[gcc r12-6439] libstdc++: Make std::variant work with Clang in C++20 mode [PR103891]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:5b417b35824fb5c15e3ee958cb86436b3409ebea

commit r12-6439-g5b417b35824fb5c15e3ee958cb86436b3409ebea
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jan 10 17:28:19 2022 +0000

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

Diff:
---
 libstdc++-v3/include/bits/c++config | 5 +++++
 libstdc++-v3/include/std/variant    | 7 ++++---
 libstdc++-v3/include/std/version    | 5 +++--
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index c64b61b3c90..b197349f976 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -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
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index d74939eddc3..c41f9f27e00 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -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
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index f421056964e..012d78e3b6b 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -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