[libc++] Implement P2445R1 (`std::forward_like`)

[fsb4000]

apply clang-format.patch and generated_output.patch

[fsb4000]

add missing - to ReleaseNotes.rst

[philnik]

libcxx/include/__utility/forward_like.h

26

Or is there a reason for the t?

27

personally I'd use decltype(auto) instead. Not sure what the consensus on that is though.

29

Is there a reason you don't want to use aliases and conditionals like the standard does? The current implementation is very hard to compare to the standard.

Something like

template <class _Ap, class _Bp>
using _CopyConst = _If<is_const_v<_Ap>, const _Bp, _Bp>;

template <class _Ap, class _Bp>
using _OverrideRef = _If<is_rvalue_reference_v<_Ap>, remove_reference_t<_Bp>&&, _Bp&>;

using _Vp = _OverrideRef<_Tp&&, _CopyConst<remove_reference_t<_Tp>, remove_reference_t<_Up>>>;

return static_cast<_Vp>(__ux);

would be a lot clearer IMO.

libcxx/include/utility

45–60

Why do you have (AFAICT non-existent) implementation details in the synopsis? You are also missing a // since C++23 after forwrward_like.

57–59

This should be copied from the standard.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.compile.pass.cpp

2–13

Two license headers seem weird. Are you sure the LLVM license header should be here?

14

I'm missing tests that ensure forward_like doesn't copy/move anything around or requires any constructors. A test that forwards the same type would also be nice. You should also check that forward_like is noexcept.

[philnik]

libcxx/include/__utility/forward_like.h

16

Please use the granularized type_traits headers instead.

[fsb4000]

fix tests

[huixie90]

libcxx/include/__utility/forward_like.h

27–28

The standard spec has a trailing return type, meaning if the return type is ill formed, the function should be SFINAE out.
I think we should keep what is in the spec rather than hard error in the function body

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.compile.pass.cpp

73–83

I think it should also run the test at runtime and the test() should also test all different combinations.

[fsb4000]

libcxx/include/__utility/forward_like.h

26

Originally they were _Ty and _Uty but I noticed that libc++ uses _Tp so I renamed y to p.
Sure, I can remove the t.

libcxx/include/utility

45–60

I copied from https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2445r1.pdf

Yes, I will add // since C++23 and I look at the standard.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.compile.pass.cpp

2–13

Maybe you're right.
Now I looked how charconv.msvc was made:

https://github.com/llvm/llvm-project/blob/main/libcxx/test/std/utilities/charconv/charconv.msvc/test.pass.cpp

and

https://github.com/llvm/llvm-project/blob/main/libcxx/test/std/utilities/charconv/charconv.msvc/test.cpp

So probably I should do the same.

[fsb4000]

applied suggestions from @philnik and @huixie90 code reviews

libcxx/include/utility

45–60

C++ standard has the same implementation details.
https://eel.is/c++draft/forward#lib:forward_like

[fsb4000]

remove #include <__iterator/iterator_traits.h>

[fsb4000]

try fix tests

[huixie90]

libcxx/include/__utility/forward_like.h

38–39

I think the return type is just _ForwardLike<_Tp, _Up>

libcxx/include/utility

45–59

I think you don't need to put implementation details here. just copy whatever in the standard

template<class T, class U>
[[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> see below;        // since C++23

[fsb4000]

apply huixie90's code review suggestions.

[fsb4000]

I tried to fix "Generated output) failed"

and it seems ninja -C build libcxx-generate-files has completely different behavior on Windows and Linux 😿
What should I do?

[fsb4000]

git apply generated_output.patch

[fsb4000]

CRLF => LF

[fsb4000]

remove a blank space.

[fsb4000]

move => std::move

[huixie90]

LGTM with nits. I will leave the final approval to others who also commented on this patch.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp

12 ↗

(On Diff #456106)

Is this test supposed to be running on gcc/clang? I am slightly confused because it is in "msvc" folder

[fsb4000]

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp

12 ↗

(On Diff #456106)

Yes, all configurations should be tested.

