[libc++][type_traits] P2674R1: A trait for implicit lifetime types (#106870)

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

---------

Co-authored-by: Hristo Hristov <[email protected]>

Author: H-G-Hristov

libcxx/docs/FeatureTestMacroTable.rst

@@ -336,6 +336,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_ios_noreplace``                                ``202207L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_is_implicit_lifetime``                         ``202302L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                               ``202011L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_mdspan``                                       ``202207L``

libcxx/docs/ReleaseNotes/20.rst

@@ -42,6 +42,7 @@ Implemented Papers
 - P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
 - ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
+- P2674R1: A trait for implicit lifetime types (`Github <https://github.com/llvm/llvm-project/issues/105259>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx23Papers.csv

@@ -113,7 +113,7 @@
 "`P2572R1 <https://wg21.link/P2572R1>`__","``std::format`` fill character allowances","2023-02 (Issaquah)","|Complete|","17.0",""
 "`P2693R1 <https://wg21.link/P2693R1>`__","Formatting ``thread::id`` and ``stacktrace``","2023-02 (Issaquah)","|Partial|","","The formatter for ``stacktrace`` is not implemented, since ``stacktrace`` is not implemented yet"
 "`P2679R2 <https://wg21.link/P2679R2>`__","Fixing ``std::start_lifetime_as`` for arrays","2023-02 (Issaquah)","","",""
-"`P2674R1 <https://wg21.link/P2674R1>`__","A trait for implicit lifetime types","2023-02 (Issaquah)","","",""
+"`P2674R1 <https://wg21.link/P2674R1>`__","A trait for implicit lifetime types","2023-02 (Issaquah)","|Complete|","20.0",""
 "`P2655R3 <https://wg21.link/P2655R3>`__","``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type","2023-02 (Issaquah)","","",""
 "`P2652R2 <https://wg21.link/P2652R2>`__","Disallow User Specialization of ``allocator_traits``","2023-02 (Issaquah)","|Complete|","19.0",""
 "`P2787R1 <https://wg21.link/P2787R1>`__","``pmr::generator`` - Promise Types are not Values","2023-02 (Issaquah)","","",""

libcxx/include/CMakeLists.txt

@@ -787,6 +787,7 @@ 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 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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);
+
+#  endif
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_IMPLICIT_LIFETIME_H

libcxx/include/module.modulemap

@@ -200,6 +200,10 @@ module std_core [system] {
       header "__type_traits/is_fundamental.h"
       export std_core.type_traits.integral_constant
     }
+    module is_implicit_lifetime {
+      header "__type_traits/is_implicit_lifetime.h"
+      export std_core.type_traits.integral_constant
+    }
     module is_implicitly_default_constructible {
       header "__type_traits/is_implicitly_default_constructible.h"
       export std_core.type_traits.integral_constant

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
@@ -516,6 +520,10 @@ 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)

libcxx/include/version

@@ -138,6 +138,7 @@ __cpp_lib_ios_noreplace                                 202207L <ios>
 __cpp_lib_is_aggregate                                  201703L <type_traits>
 __cpp_lib_is_constant_evaluated                         201811L <type_traits>
 __cpp_lib_is_final                                      201402L <type_traits>
+__cpp_lib_is_implicit_lifetime                          202302L <type_traits>
 __cpp_lib_is_invocable                                  201703L <type_traits>
 __cpp_lib_is_layout_compatible                          201907L <type_traits>
 __cpp_lib_is_nothrow_convertible                        201806L <type_traits>
@@ -473,6 +474,9 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_forward_like                         202207L
 # define __cpp_lib_invoke_r                             202106L
 # define __cpp_lib_ios_noreplace                        202207L
+# if __has_builtin(__builtin_is_implicit_lifetime)
+#   define __cpp_lib_is_implicit_lifetime               202302L
+# endif
 # define __cpp_lib_is_scoped_enum                       202011L
 # define __cpp_lib_mdspan                               202207L
 # define __cpp_lib_modules                              202207L

libcxx/modules/std/type_traits.inc

@@ -98,7 +98,9 @@ export namespace std {
 
   using std::is_nothrow_destructible;
 
-  //  using std::is_implicit_lifetime;
+#if _LIBCPP_STD_VER >= 23 && __has_builtin(__builtin_is_implicit_lifetime)
+  using std::is_implicit_lifetime;
+#endif
 
   using std::has_virtual_destructor;
 
