[libc++][type_traits] P2647R1: A trait for implicit lifetime types

Implements P2674R1: https://wg21.link/P2674R1

- https://eel.is/c++draft/type.traits
  - https://eel.is/c++draft/meta.type.synop
  - https://eel.is/c++draft/meta.unary.prop
- https://eel.is/c++draft/support.limits
  - https://eel.is/c++draft/version.syn

Implementation details:
- Uses compiler intrinsic `__builtin_is_implicit_lifetime`:
  - #101807
- Tests based on:
    - https://github.com/llvm/llvm-project/blob/d213981c80626698a07b11ce872acba098a863d4/clang/test/SemaCXX/type-traits.cpp#L1989

References:
- Implicit-lifetime
  - Implicit-lifetime types [basic.types.general]/9: https://eel.is/c++draft/basic.types.general
  - Implicit-lifetime class [class.prop]/9: https://eel.is/c++draft/class.prop
- P0593R6 Implicit creation of objects for low-level object manipulation: https://wg21.link/P0593R6
- P1010R1 Container support for implicit lifetime types: https://wg21.link/P1010R1
- P0593R6 Implicit creation of objects for low-level object manipulation: https://wg21.link/P0593R6

Closes: #105259

Author: H-G-Hristov

libcxx/include/CMakeLists.txt

@@ -778,6 +778,7 @@ set(files
   __type_traits/is_floating_point.h
   __type_traits/is_function.h
   __type_traits/is_fundamental.h
+  __type_traits/is_implicit_lifetime.h
   __type_traits/is_implicitly_default_constructible.h
   __type_traits/is_integral.h
   __type_traits/is_literal_type.h

libcxx/include/__type_traits/is_implicit_lifetime.h

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_IMPLICIT_LIFETIME_H
+#define _LIBCPP___TYPE_TRAITS_IS_IMPLICIT_LIFETIME_H
+
+#include <__config>
+#include <__type_traits/integral_constant.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+#  if __has_builtin(__builtin_is_implicit_lifetime)
+
+template <class _Tp>
+struct _LIBCPP_TEMPLATE_VIS is_implicit_lifetime : public bool_constant<__builtin_is_implicit_lifetime(_Tp)> {};
+
+template <class _Tp>
+inline constexpr bool is_implicit_lifetime_v = __builtin_is_implicit_lifetime(_Tp);
+
+#  else
+
+template <typename _Tp>
+struct is_implicit_lifetime
+    : std::disjunction< std::is_scalar<_Tp>,
+                        std::is_array<_Tp>,
+                        std::is_aggregate<_Tp>,
+                        std::conjunction< std::is_trivially_destructible<_Tp>,
+                                          std::disjunction< std::is_trivially_default_constructible<_Tp>,
+                                                            std::is_trivially_copy_constructible<_Tp>,
+                                                            std::is_trivially_move_constructible<_Tp>>>> {};
+
+template <class _Tp>
+inline constexpr bool is_implicit_lifetime_v = is_implicit_lifetime<_Tp>::value;
+
+#  endif
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_IMPLICIT_LIFETIME_H

libcxx/include/type_traits

@@ -137,6 +137,8 @@ namespace std
     template <class T>                struct is_nothrow_swappable;      // C++17
     template <class T>                struct is_nothrow_destructible;
 
+    template<class T> struct is_implicit_lifetime;                     // Since C++23
+
     template <class T> struct has_virtual_destructor;
 
     template<class T> struct has_unique_object_representations;         // C++17
@@ -374,6 +376,8 @@ namespace std
         = is_nothrow_swappable<T>::value;                               // C++17
       template <class T> inline constexpr bool is_nothrow_destructible_v
         = is_nothrow_destructible<T>::value;                             // C++17
+      template<class T>
+        constexpr bool is_implicit_lifetime_v = is_implicit_lifetime<T>::value;   // Since C++23
       template <class T> inline constexpr bool has_virtual_destructor_v
         = has_virtual_destructor<T>::value;                              // C++17
       template<class T> inline constexpr bool has_unique_object_representations_v // C++17
@@ -517,6 +521,10 @@ namespace std
 #  include <__type_traits/unwrap_ref.h>
 #endif
 
+#if _LIBCPP_STD_VER >= 23
+#  include <__type_traits/is_implicit_lifetime.h>
+#endif
+
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)