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[libc++] Implement the <type_traits> parts of P1317R2
Changes of `std::apply` are still blocked on related changes in P2165R4.
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‎libcxx/docs/ReleaseNotes/22.rst‎

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@@ -38,6 +38,8 @@ What's New in Libc++ 22.0.0?
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Implemented Papers
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------------------
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- P1317R2: Remove return type deduction in ``std::apply`` (`Github <https://github.com/llvm/llvm-project/issues/148183>`__)
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(Only components in ``<type_traits>`` are implemented.)
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- P2321R2: ``zip`` (`Github <https://github.com/llvm/llvm-project/issues/105169>`__) (The paper is partially implemented. ``zip_transform_view`` is implemented in this release)
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Improvements and New Features

‎libcxx/docs/Status/Cxx2cPapers.csv‎

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@@ -155,5 +155,5 @@
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"`P2781R9 <https://wg21.link/P2781R9>`__","``std::constant_wrapper``","2025-06 (Sofia)","","",""
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"`P3697R1 <https://wg21.link/P3697R1>`__","Minor additions to C++26 standard library hardening","2025-06 (Sofia)","","",""
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"`P3552R3 <https://wg21.link/P3552R3>`__","Add a Coroutine Task Type","2025-06 (Sofia)","","",""
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"`P1317R2 <https://wg21.link/P1317R2>`__","Remove return type deduction in ``std::apply``","2025-06 (Sofia)","","",""
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"`P1317R2 <https://wg21.link/P1317R2>`__","Remove return type deduction in ``std::apply``","2025-06 (Sofia)","|In Progress|","",""
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"","","","","",""

‎libcxx/include/CMakeLists.txt‎

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@@ -819,6 +819,7 @@ set(files
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__type_traits/is_aggregate.h
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__type_traits/is_allocator.h
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__type_traits/is_always_bitcastable.h
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__type_traits/is_applicable.h
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__type_traits/is_arithmetic.h
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__type_traits/is_array.h
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__type_traits/is_assignable.h

‎libcxx/include/__type_traits/is_applicable.h‎

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@@ -0,0 +1,115 @@
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___TYPE_TRAITS_IS_APPLICABLE_H
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#define _LIBCPP___TYPE_TRAITS_IS_APPLICABLE_H
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#include <__config>
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#include <__cstddef/size_t.h>
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#include <__fwd/get.h>
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#include <__tuple/tuple_like.h>
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#include <__tuple/tuple_size.h>
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#include <__type_traits/conjunction.h>
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#include <__type_traits/integral_constant.h>
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#include <__type_traits/invoke.h>
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#include <__type_traits/remove_reference.h>
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#include <__utility/declval.h>
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#include <__utility/integer_sequence.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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#if _LIBCPP_STD_VER >= 26
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template <class _Fn, class _Tuple>
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struct __apply_result_disabled_base {};
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template <class _Fn, class _Tuple, class _Tp>
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struct __apply_result_enabled_base {
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using type _LIBCPP_NODEBUG = _Tp;
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};
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template <bool _Applicable, bool _Nothrow, class _Tp>
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struct __applicability_traits {
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static constexpr bool __applicable = true;
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static constexpr bool __nothrow_applicable = _Nothrow;
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template <class _Fn, class _Tuple>
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using __base_type _LIBCPP_NODEBUG = __apply_result_enabled_base<_Fn, _Tuple, _Tp>;
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};
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template <bool _Nothrow, class _Tp>
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struct __applicability_traits<false, _Nothrow, _Tp> {
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static_assert(!_Nothrow, "misspecified [_Applicable = false, _Nothrow = true]");
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static constexpr bool __applicable = false;
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static constexpr bool __nothrow_applicable = false;
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template <class _Fn, class _Tuple>
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using __base_type _LIBCPP_NODEBUG = __apply_result_disabled_base<_Fn, _Tuple>;
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};
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template <class _Fn, class _Tuple, size_t... _Is>
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concept __tuple_applicable_impl = requires(_Tuple&& __tuple) {
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[](auto&&...) {}(std::get<_Is>(static_cast<_Tuple &&>(__tuple))...);
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} && __is_invocable_v<_Fn, decltype(std::get<_Is>(std::declval<_Tuple>()))...>;
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template <class _Fn, class _Tuple, size_t... _Is>
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concept __tuple_nothrow_applicable_impl = requires(_Tuple&& __tuple) {
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{
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[](auto&&...) noexcept {}(std::get<_Is>(static_cast<_Tuple &&>(__tuple))...)
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} noexcept;
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} && __is_nothrow_invocable_v<_Fn, decltype(std::get<_Is>(std::declval<_Tuple>()))...>;
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template <class _Fn, class _Tuple>
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consteval auto __applicability_traits_of() {
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if constexpr (__tuple_like<_Tuple>)
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return []<size_t... _Is>(index_sequence<_Is...>) {
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if constexpr (__tuple_applicable_impl<_Fn, _Tuple, _Is...>) {
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return __applicability_traits<
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true,
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__tuple_nothrow_applicable_impl<_Fn, _Tuple, _Is...>,
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// FIXME: Use __invoke_result_y after merging https://github.com/llvm/llvm-project/pull/151028.
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typename __invoke_result<_Fn, decltype(std::get<_Is>(std::declval<_Tuple>()))...>::type>{};
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} else
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return __applicability_traits<false, false, void>{};
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}(make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>{});
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else
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return __applicability_traits<false, false, void>{};
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}
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template <class _Fn, class _Tuple>
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struct _LIBCPP_NO_SPECIALIZATIONS is_applicable
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: bool_constant<decltype(std::__applicability_traits_of<_Fn, _Tuple>())::__applicable> {};
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template <class _Fn, class _Tuple>
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struct _LIBCPP_NO_SPECIALIZATIONS is_nothrow_applicable
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: bool_constant<decltype(std::__applicability_traits_of<_Fn, _Tuple>())::__nothrow_applicable> {};
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template <class _Fn, class _Tuple>
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_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_applicable_v =
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decltype(std::__applicability_traits_of<_Fn, _Tuple>())::__applicable;
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template <class _Fn, class _Tuple>
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_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_nothrow_applicable_v =
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decltype(std::__applicability_traits_of<_Fn, _Tuple>())::__nothrow_applicable;
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template <class _Fn, class _Tuple>
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struct _LIBCPP_NO_SPECIALIZATIONS apply_result
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: decltype(std::__applicability_traits_of<_Fn, _Tuple>())::template __base_type<_Fn, _Tuple> {};
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template <class _Fn, class _Tuple>
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using apply_result_t = apply_result<_Fn, _Tuple>::type;
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#endif // _LIBCPP_STD_VER >= 26
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___TYPE_TRAITS_IS_APPLICABLE_H