@@ -246,7 +248,9 @@ export namespace std {
   using std::is_destructible_v;
   using std::is_empty_v;
   using std::is_final_v;
-  // using std::is_implicit_lifetime_v;
+#if _LIBCPP_STD_VER >= 23 && __has_builtin(__builtin_is_implicit_lifetime)
+  using std::is_implicit_lifetime_v;
+#endif
   using std::is_move_assignable_v;
   using std::is_move_constructible_v;
   using std::is_nothrow_assignable_v;

libcxx/test/std/language.support/support.limits/support.limits.general/type_traits.version.compile.pass.cpp

@@ -23,6 +23,7 @@
     __cpp_lib_is_aggregate                         201703L [C++17]
     __cpp_lib_is_constant_evaluated                201811L [C++20]
     __cpp_lib_is_final                             201402L [C++14]
+    __cpp_lib_is_implicit_lifetime                 202302L [C++23]
     __cpp_lib_is_invocable                         201703L [C++17]
     __cpp_lib_is_layout_compatible                 201907L [C++20]
     __cpp_lib_is_nothrow_convertible               201806L [C++20]
@@ -75,6 +76,10 @@
 #   error "__cpp_lib_is_final should not be defined before c++14"
 # endif
 
+# ifdef __cpp_lib_is_implicit_lifetime
+#   error "__cpp_lib_is_implicit_lifetime should not be defined before c++23"
+# endif
+
 # ifdef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should not be defined before c++17"
 # endif
@@ -179,6 +184,10 @@
 #   error "__cpp_lib_is_final should have the value 201402L in c++14"
 # endif
 
+# ifdef __cpp_lib_is_implicit_lifetime
+#   error "__cpp_lib_is_implicit_lifetime should not be defined before c++23"
+# endif
+
 # ifdef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should not be defined before c++17"
 # endif
@@ -301,6 +310,10 @@
 #   error "__cpp_lib_is_final should have the value 201402L in c++17"
 # endif
 
+# ifdef __cpp_lib_is_implicit_lifetime
+#   error "__cpp_lib_is_implicit_lifetime should not be defined before c++23"
+# endif
+
 # ifndef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should be defined in c++17"
 # endif
@@ -444,6 +457,10 @@
 #   error "__cpp_lib_is_final should have the value 201402L in c++20"
 # endif
 
+# ifdef __cpp_lib_is_implicit_lifetime
+#   error "__cpp_lib_is_implicit_lifetime should not be defined before c++23"
+# endif
+
 # ifndef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should be defined in c++20"
 # endif
@@ -614,6 +631,19 @@
 #   error "__cpp_lib_is_final should have the value 201402L in c++23"
 # endif
 
+# if __has_builtin(__builtin_is_implicit_lifetime)
+#   ifndef __cpp_lib_is_implicit_lifetime
+#     error "__cpp_lib_is_implicit_lifetime should be defined in c++23"
+#   endif
+#   if __cpp_lib_is_implicit_lifetime != 202302L
+#     error "__cpp_lib_is_implicit_lifetime should have the value 202302L in c++23"
+#   endif
+# else
+#   ifdef __cpp_lib_is_implicit_lifetime
+#     error "__cpp_lib_is_implicit_lifetime should not be defined when the requirement '__has_builtin(__builtin_is_implicit_lifetime)' is not met!"
+#   endif
+# endif
+
 # ifndef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should be defined in c++23"
 # endif
@@ -796,6 +826,19 @@
 #   error "__cpp_lib_is_final should have the value 201402L in c++26"
 # endif
 
+# if __has_builtin(__builtin_is_implicit_lifetime)
+#   ifndef __cpp_lib_is_implicit_lifetime
+#     error "__cpp_lib_is_implicit_lifetime should be defined in c++26"
+#   endif
+#   if __cpp_lib_is_implicit_lifetime != 202302L
+#     error "__cpp_lib_is_implicit_lifetime should have the value 202302L in c++26"
+#   endif
+# else
+#   ifdef __cpp_lib_is_implicit_lifetime
+#     error "__cpp_lib_is_implicit_lifetime should not be defined when the requirement '__has_builtin(__builtin_is_implicit_lifetime)' is not met!"
+#   endif
+# endif
+
 # ifndef __cpp_lib_is_invocable
 #   error "__cpp_lib_is_invocable should be defined in c++26"
 # endif