOrOutputIterator>);
+static_assert(!HasTransformIt<cpp20_input_iterator<int*>, SentinelForNotSemiregular>);
+static_assert(!HasTransformIt<cpp20_input_iterator<int*>, InputRangeNotSentinelEqualityComparableWith>);
+
+template <class It>
+concept HasTransformOut = requires(int* it, int* sent, It out, std::array<int, 2> range) {
+  std::ranges::transform(it, sent, out, std::identity{});
+  std::ranges::transform(it, sent, it, sent, out, BinaryFunc{});
+  std::ranges::transform(range, out, std::identity{});
+  std::ranges::transform(range, range, out, BinaryFunc{});
+};
+static_assert(HasTransformOut<int*>);
+static_assert(!HasTransformOut<WeaklyIncrementableNotMovable>);
+
+// check indirectly_readable
+static_assert(HasTransformOut<char*>);
+static_assert(!HasTransformOut<int**>);
+
+struct MoveOnlyFunctor {
+  MoveOnlyFunctor(const MoveOnlyFunctor&) = delete;
+  MoveOnlyFunctor(MoveOnlyFunctor&&) = default;
+  int operator()(int);
+  int operator()(int, int);
+};
+
+template <class Func>
+concept HasTransformFuncUnary = requires(int* it, int* sent, int* out, std::array<int, 2> range, Func func) {
+  std::ranges::transform(it, sent, out, func);
+  std::ranges::transform(range, out, func);
+};
+static_assert(HasTransformFuncUnary<std::identity>);
+static_assert(!HasTransformFuncUnary<MoveOnlyFunctor>);
+
+template <class Func>
+concept HasTransformFuncBinary = requires(int* it, int* sent, int* out, std::array<int, 2> range, Func func) {
+  std::ranges::transform(it, sent, it, sent, out, func);
+  std::ranges::transform(range, range, out, func);
+};
+static_assert(HasTransformFuncBinary<BinaryFunc>);
+static_assert(!HasTransformFuncBinary<MoveOnlyFunctor>);
+
+static_assert(std::is_same_v<std::ranges::unary_transform_result<int, long>, std::ranges::in_out_result<int, long>>);
+static_assert(std::is_same_v<std::ranges::binary_transform_result<int, long, char>,
+                             std::ranges::in_in_out_result<int, long, char>>);
+
+template <class In1, class In2, class Out, class Sent1, class Sent2>
+constexpr bool test_iterators() {
+  { // simple
+    { // unary
+      {
+        int a[] = {1, 2, 3, 4, 5};
+        int b[5];
+        std::same_as<std::ranges::in_out_result<In1, Out>> decltype(auto) ret =
+          std::ranges::transform(In1(a), Sent1(In1(a + 5)), Out(b), [](int i) { return i * 2; });
+        assert((std::to_array(b) == std::array{2, 4, 6, 8, 10}));
+        assert(base(ret.in) == a + 5);
+        assert(base(ret.out) == b + 5);
+      }
+
+      {
+        int a[] = {1, 2, 3, 4, 5};
+        int b[5];
+        auto range = std::ranges::subrange(In1(a), Sent1(In1(a + 5)));
+        std::same_as<std::ranges::in_out_result<In1, Out>> decltype(auto) ret =
+          std::ranges::transform(range, Out(b), [](int i) { return i * 2; });
+        assert((std::to_array(b) == std::array{2, 4, 6, 8, 10}));
+        assert(base(ret.in) == a + 5);
+        assert(base(ret.out) == b + 5);
+      }
+    }
+
+    { // binary
+      {
+        int a[] = {1, 2, 3, 4, 5};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        std::same_as<std::ranges::in_in_out_result<In1, In2, Out>> decltype(auto) ret = std::ranges::transform(
+            In1(a), Sent1(In1(a + 5)), In2(b), Sent2(In2(b + 5)), Out(c), [](int i, int j) { return i + j; });
+
+        assert((std::to_array(c) == std::array{6, 6, 6, 6, 6}));
+        assert(base(ret.in1) == a + 5);
+        assert(base(ret.in2) == b + 5);
+        assert(base(ret.out) == c + 5);
+      }
+
+      {
+        int a[] = {1, 2, 3, 4, 5};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a + 5)));
+        auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b + 5)));
+
+        std::same_as<std::ranges::in_in_out_result<In1, In2, Out>> decltype(auto) ret = std::ranges::transform(
