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libstdc++: Make std::variant work with Clang in C++20 mode [PR103891]

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Clang has some bugs with destructors that use constraints to be
conditionally trivial, so disable the P2231R1 constexpr changes to
std::variant unless the compiler is GCC 12 or later.

If/when P2493R0 gets accepted and implemented by G++ we can remove the
__GNUC__ check and use __cpp_concepts >= 202002 instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/103891
	* include/bits/c++config (_GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS):
	Define.
	* include/std/variant (__cpp_lib_variant): Only define C++20
	value when the compiler is known to support conditionally
	trivial destructors.
	* include/std/version (__cpp_lib_variant): Likewise.
This commit is contained in:
Jonathan Wakely 2022-01-10 17:28:19 +00:00
parent 11d3e8f436
commit 5b417b3582
3 changed files with 12 additions and 5 deletions
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5
libstdc++-v3/include/bits/c++config
View File

@ -810,6 +810,11 @@ namespace std
#undef _GLIBCXX_HAS_BUILTIN
#if __cplusplus >= 202002L && __cpp_concepts && __GNUC__ >= 12
// XXX workaround for missing feature test macro for P0848R3 (see P2493R0).
# define _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS 1
#endif
// PSTL configuration
#if __cplusplus >= 201703L

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

@ -44,9 +44,10 @@
#include <bits/stl_iterator_base_funcs.h>
#include <bits/stl_construct.h>
#include <bits/utility.h> // in_place_index_t
#if __cplusplus == 201703L
#ifndef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
# include <ext/aligned_buffer.h>
#else
#endif
#if __cplusplus >= 202002L
# include <compare>
#endif
@ -54,7 +55,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 202002L && __cpp_concepts
#ifdef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
// P2231R1 constexpr needs constexpr unions and constrained destructors.
# define __cpp_lib_variant 202106L
#else

5
libstdc++-v3/include/std/version
View File

@ -173,7 +173,8 @@
# define __cpp_lib_to_chars 201611L
#endif
#define __cpp_lib_unordered_map_try_emplace 201411
#if __cplusplus == 201703L || ! __cpp_concepts // N.B. updated value in C++20
#ifndef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
// N.B. updated value in C++20
# define __cpp_lib_variant 202102L
#endif
#endif
@ -286,7 +287,7 @@
# endif
#define __cpp_lib_to_address 201711L
#define __cpp_lib_to_array 201907L
#if __cpp_concepts
#ifdef _GLIBCXX_HAVE_COND_TRIVIAL_SPECIAL_MEMBERS
# define __cpp_lib_variant 202106L
#endif
#endif