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

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.

Differential Revision: https://reviews.llvm.org/D117964

diff --git a/libcxx/docs/Status/Cxx2bIssues.csv b/libcxx/docs/Status/Cxx2bIssues.csv
index 855a1f4..f949102 100644 (file)
--- a/libcxx/docs/Status/Cxx2bIssues.csv
+++ b/libcxx/docs/Status/Cxx2bIssues.csv
@@ -114,7 +114,7 @@
 `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","",""

diff --git a/libcxx/include/__concepts/convertible_to.h b/libcxx/include/__concepts/convertible_to.h
index ec68967..795b0bd 100644 (file)
--- a/libcxx/include/__concepts/convertible_to.h
+++ b/libcxx/include/__concepts/convertible_to.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___CONCEPTS_CONVERTIBLE_TO_H
 
 #include <__config>
+#include <__utility/declval.h>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -25,8 +26,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 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)