I used https://github.com/llvm/llvm-project/tree/main/libcxx/test/std/utilities/charconv/charconv.msvc as a naming example.

I opened gcc 12/ C++ latest log: https://buildkite.com/llvm-project/libcxx-ci/builds/13210#0182deee-3853-484d-a646-79afa1e27420 and I found:

... cfg.in :: std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp
 98% [=========================================================--(B](B ETA: 00:00:04

and C++2b log: https://buildkite.com/llvm-project/libcxx-ci/builds/13210#0182deee-3841-41d9-8725-06f9a249e560

... cfg.in :: std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp
 98% [=========================================================--(B](B ETA: 00:00:08

[philnik]

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.cpp

8 ↗

(On Diff #456106)

I guess this is inherited from the MSVC testing style?

67 ↗

(On Diff #456106)

Are these changes in the upstream MSVC test? If not, I'd rather have them in a separate test to reduce divergence from upstream.

73 ↗

(On Diff #456106)

Why do you use an alias here? IMO it makes the test really hard to read.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp

12 ↗

(On Diff #456106)

This way to include the test seems quite weird to me. Why can't we just name the MSVC test test.pass.cpp?

[fsb4000]

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.cpp

8 ↗

(On Diff #456106)

Yes, MSVC using this style.

67 ↗

(On Diff #456106)

I will create a PR to the MSVC test after this patch will be landed.
I'm almost sure they will merge it without issues.

73 ↗

(On Diff #456106)

Ok, I will not use the alias.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp

12 ↗

(On Diff #456106)

Sure, we can.
I did similar to https://github.com/llvm/llvm-project/blob/main/libcxx/test/std/utilities/charconv/charconv.msvc/test.pass.cpp

If we name the test test.pass.cpp should I add LLVM licence comment too?

[Mordante]

I glossed over the code and don't see any obvious issues. However I didn't do a real review.

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.pass.cpp

12 ↗

(On Diff #456106)

This way to include the test seems quite weird to me. Why can't we just name the MSVC test test.pass.cpp?

I agree it's weird but we used this method before and it minimized the differences between our version and upstream.

I still want to look at a way to include more MSVC STL tests in our repository; however that is very low on my todo list.

[fsb4000]

I removed using NCCM = NoCtorCopyMove;

I will create a PR to the MSVC test after this patch will be landed so

And I didn't rename test.cpp to test.pass.cpp because we have 2 (or 3 if we count constexpr unique_ptr patch) such tests.

So maybe different patch for that if we decide that something like that would be better:

c++
// LLVM license

some workarounds if needed

// clang-format off
// MSVC licence
code from MSVC test file.
// clang-format on

[philnik]

LGTM.

[Mordante]

The change broke the CI. I see you added a fix in D131315.
Since it breaks the CI and it's trivial to fix next time please commit the fix directly without a review.
If the fix is non-trivial it's better to revert the patch, fix it, review if needed, and reland the patch.
I already fixed it. It seems the line-ending caused it, but that wasn't caught in the build started from Phabricator.

libcxx/docs/FeatureTestMacroTable.rst

.. table:: Current Status
     ``__cpp_lib_byteswap``                            ``202110L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_bitset``                    ``202207L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_cmath``                     *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_forward_like``                        ``202207L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_invoke_r``                            *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                      ``202011L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_move_only_function``                  *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_optional``                            ``202110L``
     ------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes.rst

 
 What's New in Libc++ 16.0.0?
 ============================
 
 Implemented Papers
 ------------------
 - P2499R0 - ``string_view`` range constructor should be ``explicit``
 - P2417R2 - A more constexpr bitset
+- P2445R1 ``std::forward_like``
 