+            range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+        assert((std::to_array(c) == std::array{6, 6, 6, 6, 6}));
+        assert(base(ret.in1) == a + 5);
+        assert(base(ret.in2) == b + 5);
+        assert(base(ret.out) == c + 5);
+      }
+    }
+  }
+
+  { // first range empty
+    { // unary
+      {
+        int a[] = {};
+        int b[5];
+        auto ret = std::ranges::transform(In1(a), Sent1(In1(a)), Out(b), [](int i) { return i * 2; });
+        assert(base(ret.in) == a);
+        assert(base(ret.out) == b);
+      }
+
+      {
+        int a[] = {};
+        int b[5];
+        auto range = std::ranges::subrange(In1(a), Sent1(In1(a)));
+        auto ret = std::ranges::transform(range, Out(b), [](int i) { return i * 2; });
+        assert(base(ret.in) == a);
+        assert(base(ret.out) == b);
+      }
+    }
+
+    { // binary
+      {
+        int a[] = {};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        auto ret = std::ranges::transform(
+            In1(a), Sent1(In1(a)), In2(b), Sent2(In2(b + 5)), Out(c), [](int i, int j) { return i + j; });
+
+        assert(base(ret.in1) == a);
+        assert(base(ret.in2) == b);
+        assert(base(ret.out) == c);
+      }
+
+      {
+        int a[] = {};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a)));
+        auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b + 5)));
+
+        auto ret = std::ranges::transform(range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+        assert(base(ret.in1) == a);
+        assert(base(ret.in2) == b);
+        assert(base(ret.out) == c);
+      }
+    }
+  }
+
+  { // second range empty (binary)
+    {
+      int a[] = {5, 4, 3, 2, 1};
+      int b[] = {};
+      int c[5];
+
+      auto ret = std::ranges::transform(
+          In1(a), Sent1(In1(a + 5)), In2(b), Sent2(In2(b)), Out(c), [](int i, int j) { return i + j; });
+
+      assert(base(ret.in1) == a);
+      assert(base(ret.in2) == b);
+      assert(base(ret.out) == c);
+    }
+
+    {
+      int a[] = {5, 4, 3, 2, 1};
+      int b[] = {};
+      int c[5];
+
+      auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a + 5)));
+      auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b)));
+
+      auto ret = std::ranges::transform(range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+      assert(base(ret.in1) == a);
+      assert(base(ret.in2) == b);
+      assert(base(ret.out) == c);
+    }
+  }
+
+  { // both ranges empty (binary)
+    {
+      int a[] = {};
+      int b[] = {};
+      int c[5];
+
+      auto ret = std::ranges::transform(
+          In1(a), Sent1(In1(a)), In2(b), Sent2(In2(b)), Out(c), [](int i, int j) { return i + j; });
+
+      assert(base(ret.in1) == a);
+      assert(base(ret.in2) == b);
+      assert(base(ret.out) == c);
+    }
+
+    {
+      int a[] = {};
+      int b[] = {};
+      int c[5];
+
+      auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a)));
+      auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b)));
+
+      auto ret = std::ranges::transform(range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+      assert(base(ret.in1) == a);
+      assert(base(ret.in2) == b);
+      assert(base(ret.out) == c);
+    }
+  }
+
+  { // first range one element
+    { // unary
+      {
+        int a[] = {2};
+        int b[5];
+        auto ret = std::ranges::transform(In1(a), Sent1(In1(a + 1)), Out(b), [](int i) { return i * 2; });
+        assert(b[0] == 4);
+        assert(base(ret.in) == a + 1);
+        assert(base(ret.out) == b + 1);
+      }
+
+      {
+        int a[] = {2};
+        int b[5];
+        auto range = std::ranges::subrange(In1(a), Sent1(In1(a + 1)));
+        auto ret = std::ranges::transform(range, Out(b), [](int i) { return i * 2; });
+        assert(b[0] == 4);
+        assert(base(ret.in) == a + 1);
+        assert(base(ret.out) == b + 1);
+      }
