[libc++][NFC] Use __construct_at and __destroy_at instead of using preprocessor conditionals (#70866)

Author: philnik777

libcxx/include/__memory/allocator_traits.h

@@ -300,11 +300,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
         __enable_if_t<!__has_construct<allocator_type, _Tp*, _Args...>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
     static void construct(allocator_type&, _Tp* __p, _Args&&... __args) {
-#if _LIBCPP_STD_VER >= 20
-        _VSTD::construct_at(__p, _VSTD::forward<_Args>(__args)...);
-#else
-        ::new ((void*)__p) _Tp(_VSTD::forward<_Args>(__args)...);
-#endif
+        std::__construct_at(__p, std::forward<_Args>(__args)...);
     }
 
     template <class _Tp, class =
@@ -319,11 +315,7 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits
         __enable_if_t<!__has_destroy<allocator_type, _Tp*>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
     static void destroy(allocator_type&, _Tp* __p) {
-#if _LIBCPP_STD_VER >= 20
-        _VSTD::destroy_at(__p);
-#else
-        __p->~_Tp();
-#endif
+        std::__destroy_at(__p);
     }
 
     template <class _Ap = _Alloc, class =

libcxx/include/optional

@@ -416,11 +416,7 @@ struct __optional_storage_base : __optional_destruct_base<_Tp>
     _LIBCPP_CONSTEXPR_SINCE_CXX20 void __construct(_Args&&... __args)
     {
         _LIBCPP_ASSERT_INTERNAL(!has_value(), "__construct called for engaged __optional_storage");
-#if _LIBCPP_STD_VER >= 20
-        _VSTD::construct_at(_VSTD::addressof(this->__val_), _VSTD::forward<_Args>(__args)...);
-#else
-        ::new ((void*)_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
-#endif
+        std::__construct_at(std::addressof(this->__val_), std::forward<_Args>(__args)...);
         this->__engaged_ = true;
     }
 

libcxx/test/std/containers/sequences/vector/vector.cons/copy.move_only.verify.cpp

@@ -16,5 +16,5 @@
 
 void f() {
     std::vector<MoveOnly> v;
-    std::vector<MoveOnly> copy = v; // expected-error-re@* {{{{(no matching function for call to 'construct_at')|(call to implicitly-deleted copy constructor of 'MoveOnly')|(call to deleted constructor of 'MoveOnly')}}}}
+    std::vector<MoveOnly> copy = v; // expected-error-re@* {{{{(no matching function for call to '__construct_at')|(call to deleted constructor of 'MoveOnly')}}}}
 }

libcxx/test/std/containers/sequences/vector/vector.modifiers/resize_not_move_insertable.verify.cpp

@@ -33,7 +33,7 @@ int main(int, char**) {
 
   // Other diagnostics that might be seen as Clang tries to continue compiling:
   // expected-error@* 0-2 {{call to deleted constructor}}
-  // expected-error@* 0-2 {{no matching function for call to 'construct_at'}}
+  // expected-error@* 0-2 {{no matching function for call to '__construct_at'}}
   {
 
     std::vector<BadUserNoCookie<1> > x;