 Improvements and New Features
 -----------------------------
 
 Deprecations and Removals
 -------------------------
 
 Upcoming Deprecations and Removals

libcxx/docs/Status/Cxx2bPapers.csv

 "`P2302R4 <https://wg21.link/P2302R4>`__","LWG","``std::ranges::contains``","July 2022","",""
 "`P2322R6 <https://wg21.link/P2322R6>`__","LWG","``ranges::fold``","July 2022","",""
 "`P2374R4 <https://wg21.link/P2374R4>`__","LWG","``views::cartesian_product``","July 2022","",""
 "`P2404R3 <https://wg21.link/P2404R3>`__","LWG","Move-only types for ``equality_comparable_with``, ``totally_ordered_with``, and ``three_way_comparable_with``","July 2022","",""
 "`P2408R5 <https://wg21.link/P2408R5>`__","LWG","Ranges iterators as inputs to non-Ranges algorithms","July 2022","",""
 "`P2417R2 <https://wg21.link/P2417R2>`__","LWG","A more ``constexpr`` ``bitset``","July 2022","|Complete|","16.0"
 "`P2419R2 <https://wg21.link/P2419R2>`__","LWG","Clarify handling of encodings in localized formatting of chrono types","July 2022","",""
 "`P2438R2 <https://wg21.link/P2438R2>`__","LWG","``std::string::substr() &&``","July 2022","",""
-"`P2445R1 <https://wg21.link/P2445R1>`__","LWG","``forward_like``","July 2022","",""
+"`P2445R1 <https://wg21.link/P2445R1>`__","LWG","``forward_like``","July 2022","|Complete|","16.0"
 "`P2446R2 <https://wg21.link/P2446R2>`__","LWG","``views::as_rvalue``","July 2022","",""
 "`P2460R2 <https://wg21.link/P2460R2>`__","LWG","Relax requirements on ``wchar_t`` to match existing practices","July 2022","",""
 "`P2465R3 <https://wg21.link/P2465R3>`__","LWG","Standard Library Modules ``std`` and ``std.compat``","July 2022","",""
 "`P2467R1 <https://wg21.link/P2467R1>`__","LWG","Support exclusive mode for ``fstreams``","July 2022","",""
 "`P2474R2 <https://wg21.link/P2474R2>`__","LWG","``views::repeat``","July 2022","",""
 "`P2494R2 <https://wg21.link/P2494R2>`__","LWG","Relaxing range adaptors to allow for move only types","July 2022","",""
 "`P2499R0 <https://wg21.link/P2499R0>`__","LWG","``string_view`` range constructor should be ``explicit``","July 2022","|Complete|","16.0"
 "`P2502R2 <https://wg21.link/P2502R2>`__","LWG","``std::generator``: Synchronous Coroutine Generator for Ranges","July 2022","",""

libcxx/include/CMakeLists.txt

set(files
   __undef_macros
   __utility/as_const.h
   __utility/auto_cast.h
   __utility/cmp.h
   __utility/convert_to_integral.h
   __utility/declval.h
   __utility/exchange.h
   __utility/forward.h
+  __utility/forward_like.h
   __utility/in_place.h
   __utility/integer_sequence.h
   __utility/move.h
   __utility/pair.h
   __utility/piecewise_construct.h
   __utility/priority_tag.h
   __utility/rel_ops.h
   __utility/swap.h

libcxx/include/__utility/forward_like.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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___UTILITY_FORWARD_LIKE_H
+#define _LIBCPP___UTILITY_FORWARD_LIKE_H
+
+#include <__config>
+#include <__iterator/iterator_traits.h> // for __can_reference
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 20
+
+template <class _Tp, class _Utp>
+[[nodiscard]] constexpr auto&& forward_like(_Utp&& __ux) noexcept {
+    static_assert(__can_reference<_Tp>, "std::forward_like's first template argument must be a referenceable type.");
+
+    using __unrefT = remove_reference_t<_Tp>;
+    using __unrefU = remove_reference_t<_Utp>;
+    if constexpr (is_const_v<__unrefT>) {
+        if constexpr (is_lvalue_reference_v<_Tp>) {
+            return static_cast<const __unrefU&>(__ux);
+        } else {
+            return static_cast<const __unrefU&&>(__ux);
+        }
+    } else {
+        if constexpr (is_lvalue_reference_v<_Tp>) {
+            return static_cast<__unrefU&>(__ux);
+        } else {
+            return static_cast<__unrefU&&>(__ux);
+        }
+    }
+}
+
+#endif // _LIBCPP_STD_VER > 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___UTILITY_FORWARD_LIKE_H

libcxx/include/utility

 
 template <class T, size_t N>
 void
 swap(T (&a)[N], T (&b)[N]) noexcept(noexcept(swap(*a, *b)));
 
 template <class T> T&& forward(typename remove_reference<T>::type& t) noexcept;  // constexpr in C++14
 template <class T> T&& forward(typename remove_reference<T>::type&& t) noexcept; // constexpr in C++14
 