+    }
+
+    { // binary
+      {
+        int a[] = {2};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        auto ret = std::ranges::transform(
+            In1(a), Sent1(In1(a + 1)), In2(b), Sent2(In2(b + 5)), Out(c), [](int i, int j) { return i + j; });
+
+        assert(c[0] == 7);
+        assert(base(ret.in1) == a + 1);
+        assert(base(ret.in2) == b + 1);
+        assert(base(ret.out) == c + 1);
+      }
+
+      {
+        int a[] = {2};
+        int b[] = {5, 4, 3, 2, 1};
+        int c[5];
+
+        auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a + 1)));
+        auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b + 5)));
+
+        auto ret = std::ranges::transform(range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+        assert(c[0] == 7);
+        assert(base(ret.in1) == a + 1);
+        assert(base(ret.in2) == b + 1);
+        assert(base(ret.out) == c + 1);
+      }
+    }
+  }
+
+  { // second range contains one element (binary)
+    {
+      int a[] = {5, 4, 3, 2, 1};
+      int b[] = {4};
+      int c[5];
+
+      auto ret = std::ranges::transform(
+          In1(a), Sent1(In1(a + 5)), In2(b), Sent2(In2(b + 1)), Out(c), [](int i, int j) { return i + j; });
+
+      assert(c[0] == 9);
+      assert(base(ret.in1) == a + 1);
+      assert(base(ret.in2) == b + 1);
+      assert(base(ret.out) == c + 1);
+    }
+
+    {
+      int a[] = {5, 4, 3, 2, 1};
+      int b[] = {4};
+      int c[5];
+
+      auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a + 5)));
+      auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b + 1)));
+
+      auto ret = std::ranges::transform(range1, range2, Out(c), [](int i, int j) { return i + j; });
+
+      assert(c[0] == 9);
+      assert(base(ret.in1) == a + 1);
+      assert(base(ret.in2) == b + 1);
+      assert(base(ret.out) == c + 1);
+    }
+  }
+
+  { // check that the transform function and projection call counts are correct
+    { // unary
+      {
+        int predCount = 0;
+        int projCount = 0;
+        auto pred = [&](int) { ++predCount; return 1; };
+        auto proj = [&](int) { ++projCount; return 0; };
+        int a[] = {1, 2, 3, 4};
+        std::array<int, 4> c;
+        std::ranges::transform(In1(a), Sent1(In1(a + 4)), Out(c.data()), pred, proj);
+        assert(predCount == 4);
+        assert(projCount == 4);
+        assert((c == std::array{1, 1, 1, 1}));
+      }
+      {
+        int predCount = 0;
+        int projCount = 0;
+        auto pred = [&](int) { ++predCount; return 1; };
+        auto proj = [&](int) { ++projCount; return 0; };
+        int a[] = {1, 2, 3, 4};
+        std::array<int, 4> c;
+        auto range = std::ranges::subrange(In1(a), Sent1(In1(a + 4)));
+        std::ranges::transform(range, Out(c.data()), pred, proj);
+        assert(predCount == 4);
+        assert(projCount == 4);
+        assert((c == std::array{1, 1, 1, 1}));
+      }
+    }
+    { // binary
+      {
+        int predCount = 0;
+        int proj1Count = 0;
+        int proj2Count = 0;
+        auto pred = [&](int, int) { ++predCount; return 1; };
+        auto proj1 = [&](int) { ++proj1Count; return 0; };
+        auto proj2 = [&](int) { ++proj2Count; return 0; };
+        int a[] = {1, 2, 3, 4};
+        int b[] = {1, 2, 3, 4};
+        std::array<int, 4> c;
+        std::ranges::transform(In1(a), Sent1(In1(a + 4)), In2(b), Sent2(In2(b + 4)), Out(c.data()), pred, proj1, proj2);
+        assert(predCount == 4);
+        assert(proj1Count == 4);
+        assert(proj2Count == 4);
+        assert((c == std::array{1, 1, 1, 1}));
+      }
+      {
+        int predCount = 0;
+        int proj1Count = 0;
+        int proj2Count = 0;
+        auto pred = [&](int, int) { ++predCount; return 1; };
+        auto proj1 = [&](int) { ++proj1Count; return 0; };
