Implement LWG-3683: `operator==` for `polymorphic_allocator` cannot deduce template argument in common cases (#2879)

stl/inc/xpolymorphic_allocator.h

@@ -282,6 +282,18 @@ namespace pmr {
             return _Resource;
         }
 
+        _NODISCARD_FRIEND bool operator==(
+            const polymorphic_allocator& _Lhs, const polymorphic_allocator& _Rhs) noexcept {
+            return *_Lhs._Resource == *_Rhs._Resource;
+        }
+
+#if !_HAS_CXX20
+        _NODISCARD_FRIEND bool operator!=(
+            const polymorphic_allocator& _Lhs, const polymorphic_allocator& _Rhs) noexcept {
+            return *_Lhs._Resource != *_Rhs._Resource;
+        }
+#endif // !_HAS_CXX20
+
     private:
         memory_resource* _Resource = _STD pmr::get_default_resource();
     };

tests/std/tests/P0220R1_polymorphic_memory_resources/test.cpp

@@ -10,6 +10,7 @@
 #include <cstdlib>
 #include <deque>
 #include <forward_list>
+#include <functional>
 #include <list>
 #include <malloc.h>
 #include <map>
@@ -946,6 +947,45 @@ namespace {
             }
         } // namespace eq
 
+        namespace eq_cvt {
+            void test() {
+                const auto pres                        = std::pmr::get_default_resource();
+                std::pmr::polymorphic_allocator<int> a = pres;
+                const auto ra                          = std::ref(a);
+                const auto cra                         = std::cref(a);
+
+                CHECK(a == pres);
+                CHECK(a == ra);
+                CHECK(a == cra);
+                CHECK(pres == a);
+                CHECK(pres == ra);
+                CHECK(pres == cra);
+                CHECK(ra == a);
+                CHECK(ra == pres);
+                CHECK(ra == ra);
+                CHECK(ra == cra);
+                CHECK(cra == a);
+                CHECK(cra == pres);
+                CHECK(cra == ra);
+                CHECK(cra == cra);
+
+                CHECK(!(a != pres));
+                CHECK(!(a != ra));
+                CHECK(!(a != cra));
+                CHECK(!(pres != a));
+                CHECK(!(pres != ra));
+                CHECK(!(pres != cra));
+                CHECK(!(ra != a));
+                CHECK(!(ra != pres));
+                CHECK(!(ra != ra));
+                CHECK(!(ra != cra));
+                CHECK(!(cra != a));
+                CHECK(!(cra != pres));
+                CHECK(!(cra != ra));
+                CHECK(!(cra != cra));
+            }
+        } // namespace eq_cvt
+
         namespace destroy {
             void test() {
                 bool destroyed = false;
@@ -1505,6 +1545,7 @@ int main() {
     polymorphic_allocator::mem::select_on_container_copy_construction::test();
     polymorphic_allocator::mem::resource::test();
     polymorphic_allocator::eq::test();
+    polymorphic_allocator::eq_cvt::test();
     polymorphic_allocator::destroy::test();
 
     monotonic::ctor::buffer_upstream::test();