+template <typename T, typename U>
+using __override_ref_t = std::conditional_t<std::is_rvalue_reference_v<T>,
+                                            std::remove_reference_t<U> &&, U &>;
+template <typename T, typename U>
+using __copy_const_t =
+    std::conditional_t<std::is_const_v<std::remove_reference_t<T>>,
+                       U const, U>;
+template <typename T, typename U>
+using __forward_like_t = __override_ref_t<
+    T &&,
+    __copy_const_t<T, std::remove_reference_t<U>>>;
+
+template <typename T>
+[[nodiscard]] constexpr
+auto forward_like(auto&& x) noexcept -> __forward_like_t<T, decltype(x)>;
+
 template <class T> typename remove_reference<T>::type&& move(T&&) noexcept;      // constexpr in C++14
 
 template <class T>
     typename conditional
     <
         !is_nothrow_move_constructible<T>::value && is_copy_constructible<T>::value,
         const T&,
         T&&

 #include <__config>
 #include <__tuple>
 #include <__utility/as_const.h>
 #include <__utility/auto_cast.h>
 #include <__utility/cmp.h>
 #include <__utility/declval.h>
 #include <__utility/exchange.h>
 #include <__utility/forward.h>
+#include <__utility/forward_like.h>
 #include <__utility/in_place.h>
 #include <__utility/integer_sequence.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
 #include <__utility/piecewise_construct.h>
 #include <__utility/priority_tag.h>
 #include <__utility/rel_ops.h>
 #include <__utility/swap.h>

libcxx/include/version

 __cpp_lib_erase_if                                      202002L <deque> <forward_list> <list>
                                                                 <map> <set> <string>
                                                                 <unordered_map> <unordered_set> <vector>
 __cpp_lib_exchange_function                             201304L <utility>
 __cpp_lib_execution                                     201902L <execution>
                                                         201603L // C++17
 __cpp_lib_filesystem                                    201703L <filesystem>
 __cpp_lib_format                                        202106L <format>
+__cpp_lib_forward_like                                  202207L <utility>
 __cpp_lib_gcd_lcm                                       201606L <numeric>
 __cpp_lib_generic_associative_lookup                    201304L <map> <set>
 __cpp_lib_generic_unordered_lookup                      201811L <unordered_map> <unordered_set>
 __cpp_lib_hardware_interference_size                    201703L <new>
 __cpp_lib_has_unique_object_representations             201606L <type_traits>
 __cpp_lib_hypot                                         201603L <cmath>
 __cpp_lib_incomplete_container_elements                 201505L <forward_list> <list> <vector>
 __cpp_lib_int_pow2                                      202002L <bit>

 # define __cpp_lib_adaptor_iterator_pair_constructor    202106L
 # define __cpp_lib_allocate_at_least                    202106L
 // # define __cpp_lib_associative_heterogeneous_erasure    202110L
 // # define __cpp_lib_bind_back                            202202L
 # define __cpp_lib_byteswap                             202110L
 # define __cpp_lib_constexpr_bitset                     202207L
 // # define __cpp_lib_constexpr_cmath                      202202L
 // # define __cpp_lib_constexpr_typeinfo                   202106L
+# define __cpp_lib_forward_like                         202207L
 // # define __cpp_lib_invoke_r                             202106L
 # define __cpp_lib_is_scoped_enum                       202011L
 // # define __cpp_lib_move_only_function                   202110L
 # undef  __cpp_lib_optional
 # define __cpp_lib_optional                             202110L
 // # define __cpp_lib_out_ptr                              202106L
 // # define __cpp_lib_ranges_chunk                         202202L
 // # define __cpp_lib_ranges_chunk_by                      202202L

libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.compile.pass.cpp

 // <utility>
 
 // Test the feature test macros defined by <utility>
 
 /*  Constant                                  Value
     __cpp_lib_as_const                        201510L [C++17]
     __cpp_lib_constexpr_utility               201811L [C++20]
     __cpp_lib_exchange_function               201304L [C++14]
+    __cpp_lib_forward_like                    202207L [C++2b]
     __cpp_lib_integer_comparison_functions    202002L [C++20]
     __cpp_lib_integer_sequence                201304L [C++14]
     __cpp_lib_ranges_zip                      202110L [C++2b]
     __cpp_lib_to_underlying                   202102L [C++2b]
     __cpp_lib_tuples_by_type                  201304L [C++14]
     __cpp_lib_unreachable                     202202L [C++2b]
 */
 

 # ifdef __cpp_lib_constexpr_utility
 #   error "__cpp_lib_constexpr_utility should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_exchange_function
 #   error "__cpp_lib_exchange_function should not be defined before c++14"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifdef __cpp_lib_integer_comparison_functions
 #   error "__cpp_lib_integer_comparison_functions should not be defined before c++20"
 # endif
 
 # ifdef __cpp_lib_integer_sequence
 #   error "__cpp_lib_integer_sequence should not be defined before c++14"
 # endif
 

 
 # ifndef __cpp_lib_exchange_function
 #   error "__cpp_lib_exchange_function should be defined in c++14"
 # endif
 # if __cpp_lib_exchange_function != 201304L
 #   error "__cpp_lib_exchange_function should have the value 201304L in c++14"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifdef __cpp_lib_integer_comparison_functions
 #   error "__cpp_lib_integer_comparison_functions should not be defined before c++20"
 # endif
 
 # ifndef __cpp_lib_integer_sequence
 #   error "__cpp_lib_integer_sequence should be defined in c++14"
 # endif
 # if __cpp_lib_integer_sequence != 201304L

 
 # ifndef __cpp_lib_exchange_function
 #   error "__cpp_lib_exchange_function should be defined in c++17"
 # endif
 # if __cpp_lib_exchange_function != 201304L
 #   error "__cpp_lib_exchange_function should have the value 201304L in c++17"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifdef __cpp_lib_integer_comparison_functions
 #   error "__cpp_lib_integer_comparison_functions should not be defined before c++20"
 # endif
 
 # ifndef __cpp_lib_integer_sequence
 #   error "__cpp_lib_integer_sequence should be defined in c++17"
 # endif
 # if __cpp_lib_integer_sequence != 201304L

 
 # ifndef __cpp_lib_exchange_function
 #   error "__cpp_lib_exchange_function should be defined in c++20"
 # endif
 # if __cpp_lib_exchange_function != 201304L
 #   error "__cpp_lib_exchange_function should have the value 201304L in c++20"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifndef __cpp_lib_integer_comparison_functions
 #   error "__cpp_lib_integer_comparison_functions should be defined in c++20"
 # endif
 # if __cpp_lib_integer_comparison_functions != 202002L
 #   error "__cpp_lib_integer_comparison_functions should have the value 202002L in c++20"
 # endif
 
 # ifndef __cpp_lib_integer_sequence

 
 # ifndef __cpp_lib_exchange_function
 #   error "__cpp_lib_exchange_function should be defined in c++2b"
 # endif
 # if __cpp_lib_exchange_function != 201304L
 #   error "__cpp_lib_exchange_function should have the value 201304L in c++2b"
 # endif
 
+# ifndef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should be defined in c++2b"
+# endif
+# if __cpp_lib_forward_like != 202207L
+#   error "__cpp_lib_forward_like should have the value 202207L in c++2b"
+# endif
+
 # ifndef __cpp_lib_integer_comparison_functions
 #   error "__cpp_lib_integer_comparison_functions should be defined in c++2b"
 # endif
 # if __cpp_lib_integer_comparison_functions != 202002L
 #   error "__cpp_lib_integer_comparison_functions should have the value 202002L in c++2b"
 # endif
 