+        auto proj2 = [&](int) { ++proj2Count; return 0; };
+        int a[] = {1, 2, 3, 4};
+        int b[] = {1, 2, 3, 4};
+        std::array<int, 4> c;
+        auto range1 = std::ranges::subrange(In1(a), Sent1(In1(a + 4)));
+        auto range2 = std::ranges::subrange(In2(b), Sent2(In2(b + 4)));
+        std::ranges::transform(range1, range2, Out(c.data()), pred, proj1, proj2);
+        assert(predCount == 4);
+        assert(proj1Count == 4);
+        assert(proj2Count == 4);
+        assert((c == std::array{1, 1, 1, 1}));
+      }
+    }
+  }
+
+  return true;
+}
+
+template <class In2, class Out, class Sent2 = In2>
+constexpr void test_iterator_in1() {
+  test_iterators<cpp17_input_iterator<int*>, In2, Out, sentinel_wrapper<cpp17_input_iterator<int*>>, Sent2>();
+  test_iterators<cpp20_input_iterator<int*>, In2, Out, sentinel_wrapper<cpp20_input_iterator<int*>>, Sent2>();
+  test_iterators<forward_iterator<int*>, In2, Out, forward_iterator<int*>, Sent2>();
+  test_iterators<bidirectional_iterator<int*>, In2, Out, bidirectional_iterator<int*>, Sent2>();
+  test_iterators<random_access_iterator<int*>, In2, Out, random_access_iterator<int*>, Sent2>();
+  test_iterators<contiguous_iterator<int*>, In2, Out, contiguous_iterator<int*>, Sent2>();
+  test_iterators<int*, In2, Out, int*, Sent2>();
+  // static_asserting here to avoid hitting the constant evaluation step limit
+  static_assert(test_iterators<cpp17_input_iterator<int*>, In2, Out, sentinel_wrapper<cpp17_input_iterator<int*>>, Sent2>());
+  static_assert(test_iterators<cpp20_input_iterator<int*>, In2, Out, sentinel_wrapper<cpp20_input_iterator<int*>>, Sent2>());
+  static_assert(test_iterators<forward_iterator<int*>, In2, Out, forward_iterator<int*>, Sent2>());
+  static_assert(test_iterators<bidirectional_iterator<int*>, In2, Out, bidirectional_iterator<int*>, Sent2>());
+  static_assert(test_iterators<random_access_iterator<int*>, In2, Out, random_access_iterator<int*>, Sent2>());
+  static_assert(test_iterators<contiguous_iterator<int*>, In2, Out, contiguous_iterator<int*>, Sent2>());
+  static_assert(test_iterators<int*, In2, Out, int*, Sent2>());
+}
+
+template <class Out>
+void test_iterators_in1_in2() {
+  test_iterator_in1<cpp17_input_iterator<int*>, Out, sentinel_wrapper<cpp17_input_iterator<int*>>>();
+  test_iterator_in1<cpp20_input_iterator<int*>, Out, sentinel_wrapper<cpp20_input_iterator<int*>>>();
+  test_iterator_in1<forward_iterator<int*>, Out>();
+  test_iterator_in1<bidirectional_iterator<int*>, Out>();
+  test_iterator_in1<random_access_iterator<int*>, Out>();
+  test_iterator_in1<contiguous_iterator<int*>, Out>();
+  test_iterator_in1<int*, Out>();
+}
+
+constexpr bool test() {
+  { // check that std::ranges::dangling is returned properly
+    { // unary
+      std::array<int, 5> b;
+      std::same_as<std::ranges::in_out_result<std::ranges::dangling, int*>> auto ret =
+          std::ranges::transform(std::array{1, 2, 3, 5, 4}, b.data(), [](int i) { return i * i; });
+      assert((b == std::array{1, 4, 9, 25, 16}));
+      assert(ret.out == b.data() + b.size());
+    }
+    // binary
+    {
+      int b[] = {2, 5, 4, 3, 1};
+      std::array<int, 5> c;
+      std::same_as<std::ranges::in_in_out_result<std::ranges::dangling, int*, int*>> auto ret =
+          std::ranges::transform(std::array{1, 2, 3, 5, 4}, b, c.data(), [](int i, int j) { return i * j; });
+      assert((c == std::array{2, 10, 12, 15, 4}));
+      assert(ret.in2 == b + 5);
+      assert(ret.out == c.data() + c.size());
+    }
+    {
+      int a[] = {2, 5, 4, 3, 1, 4, 5, 6};
+      std::array<int, 8> c;
+      std::same_as<std::ranges::in_in_out_result<int*, std::ranges::dangling, int*>> auto ret =
+          std::ranges::transform(a, std::array{1, 2, 3, 5, 4, 5, 6, 7}, c.data(), [](int i, int j) { return i * j; });
+      assert((c == std::array{2, 10, 12, 15, 4, 20, 30, 42}));
