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libstdc++: Implement P2655R3 changes to common_reference bullet 1 

We implement this paper as a DR against C++20 (as do MSVC and libc++).

	PR libstdc++/120446

libstdc++-v3/ChangeLog:

	* include/bits/version.def (common_reference): New.
	* include/bits/version.h: Regenerate.
	* include/std/type_traits (__glibcxx_want_common_reference):
	Define.
	(__common_reference_impl<T1, T2, 1>): Add pointer convertibility
	constraints as per P2655R3.
	* testsuite/20_util/common_reference/p2655r3.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
This commit is contained in:
Patrick Palka 2025-12-05 13:43:29 -05:00
parent cbdbbdd1fc
commit a9fd651fbb
4 changed files with 40 additions and 0 deletions
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8
libstdc++-v3/include/bits/version.def
View File

@ -1821,6 +1821,14 @@ ftms = {
};
};
ftms = {
name = common_reference;
values = {
v = 202302;
cxxmin = 20; // We treat P2655R3 as a DR against C++20.
};
};
ftms = {
name = formatters;
values = {

10
libstdc++-v3/include/bits/version.h
View File

@ -2030,6 +2030,16 @@
#endif /* !defined(__cpp_lib_flat_set) */
#undef __glibcxx_want_flat_set
#if !defined(__cpp_lib_common_reference)
# if (__cplusplus >= 202002L)
# define __glibcxx_common_reference 202302L
# if defined(__glibcxx_want_all) || defined(__glibcxx_want_common_reference)
# define __cpp_lib_common_reference 202302L
# endif
# endif
#endif /* !defined(__cpp_lib_common_reference) && defined(__glibcxx_want_common_reference) */
#undef __glibcxx_want_common_reference
#if !defined(__cpp_lib_formatters)
# if (__cplusplus >= 202100L) && _GLIBCXX_HOSTED
# define __glibcxx_formatters 202302L

7
libstdc++-v3/include/std/type_traits
View File

@ -41,6 +41,7 @@
#define __glibcxx_want_bool_constant
#define __glibcxx_want_bounded_array_traits
#define __glibcxx_want_common_reference
#define __glibcxx_want_constant_wrapper
#define __glibcxx_want_has_unique_object_representations
#define __glibcxx_want_integral_constant_callable
@ -4240,6 +4241,12 @@ template<typename _Ret, typename _Fn, typename... _Args>
template<typename _Tp1, typename _Tp2>
requires is_reference_v<_Tp1> && is_reference_v<_Tp2>
&& requires { typename __common_ref<_Tp1, _Tp2>; }
#if __cpp_lib_common_reference // C++ >= 20
&& is_convertible_v<add_pointer_t<_Tp1>,
add_pointer_t<__common_ref<_Tp1, _Tp2>>>
&& is_convertible_v<add_pointer_t<_Tp2>,
add_pointer_t<__common_ref<_Tp1, _Tp2>>>
#endif
struct __common_reference_impl<_Tp1, _Tp2, 1>
{ using type = __common_ref<_Tp1, _Tp2>; };

15
libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc Normal file
View File

@ -0,0 +1,15 @@
// P2655R3 - common_reference_t of reference_wrapper Should Be a Reference Type
// Implemented as a DR against C++20
// { dg-do compile { target c++20 } }
#include <type_traits>
#if __cpp_lib_common_reference != 202302L
# error "Feature-test macro __cpp_lib_common_reference has wrong value in <type_traits>"
#endif
struct A { };
struct B { operator A&() const; };
static_assert( std::is_same_v<std::common_reference_t<A&, const B&>, A&> );
static_assert( std::is_same_v<std::common_reference_t<const B&, A&>, A&> );