[libc++] Simplify the implementation of remove_reference (llvm#85207)

GCC 13 introduced the type trait `__remove_reference`. We can simplify
the implementation of `remove_reference` a bit by using it.

Author: philnik777

libcxx/include/__type_traits/remove_reference.h

@@ -10,7 +10,6 @@
 #define _LIBCPP___TYPE_TRAITS_REMOVE_REFERENCE_H
 
 #include <__config>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -26,15 +25,16 @@ struct remove_reference {
 
 template <class _Tp>
 using __libcpp_remove_reference_t = __remove_reference_t(_Tp);
-#else
-// clang-format off
-template <class _Tp> struct _LIBCPP_TEMPLATE_VIS remove_reference        {typedef _LIBCPP_NODEBUG _Tp type;};
-template <class _Tp> struct _LIBCPP_TEMPLATE_VIS remove_reference<_Tp&>  {typedef _LIBCPP_NODEBUG _Tp type;};
-template <class _Tp> struct _LIBCPP_TEMPLATE_VIS remove_reference<_Tp&&> {typedef _LIBCPP_NODEBUG _Tp type;};
-// clang-format on
+#elif __has_builtin(__remove_reference)
+template <class _Tp>
+struct remove_reference {
+  using type _LIBCPP_NODEBUG = __remove_reference(_Tp);
+};
 
 template <class _Tp>
 using __libcpp_remove_reference_t = typename remove_reference<_Tp>::type;
+#else
+#  error "remove_reference not implemented!"
 #endif // __has_builtin(__remove_reference_t)
 
 #if _LIBCPP_STD_VER >= 14

libcxx/include/cwchar

@@ -254,4 +254,8 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_wmemchr(_Tp
 
 _LIBCPP_END_NAMESPACE_STD
 
+#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
+#  include <cstddef>
+#endif
+
 #endif // _LIBCPP_CWCHAR

libcxx/include/execution

@@ -142,4 +142,8 @@ _LIBCPP_END_NAMESPACE_STD
 
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
 
+#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
+#  include <cstddef>
+#endif
+
 #endif // _LIBCPP_EXECUTION

libcxx/test/libcxx/transitive_includes/cxx23.csv

@@ -142,7 +142,6 @@ coroutine version
 cstddef version
 ctgmath ccomplex
 ctgmath cmath
-cwchar cstddef
 cwchar cwctype
 cwctype cctype
 deque compare
@@ -161,7 +160,6 @@ exception cstdlib
 exception new
 exception typeinfo
 exception version
-execution cstddef
 execution version
 expected cstddef
 expected initializer_list

libcxx/test/libcxx/transitive_includes/cxx26.csv

@@ -142,7 +142,6 @@ coroutine version
 cstddef version
 ctgmath ccomplex
 ctgmath cmath
-cwchar cstddef
 cwchar cwctype
 cwctype cctype
 deque compare
@@ -161,7 +160,6 @@ exception cstdlib
 exception new
 exception typeinfo
 exception version
-execution cstddef
 execution version
 expected cstddef
 expected initializer_list