 # ifndef __cpp_lib_integer_sequence

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

/*  Constant                                       Value
     __cpp_lib_enable_shared_from_this              201603L [C++17]
     __cpp_lib_endian                               201907L [C++20]
     __cpp_lib_erase_if                             202002L [C++20]
     __cpp_lib_exchange_function                    201304L [C++14]
     __cpp_lib_execution                            201603L [C++17]
                                                    201902L [C++20]
     __cpp_lib_filesystem                           201703L [C++17]
     __cpp_lib_format                               202106L [C++20]
+    __cpp_lib_forward_like                         202207L [C++2b]
     __cpp_lib_gcd_lcm                              201606L [C++17]
     __cpp_lib_generic_associative_lookup           201304L [C++14]
     __cpp_lib_generic_unordered_lookup             201811L [C++20]
     __cpp_lib_hardware_interference_size           201703L [C++17]
     __cpp_lib_has_unique_object_representations    201606L [C++17]
     __cpp_lib_hypot                                201603L [C++17]
     __cpp_lib_incomplete_container_elements        201505L [C++17]
     __cpp_lib_int_pow2                             202002L [C++20]

 # ifdef __cpp_lib_filesystem
 #   error "__cpp_lib_filesystem should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_format
 #   error "__cpp_lib_format should not be defined before c++20"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifdef __cpp_lib_gcd_lcm
 #   error "__cpp_lib_gcd_lcm should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_generic_associative_lookup
 #   error "__cpp_lib_generic_associative_lookup should not be defined before c++14"
 # endif
 

 # ifdef __cpp_lib_filesystem
 #   error "__cpp_lib_filesystem should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_format
 #   error "__cpp_lib_format should not be defined before c++20"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifdef __cpp_lib_gcd_lcm
 #   error "__cpp_lib_gcd_lcm should not be defined before c++17"
 # endif
 
 # ifndef __cpp_lib_generic_associative_lookup
 #   error "__cpp_lib_generic_associative_lookup should be defined in c++14"
 # endif
 # if __cpp_lib_generic_associative_lookup != 201304L

 #     error "__cpp_lib_filesystem should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_filesystem) is not defined!"
 #   endif
 # endif
 
 # ifdef __cpp_lib_format
 #   error "__cpp_lib_format should not be defined before c++20"
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifndef __cpp_lib_gcd_lcm
 #   error "__cpp_lib_gcd_lcm should be defined in c++17"
 # endif
 # if __cpp_lib_gcd_lcm != 201606L
 #   error "__cpp_lib_gcd_lcm should have the value 201606L in c++17"
 # endif
 
 # ifndef __cpp_lib_generic_associative_lookup

 #     error "__cpp_lib_format should have the value 202106L in c++20"
 #   endif
 # else
 #   ifdef __cpp_lib_format
 #     error "__cpp_lib_format should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT) is not defined!"
 #   endif
 # endif
 
+# ifdef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should not be defined before c++2b"
+# endif
+
 # ifndef __cpp_lib_gcd_lcm
 #   error "__cpp_lib_gcd_lcm should be defined in c++20"
 # endif
 # if __cpp_lib_gcd_lcm != 201606L
 #   error "__cpp_lib_gcd_lcm should have the value 201606L in c++20"
 # endif
 
 # ifndef __cpp_lib_generic_associative_lookup

 #     error "__cpp_lib_format should have the value 202106L in c++2b"
 #   endif
 # else
 #   ifdef __cpp_lib_format
 #     error "__cpp_lib_format should not be defined when !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT) is not defined!"
 #   endif
 # endif
 
+# ifndef __cpp_lib_forward_like
+#   error "__cpp_lib_forward_like should be defined in c++2b"
+# endif
+# if __cpp_lib_forward_like != 202207L
+#   error "__cpp_lib_forward_like should have the value 202207L in c++2b"
+# endif
+
 # ifndef __cpp_lib_gcd_lcm
 #   error "__cpp_lib_gcd_lcm should be defined in c++2b"
 # endif
 # if __cpp_lib_gcd_lcm != 201606L
 #   error "__cpp_lib_gcd_lcm should have the value 201606L in c++2b"
 # endif
 