+      assert(ret.in1 == a + 8);
+      assert(ret.out == c.data() + c.size());
+    }
+    {
+      std::array<int, 3> c;
+      std::same_as<std::ranges::in_in_out_result<std::ranges::dangling, std::ranges::dangling, int*>> auto ret =
+          std::ranges::transform(std::array{4, 4, 4}, std::array{4, 4, 4}, c.data(), [](int i, int j) { return i * j; });
+      assert((c == std::array{16, 16, 16}));
+      assert(ret.out == c.data() + c.size());
+    }
+  }
+
+  { // check that returning another type from the projection works
+    { // unary
+      {
+        struct S { int i; int other; };
+        S a[] = { S{0, 0}, S{1, 0}, S{3, 0}, S{10, 0} };
+        std::array<int, 4> b;
+        std::ranges::transform(a, a + 4, b.begin(), [](S s) { return s.i; });
+        assert((b == std::array{0, 1, 3, 10}));
+      }
+      {
+        struct S { int i; int other; };
+        S a[] = { S{0, 0}, S{1, 0}, S{3, 0}, S{10, 0} };
+        std::array<int, 4> b;
+        std::ranges::transform(a, b.begin(), [](S s) { return s.i; });
+        assert((b == std::array{0, 1, 3, 10}));
+      }
+    }
+    { // binary
+      {
+        struct S { int i; int other; };
+        S a[] = { S{0, 0}, S{1, 0}, S{3, 0}, S{10, 0} };
+        S b[] = { S{0, 10}, S{1, 20}, S{3, 30}, S{10, 40} };
+        std::array<int, 4> c;
+        std::ranges::transform(a, a + 4, b, b + 4, c.begin(), [](S s1, S s2) { return s1.i + s2.other; });
+        assert((c == std::array{10, 21, 33, 50}));
+      }
+      {
+        struct S { int i; int other; };
+        S a[] = { S{0, 0}, S{1, 0}, S{3, 0}, S{10, 0} };
+        S b[] = { S{0, 10}, S{1, 20}, S{3, 30}, S{10, 40} };
+        std::array<int, 4> c;
+        std::ranges::transform(a, b, c.begin(), [](S s1, S s2) { return s1.i + s2.other; });
+        assert((c == std::array{10, 21, 33, 50}));
+      }
+    }
+  }
+
+  { // check that std::invoke is used
+    { // unary
+      struct S { int i; };
+      S a[] = { S{1}, S{3}, S{2} };
+      std::array<int, 3> b;
+      auto ret = std::ranges::transform(a, b.data(), [](int i) { return i; }, &S::i);
+      assert((b == std::array{1, 3, 2}));
+      assert(ret.out == b.data() + 3);
+    }
+    { // binary
+      struct S { int i; };
+      S a[] = { S{1}, S{3}, S{2} };
+      S b[] = { S{2}, S{5}, S{3} };
+      std::array<int, 3> c;
+      auto ret = std::ranges::transform(a, b, c.data(), [](int i, int j) { return i + j + 2; }, &S::i, &S::i);
+      assert((c == std::array{5, 10, 7}));
+      assert(ret.out == c.data() + 3);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test_iterators_in1_in2<cpp17_output_iterator<int*>>();
+  test_iterators_in1_in2<forward_iterator<int*>>();
+  test_iterators_in1_in2<bidirectional_iterator<int*>>();
+  test_iterators_in1_in2<random_access_iterator<int*>>();
+  test_iterators_in1_in2<contiguous_iterator<int*>>();
+  test_iterators_in1_in2<int*>();
+  test();
+  static_assert(test());
+
+  return 0;
+}

libcxx/test/support/almost_satisfies_types.h

 
 using InputRangeNotSentinelEqualityComparableWith =
   UncheckedRange<cpp20_input_iterator<int*>, SentinelForNotWeaklyEqualityComparableWith>;
 
 static_assert(std::input_or_output_iterator<SentinelForNotWeaklyEqualityComparableWith>);
 static_assert(std::semiregular<SentinelForNotWeaklyEqualityComparableWith>);
 static_assert(!std::sentinel_for<SentinelForNotWeaklyEqualityComparableWith, cpp20_input_iterator<int*>>);
 
+class WeaklyIncrementableNotMovable {
+public:
+  using difference_type = long;
+  WeaklyIncrementableNotMovable& operator++();
+  void operator++(int);
+  WeaklyIncrementableNotMovable(const WeaklyIncrementableNotMovable&) = delete;
+};
+
+static_assert(!std::movable<WeaklyIncrementableNotMovable>);
+static_assert(!std::weakly_incrementable<WeaklyIncrementableNotMovable>);
+
 #endif // ALMOST_SATISFIES_TYPES_H