[libc++] Fix LWG3557 "static_cast expression in convertible_to has the wrong operand"

[• Quuxplusone]

Summary

https://cplusplus.github.io/LWG/issue3557

I think the code change is unobservable, so we could just close this as "Nothing To Do" instead; but it seems appropriate to follow the Standard's wording here as closely as possible.

[Mordante]

+1 for following the Standard's wording.

LGTM!

libcxx/docs/Status/Cxx2bIssues.csv

 `3392 <https://wg21.link/LWG3392>`__,"``ranges::distance()`` cannot be used on a move-only iterator with a sized sentinel","October 2021","","","|ranges|"
 `3407 <https://wg21.link/LWG3407>`__,"Some problems with the wording changes of P1739R4","October 2021","","","|ranges|"
 `3422 <https://wg21.link/LWG3422>`__,"Issues of ``seed_seq``'s constructors","October 2021","|Complete|","14.0"
 `3470 <https://wg21.link/LWG3470>`__,"``convertible-to-non-slicing`` seems to reject valid case","October 2021","|Complete|","14.0","|ranges|"
 `3480 <https://wg21.link/LWG3480>`__,"``directory_iterator`` and ``recursive_directory_iterator`` are not C++20 ranges","October 2021","|Complete|","14.0","|ranges|"
 `3498 <https://wg21.link/LWG3498>`__,"Inconsistent ``noexcept``-specifiers for ``basic_syncbuf``","October 2021","",""
 `3535 <https://wg21.link/LWG3535>`__,"``join_view::iterator::iterator_category`` and ``::iterator_concept`` lie","October 2021","","","|ranges|"
 `3554 <https://wg21.link/LWG3554>`__,"``chrono::parse`` needs ``const charT*`` and ``basic_string_view<charT>`` overloads","October 2021","","","|chrono|"
-`3557 <https://wg21.link/LWG3557>`__,"The ``static_cast`` expression in ``convertible_to`` has the wrong operand","October 2021","",""
+`3557 <https://wg21.link/LWG3557>`__,"The ``static_cast`` expression in ``convertible_to`` has the wrong operand","October 2021","|Complete|","14.0"
 `3559 <https://wg21.link/LWG3559>`__,"Semantic requirements of ``sized_range`` is circular","October 2021","","","|ranges|"
 `3560 <https://wg21.link/LWG3560>`__,"``ranges::equal`` and ``ranges::is_permutation`` should short-circuit for ``sized_ranges``","October 2021","","","|ranges|"
 `3561 <https://wg21.link/LWG3561>`__,"Issue with internal counter in ``discard_block_engine``","October 2021","",""
 `3563 <https://wg21.link/LWG3563>`__,"``keys_view`` example is broken","October 2021","","","|ranges|"
 `3566 <https://wg21.link/LWG3566>`__,"Constraint recursion for ``operator<=>(optional<T>, U)``","October 2021","","","|spaceship|"
 `3567 <https://wg21.link/LWG3567>`__,"Formatting move-only iterators take two","October 2021","","","|format|"
 `3568 <https://wg21.link/LWG3568>`__,"``basic_istream_view`` needs to initialize ``value_``","October 2021","","","|ranges|"
 `3570 <https://wg21.link/LWG3570>`__,"``basic_osyncstream::emit`` should be an unformatted output function","October 2021","",""

libcxx/include/__concepts/convertible_to.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___CONCEPTS_CONVERTIBLE_TO_H
 #define _LIBCPP___CONCEPTS_CONVERTIBLE_TO_H
 
 #include <__config>
+#include <__utility/declval.h>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 // [concept.convertible]
 
 template<class _From, class _To>
 concept convertible_to =
   is_convertible_v<_From, _To> &&
-  requires (add_rvalue_reference_t<_From> (&__f)()) {
-    static_cast<_To>(__f());
+  requires {
+    static_cast<_To>(declval<_From>());
   };
 
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CONCEPTS_CONVERTIBLE_TO_H