Revert "libcxx-compat: revert llvmorg-19-init-17728-g30cc12cd818d:"

This reverts commit 198b947ebc6834eade6acc52c5441a38693b8822, in
preparation for merging llvm 21.

PR:		292067
MFC after:	1 month

contrib/llvm-project/libcxx/include/__type_traits/is_fundamental.h

@@ -34,7 +34,7 @@ inline constexpr bool is_fundamental_v = __is_fundamental(_Tp);
 
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_fundamental
-    : public integral_constant<bool, is_void<_Tp>::value || __is_nullptr_t<_Tp>::value || is_arithmetic<_Tp>::value> {};
+    : public integral_constant<bool, is_void<_Tp>::value || __is_null_pointer_v<_Tp> || is_arithmetic<_Tp>::value> {};
 
 #  if _LIBCPP_STD_VER >= 17
 template <class _Tp>

contrib/llvm-project/libcxx/include/__type_traits/is_null_pointer.h

@@ -11,7 +11,6 @@
 
 #include <__config>
 #include <__type_traits/integral_constant.h>
-#include <__type_traits/remove_cv.h>
 #include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -21,20 +20,15 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-struct __is_nullptr_t_impl : public false_type {};
+inline const bool __is_null_pointer_v = __is_same(__remove_cv(_Tp), nullptr_t);
-template <>
-struct __is_nullptr_t_impl<nullptr_t> : public true_type {};
-
-template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS __is_nullptr_t : public __is_nullptr_t_impl<__remove_cv_t<_Tp> > {};
 
 #if _LIBCPP_STD_VER >= 14
 template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS is_null_pointer : public __is_nullptr_t_impl<__remove_cv_t<_Tp> > {};
+struct _LIBCPP_TEMPLATE_VIS is_null_pointer : integral_constant<bool, __is_null_pointer_v<_Tp>> {};
 
 #  if _LIBCPP_STD_VER >= 17
 template <class _Tp>
-inline constexpr bool is_null_pointer_v = is_null_pointer<_Tp>::value;
+inline constexpr bool is_null_pointer_v = __is_null_pointer_v<_Tp>;
 #  endif
 #endif // _LIBCPP_STD_VER >= 14
 

contrib/llvm-project/libcxx/include/__type_traits/is_scalar.h

@@ -49,7 +49,7 @@ struct _LIBCPP_TEMPLATE_VIS is_scalar
           bool, is_arithmetic<_Tp>::value ||
                 is_member_pointer<_Tp>::value ||
                 is_pointer<_Tp>::value ||
-                __is_nullptr_t<_Tp>::value ||
+                __is_null_pointer_v<_Tp> ||
                 __is_block<_Tp>::value ||
                 is_enum<_Tp>::value> {};
 // clang-format on