[libc++][NFC] Merge add_{const, cv, volatile}.h into a single header (#115610)

There isn't much benefit in having granular headers for only a few
simple lines of code.

Author: philnik777

libcxx/include/CMakeLists.txt

@@ -738,12 +738,10 @@ set(files
   __tuple/tuple_like_no_subrange.h
   __tuple/tuple_size.h
   __tuple/tuple_types.h
-  __type_traits/add_const.h
-  __type_traits/add_cv.h
+  __type_traits/add_cv_quals.h
   __type_traits/add_lvalue_reference.h
   __type_traits/add_pointer.h
   __type_traits/add_rvalue_reference.h
-  __type_traits/add_volatile.h
   __type_traits/aligned_storage.h
   __type_traits/aligned_union.h
   __type_traits/alignment_of.h

libcxx/include/__type_traits/add_const.h

@@ -17,6 +17,16 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Tp>
+struct _LIBCPP_TEMPLATE_VIS add_const {
+  typedef _LIBCPP_NODEBUG const _Tp type;
+};
+
+#if _LIBCPP_STD_VER >= 14
+template <class _Tp>
+using add_const_t = typename add_const<_Tp>::type;
+#endif
+
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS add_cv {
   typedef _LIBCPP_NODEBUG const volatile _Tp type;
@@ -27,6 +37,16 @@ template <class _Tp>
 using add_cv_t = typename add_cv<_Tp>::type;
 #endif
 
+template <class _Tp>
+struct _LIBCPP_TEMPLATE_VIS add_volatile {
+  typedef _LIBCPP_NODEBUG volatile _Tp type;
+};
+
+#if _LIBCPP_STD_VER >= 14
+template <class _Tp>
+using add_volatile_t = typename add_volatile<_Tp>::type;
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_ADD_CV_H

libcxx/include/__type_traits/add_cv.h renamed to libcxx/include/__type_traits/add_cv_quals.h

@@ -10,7 +10,6 @@
 #define _LIBCPP___TYPE_TRAITS_IS_TRIVIALLY_ASSIGNABLE_H
 
 #include <__config>
-#include <__type_traits/add_const.h>
 #include <__type_traits/add_lvalue_reference.h>
 #include <__type_traits/add_rvalue_reference.h>
 #include <__type_traits/integral_constant.h>

libcxx/include/__type_traits/add_volatile.h

@@ -10,9 +10,6 @@
 #define _LIBCPP___UTILITY_AS_CONST_H
 
 #include <__config>
-#include <__type_traits/add_const.h>
-#include <__utility/forward.h>
-#include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -22,7 +19,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17
 template <class _Tp>
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& as_const(_Tp& __t) noexcept {
   return __t;
 }
 

libcxx/include/__type_traits/is_trivially_assignable.h

@@ -85,7 +85,7 @@ namespace std {
 #include <__memory/allocator_destructor.h>
 #include <__memory/allocator_traits.h>
 #include <__memory/unique_ptr.h>
-#include <__type_traits/add_const.h>
+#include <__type_traits/add_cv_quals.h>
 #include <__type_traits/add_pointer.h>
 #include <__type_traits/aligned_storage.h>
 #include <__type_traits/conditional.h>

libcxx/include/__utility/as_const.h

@@ -63,12 +63,10 @@ module std_core [system] {
   }
 
   module type_traits {
-    module add_const                                  { header "__type_traits/add_const.h" }
-    module add_cv                                     { header "__type_traits/add_cv.h" }
+    module add_cv_quals                               { header "__type_traits/add_cv_quals.h" }
     module add_lvalue_reference                       { header "__type_traits/add_lvalue_reference.h" }
     module add_pointer                                { header "__type_traits/add_pointer.h" }
     module add_rvalue_reference                       { header "__type_traits/add_rvalue_reference.h" }
-    module add_volatile                               { header "__type_traits/add_volatile.h" }
     module aligned_storage                            { header "__type_traits/aligned_storage.h" }
     module aligned_union                              { header "__type_traits/aligned_union.h" }
     module alignment_of                               { header "__type_traits/alignment_of.h" }

libcxx/include/any

@@ -425,12 +425,10 @@ namespace std
 */
 
 #include <__config>
-#include <__type_traits/add_const.h>
-#include <__type_traits/add_cv.h>
+#include <__type_traits/add_cv_quals.h>
 #include <__type_traits/add_lvalue_reference.h>
 #include <__type_traits/add_pointer.h>
 #include <__type_traits/add_rvalue_reference.h>
-#include <__type_traits/add_volatile.h>
 #include <__type_traits/aligned_storage.h>
 #include <__type_traits/aligned_union.h>
 #include <__type_traits/alignment_of.h>

libcxx/include/module.modulemap

@@ -226,10 +226,8 @@ namespace std {
 #include <__memory/construct_at.h>
 #include <__tuple/find_index.h>
 #include <__tuple/sfinae_helpers.h>
-#include <__type_traits/add_const.h>
-#include <__type_traits/add_cv.h>
+#include <__type_traits/add_cv_quals.h>
 #include <__type_traits/add_pointer.h>
-#include <__type_traits/add_volatile.h>
 #include <__type_traits/common_type.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/conjunction.h>