 # ifndef __cpp_lib_generic_associative_lookup

libcxx/test/std/utilities/utility/forward_like/forward_like.msvc/test.compile.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
+
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+using namespace std;
+
+struct U {}; // class type so const-qualification is not stripped from a prvalue
+using CU = const U;
+using T  = int;
+using CT = const T;
+
+U u{};
+const U& cu = u;
+
+static_assert(is_same_v<decltype(forward_like<T>(U{})), U&&>);
+static_assert(is_same_v<decltype(forward_like<T>(CU{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<T>(u)), U&&>);
+static_assert(is_same_v<decltype(forward_like<T>(cu)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<T>(move(u))), U&&>);
+static_assert(is_same_v<decltype(forward_like<T>(move(cu))), CU&&>);
+
+static_assert(is_same_v<decltype(forward_like<CT>(U{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT>(CU{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT>(u)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT>(cu)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT>(move(u))), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT>(move(cu))), CU&&>);
+
+static_assert(is_same_v<decltype(forward_like<T&>(U{})), U&>);
+static_assert(is_same_v<decltype(forward_like<T&>(CU{})), CU&>);
+static_assert(is_same_v<decltype(forward_like<T&>(u)), U&>);
+static_assert(is_same_v<decltype(forward_like<T&>(cu)), CU&>);
+static_assert(is_same_v<decltype(forward_like<T&>(move(u))), U&>);
+static_assert(is_same_v<decltype(forward_like<T&>(move(cu))), CU&>);
+
+static_assert(is_same_v<decltype(forward_like<CT&>(U{})), CU&>);
+static_assert(is_same_v<decltype(forward_like<CT&>(CU{})), CU&>);
+static_assert(is_same_v<decltype(forward_like<CT&>(u)), CU&>);
+static_assert(is_same_v<decltype(forward_like<CT&>(cu)), CU&>);
+static_assert(is_same_v<decltype(forward_like<CT&>(move(u))), CU&>);
+static_assert(is_same_v<decltype(forward_like<CT&>(move(cu))), CU&>);
+
+static_assert(is_same_v<decltype(forward_like<T&&>(U{})), U&&>);
+static_assert(is_same_v<decltype(forward_like<T&&>(CU{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<T&&>(u)), U&&>);
+static_assert(is_same_v<decltype(forward_like<T&&>(cu)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<T&&>(move(u))), U&&>);
+static_assert(is_same_v<decltype(forward_like<T&&>(move(cu))), CU&&>);
+
+static_assert(is_same_v<decltype(forward_like<CT&&>(U{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT&&>(CU{})), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT&&>(u)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT&&>(cu)), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT&&>(move(u))), CU&&>);
+static_assert(is_same_v<decltype(forward_like<CT&&>(move(cu))), CU&&>);
+
+static_assert(noexcept(forward_like<T>(u)));
+
+constexpr bool test() {
+    int val       = 1729;
+    auto&& result = forward_like<const double&>(val);
+    static_assert(is_same_v<decltype(result), const int&>);
+    assert(&result == &val);
+    return true;
+}
+
+static_assert(test());
+
+int main() {} // COMPILE-ONLY

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = [ add_version_header(x) for x in [
     # value as is.
     # TODO FMT Set P2508's feature-test macro.
     #"values": { "c++20": 202106, "c++23": 202207" },
     "values": { "c++20": 202106 },
     "headers": ["format"],
     "test_suite_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)",
     "libcxx_guard": "!defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_format) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)",
   }, {
+    "name": "__cpp_lib_forward_like",
+    "values": { "c++2b": 202207 },
+    "headers": ["utility"],
+  }, {
     "name": "__cpp_lib_gcd_lcm",
     "values": { "c++17": 201606 },
     "headers": ["numeric"],
   }, {
     "name": "__cpp_lib_generic_associative_lookup",
     "values": { "c++14": 201304 },
     "headers": ["map", "set"],
   }, {