[gcc r12-8424] libstdc++: Implement LWG 3683 for pmr::polymorphic_allocator

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

https://gcc.gnu.org/g:5647e401bb8656d1d9897c7a536bf00ff50fd20b

commit r12-8424-g5647e401bb8656d1d9897c7a536bf00ff50fd20b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 19 13:26:49 2022 +0100

    libstdc++: Implement LWG 3683 for pmr::polymorphic_allocator
    
    This issue has recently been moved to Tentatively Ready, and seems
    uncontroversial. This allows equality comparison with types that are
    convertible to pmr::polymorphic_allocator, which fail deduction for the
    existing equality operator.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/memory_resource (polymorphic_allocator): Add
            non-template equality operator, as proposed for LWG 3683.
            * testsuite/20_util/polymorphic_allocator/lwg3683.cc: New test.
    
    (cherry picked from commit f13f9c99dbee9c495955a430dd10bdb24a16f24a)

Diff:
---
 libstdc++-v3/include/std/memory_resource                 | 16 ++++++++++++++++
 .../testsuite/20_util/polymorphic_allocator/lwg3683.cc   | 13 +++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/libstdc++-v3/include/std/memory_resource b/libstdc++-v3/include/std/memory_resource
index 4a18d3e8598..5333765de05 100644
--- a/libstdc++-v3/include/std/memory_resource
+++ b/libstdc++-v3/include/std/memory_resource
@@ -338,6 +338,22 @@ namespace pmr
       __attribute__((__returns_nonnull__))
       { return _M_resource; }
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3683. operator== for polymorphic_allocator cannot deduce template arg
+      [[nodiscard]]
+      friend bool
+      operator==(const polymorphic_allocator& __a,
+		 const polymorphic_allocator& __b) noexcept
+      { return *__a.resource() == *__b.resource(); }
+
+#if __cpp_impl_three_way_comparison < 201907L
+      [[nodiscard]]
+      friend bool
+      operator!=(const polymorphic_allocator& __a,
+		 const polymorphic_allocator& __b) noexcept
+      { return !(__a == __b); }
+#endif
+
     private:
 #if ! __cpp_lib_make_obj_using_allocator
       using __uses_alloc1_ = __uses_alloc1<polymorphic_allocator>;
diff --git a/libstdc++-v3/testsuite/20_util/polymorphic_allocator/lwg3683.cc b/libstdc++-v3/testsuite/20_util/polymorphic_allocator/lwg3683.cc
new file mode 100644
index 00000000000..acc91f540b5
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/polymorphic_allocator/lwg3683.cc
@@ -0,0 +1,13 @@
+// { dg-do compile { target c++17 } }
+
+#include <memory_resource>
+
+bool
+test_lwg3683(const std::pmr::polymorphic_allocator<int>& a)
+{
+  if (a == std::pmr::get_default_resource())
+    return true;
+  if (std::pmr::get_default_resource() != a)
+    return false;
+  throw a;
+}