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>

4 files changed, 40 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index d20e085..91c1ede 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1822,6 +1822,14 @@ ftms = {
 };
 
 ftms = {
+  name = common_reference;
+  values = {
+    v = 202302;
+    cxxmin = 20; // We treat P2655R3 as a DR against C++20.
+  };
+};
+
+ftms = {
   name = formatters;
   values = {
     v = 202302;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index c75368d4..f5ddc86 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -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
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 418e218..7c157ea 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -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>; };
 
diff --git a/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc
new file mode 100644
index 0000000..4188dd2
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc
@@ -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&> );