libstdc++: uninitialized_construct_using_allocator should use construct_at (LWG 3321)

	* include/std/memory (uninitialized_construct_using_allocator): Use
	std::construct_at (LWG 3321).

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/memory (uninitialized_construct_using_allocator): Use
+	std::construct_at (LWG 3321).
+
 	* include/std/memory_resource (polymorphic_allocator::allocate_bytes)
 	(polymorphic_allocator::allocate_object)
 	(polymorphic_allocator::new_object): Add nodiscard attribute (LWG3304).

libstdc++-v3/include/std/memory

@@ -387,9 +387,10 @@ get_pointer_safety() noexcept { return pointer_safety::relaxed; }
     uninitialized_construct_using_allocator(_Tp* __p, const _Alloc& __a,
 					    _Args&&... __args)
     {
-      void* __vp = const_cast<void*>(static_cast<const volatile void*>(__p));
-      return ::new(__vp) _Tp(std::make_obj_using_allocator<_Tp>(__a,
-	    std::forward<_Args>(__args)...));
+      return std::apply([&](auto&&... __xs) {
+	return std::construct_at(__p, std::forward<decltype(__xs)>(__xs)...);
+      }, std::uses_allocator_construction_args<_Tp>(__a,
+	std::forward<_Args>(__args)...));
     }
 // @}