[committed] libstdc++: Use feature test macro for pmr::polymorphic_allocator<>

[Jonathan Wakely]

Check the __glibcxx_polymorphic_allocator macro instead of just checking
whether __cplusplus > 201703L.

libstdc++-v3/ChangeLog:

        * include/bits/memory_resource.h (polymoprhic_allocator): Use
        feature test macro for P0339R6 features.
---

Tested x86_64-linux. Pushed to trunk.

 libstdc++-v3/include/bits/memory_resource.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/memory_resource.h 
b/libstdc++-v3/include/bits/memory_resource.h
index a40c9729c57..efcb1bda6e8 100644
--- a/libstdc++-v3/include/bits/memory_resource.h
+++ b/libstdc++-v3/include/bits/memory_resource.h
@@ -170,7 +170,7 @@ namespace pmr
       __attribute__((__nonnull__))
       { _M_resource->deallocate(__p, __n * sizeof(_Tp), alignof(_Tp)); }
 
-#if __cplusplus > 201703L
+#ifdef __glibcxx_polymorphic_allocator // >= C++20
       [[nodiscard]] void*
       allocate_bytes(size_t __nbytes,
                     size_t __alignment = alignof(max_align_t))
@@ -220,9 +220,9 @@ namespace pmr
          __p->~_Up();
          deallocate_object(__p);
        }
-#endif // C++2a
+#endif // C++20
 
-#if ! __glibcxx_make_obj_using_allocator
+#if ! __glibcxx_make_obj_using_allocator // >= C++20
       template<typename _Tp1, typename... _Args>
        __attribute__((__nonnull__))
        typename __not_pair<_Tp1>::type
@@ -338,7 +338,7 @@ namespace pmr
 #endif
 
     private:
-#if ! __glibcxx_make_obj_using_allocator
+#if ! __glibcxx_make_obj_using_allocator // >= C++20
       using __uses_alloc1_ = __uses_alloc1<polymorphic_allocator>;
       using __uses_alloc2_ = __uses_alloc2<polymorphic_allocator>;
 
-- 
2.47.1