[libc++] Use [[clang::no_specializations]] to diagnose invalid user specializations (#118167)

Some templates in the standard library are illegal to specialize for users
(even if the specialization contains user-defined types). The [[clang::no_specializations]]
attribute allows marking such base templates so that the compiler will
diagnose if users try adding a specialization.

Author: philnik777

libcxx/include/__compare/compare_three_way_result.h

@@ -33,7 +33,8 @@ struct _LIBCPP_HIDE_FROM_ABI __compare_three_way_result<
 };
 
 template <class _Tp, class _Up = _Tp>
-struct _LIBCPP_TEMPLATE_VIS compare_three_way_result : __compare_three_way_result<_Tp, _Up, void> {};
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS compare_three_way_result
+    : __compare_three_way_result<_Tp, _Up, void> {};
 
 template <class _Tp, class _Up = _Tp>
 using compare_three_way_result_t = typename compare_three_way_result<_Tp, _Up>::type;

libcxx/include/__config

@@ -1168,6 +1168,13 @@ typedef __char32_t char32_t;
 
 #  define _LIBCPP_NODEBUG [[__gnu__::__nodebug__]]
 
+#  if __has_cpp_attribute(_Clang::__no_specializations__)
+#    define _LIBCPP_NO_SPECIALIZATIONS                                                                                 \
+      [[_Clang::__no_specializations__("Users are not allowed to specialize this standard library entity")]]
+#  else
+#    define _LIBCPP_NO_SPECIALIZATIONS
+#  endif
+
 #  if __has_attribute(__standalone_debug__)
 #    define _LIBCPP_STANDALONE_DEBUG __attribute__((__standalone_debug__))
 #  else

libcxx/include/__format/format_arg.h

@@ -277,7 +277,7 @@ class __basic_format_arg_value {
 };
 
 template <class _Context>
-class _LIBCPP_TEMPLATE_VIS basic_format_arg {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS basic_format_arg {
 public:
   class _LIBCPP_TEMPLATE_VIS handle;
 

libcxx/include/__ranges/range_adaptor.h

@@ -85,7 +85,7 @@ template <_RangeAdaptorClosure _Closure, _RangeAdaptorClosure _OtherClosure>
 #  if _LIBCPP_STD_VER >= 23
 template <class _Tp>
   requires is_class_v<_Tp> && same_as<_Tp, remove_cv_t<_Tp>>
-class range_adaptor_closure : public __range_adaptor_closure<_Tp> {};
+class _LIBCPP_NO_SPECIALIZATIONS range_adaptor_closure : public __range_adaptor_closure<_Tp> {};
 #  endif // _LIBCPP_STD_VER >= 23
 
 } // namespace ranges

libcxx/include/__type_traits/add_cv_quals.h

@@ -18,7 +18,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS add_const {
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS add_const {
   using type _LIBCPP_NODEBUG = const _Tp;
 };
 
@@ -28,7 +28,7 @@ using add_const_t = typename add_const<_Tp>::type;
 #endif
 
 template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS add_cv {
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS add_cv {
   using type _LIBCPP_NODEBUG = const volatile _Tp;
 };
 
@@ -38,7 +38,7 @@ using add_cv_t = typename add_cv<_Tp>::type;
 #endif
 
 template <class _Tp>
-struct _LIBCPP_TEMPLATE_VIS add_volatile {
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS add_volatile {
   using type _LIBCPP_NODEBUG = volatile _Tp;
 };
 

libcxx/include/__type_traits/add_lvalue_reference.h

@@ -40,7 +40,7 @@ using __add_lvalue_reference_t = typename __add_lvalue_reference_impl<_Tp>::type
 #endif // __has_builtin(__add_lvalue_reference)
 
 template <class _Tp>
-struct add_lvalue_reference {
+struct _LIBCPP_NO_SPECIALIZATIONS add_lvalue_reference {
   using type _LIBCPP_NODEBUG = __add_lvalue_reference_t<_Tp>;
 };
 

libcxx/include/__type_traits/add_pointer.h

@@ -41,7 +41,7 @@ using __add_pointer_t = typename __add_pointer_impl<_Tp>::type;
 #endif // !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer)
 
 template <class _Tp>
-struct add_pointer {
+struct _LIBCPP_NO_SPECIALIZATIONS add_pointer {
   using type _LIBCPP_NODEBUG = __add_pointer_t<_Tp>;
 };
 

libcxx/include/__type_traits/add_rvalue_reference.h

@@ -40,7 +40,7 @@ using __add_rvalue_reference_t = typename __add_rvalue_reference_impl<_Tp>::type
 #endif // __has_builtin(__add_rvalue_reference)
 
 template <class _Tp>
-struct add_rvalue_reference {
+struct _LIBCPP_NO_SPECIALIZATIONS add_rvalue_reference {
   using type = __add_rvalue_reference_t<_Tp>;
 };
 

libcxx/include/__type_traits/aligned_storage.h

@@ -68,7 +68,7 @@ struct __find_max_align<__type_list<_Head, _Tail...>, _Len>
           __select_align<_Len, _Head::value, __find_max_align<__type_list<_Tail...>, _Len>::value>::value> {};
 
 template <size_t _Len, size_t _Align = __find_max_align<__all_types, _Len>::value>
-struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_TEMPLATE_VIS aligned_storage {
+struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS aligned_storage {
   union _ALIGNAS(_Align) type {
     unsigned char __data[(_Len + _Align - 1) / _Align * _Align];
   };

libcxx/include/__type_traits/aligned_union.h

@@ -33,7 +33,7 @@ struct __static_max<_I0, _I1, _In...> {
 };
 
 template <size_t _Len, class _Type0, class... _Types>
-struct _LIBCPP_DEPRECATED_IN_CXX23 aligned_union {
+struct _LIBCPP_DEPRECATED_IN_CXX23 _LIBCPP_NO_SPECIALIZATIONS aligned_union {
   static const size_t alignment_value =
       __static_max<_LIBCPP_PREFERRED_ALIGNOF(_Type0), _LIBCPP_PREFERRED_ALIGNOF(_Types)...>::value;
   static const size_t __len = __static_max<_Len, sizeof(_Type0), sizeof(_Types)...>::value;