‎libcxx/include/module.modulemap.in‎

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@@ -112,6 +112,10 @@ module std_core [system] {
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header "__type_traits/is_always_bitcastable.h"
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export std_core.type_traits.integral_constant
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}
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module is_applicable {
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header "__type_traits/is_applicable.h"
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export std_core.type_traits.integral_constant
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}
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module is_arithmetic {
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header "__type_traits/is_arithmetic.h"
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export std_core.type_traits.integral_constant

‎libcxx/include/type_traits‎

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template <class R, class Fn, class... ArgTypes>
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struct is_nothrow_invocable_r; // since C++17
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template<class Fn, class Tuple> struct is_applicable; // since C++26
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template<class Fn, class Tuple>
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struct is_nothrow_applicable; // since C++26
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// Alignment properties and transformations:
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template <class T> struct alignment_of;
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template <size_t Len, size_t Align = most_stringent_alignment_requirement>
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struct result_of<Fn(ArgTypes...)>; // deprecated in C++17; removed in C++20
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template <class Fn, class... ArgTypes>
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struct invoke_result; // since C++17
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template<class Fn, class Tuple> struct apply_result; // since C++26
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// const-volatile modifications:
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template <class T>
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template <class Fn, class... ArgTypes>
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using invoke_result_t
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= typename invoke_result<Fn, ArgTypes...>::type; // since C++17
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template <class Fn, class... ArgTypes>
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using apply_result_t
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= typename invoke_result<Fn, ArgTypes...>::type; // since C++26
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template <class...>
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using void_t = void; // since C++17
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= is_nothrow_invocable<Fn, ArgTypes...>::value; // since C++17
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template <class R, class Fn, class... ArgTypes> inline constexpr bool is_nothrow_invocable_r_v
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= is_nothrow_invocable_r<R, Fn, ArgTypes...>::value; // since C++17
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template<class Fn, class Tuple> constexpr bool is_applicable_v
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= is_applicable<Fn, Tuple>::value; // since C++26
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template<class Fn, class Tuple> constexpr bool is_nothrow_applicable_v
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= is_nothrow_applicable<Fn, Tuple>::value; // since C++26
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// [meta.logical], logical operator traits:
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template<class... B> struct conjunction; // since C++17
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# include <__type_traits/reference_converts_from_temporary.h>
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# endif
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# if _LIBCPP_STD_VER >= 26
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# include <__type_traits/is_applicable.h>
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# endif
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# include <version>
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# if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

‎libcxx/modules/std/type_traits.inc‎

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using std::rank;
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// [meta.rel], type relations
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#if _LIBCPP_STD_VER >= 26
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using std::is_applicable;
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#endif
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using std::is_base_of;
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#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_virtual_base_of)
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using std::is_virtual_base_of;
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using std::is_invocable;
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using std::is_invocable_r;
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#if _LIBCPP_STD_VER >= 26
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using std::is_nothrow_applicable;
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#endif
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using std::is_nothrow_invocable;
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using std::is_nothrow_invocable_r;
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@@ -183,6 +189,9 @@ export namespace std {
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using std::remove_pointer_t;
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// [meta.trans.other], other transformations
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#if _LIBCPP_STD_VER >= 26
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using std::apply_result;
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#endif
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using std::basic_common_reference;
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using std::common_reference;
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using std::common_type;
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using std::unwrap_ref_decay;
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using std::unwrap_reference;
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#if _LIBCPP_STD_VER >= 26
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using std::apply_result_t;
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#endif
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using std::common_reference_t;
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using std::common_type_t;
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using std::conditional_t;
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using std::rank_v;
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// [meta.rel], type relations
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#if _LIBCPP_STD_VER >= 26
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using std::is_applicable_v;
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#endif
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using std::is_base_of_v;
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#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_virtual_base_of)
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using std::is_virtual_base_of_v;
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using std::is_invocable_r_v;
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using std::is_invocable_v;
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// using std::is_layout_compatible_v;
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#if _LIBCPP_STD_VER >= 26
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using std::is_nothrow_applicable_v;
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#endif
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using std::is_nothrow_convertible_v;
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using std::is_nothrow_invocable_r_v;
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using std::is_nothrow_invocable_v;

‎libcxx/test/libcxx/type_traits/no_specializations.verify.cpp‎

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@@ -53,6 +53,11 @@ SPECIALIZE_TRAIT(unwrap_reference); // expected-error {{cannot be specialized}}
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SPECIALIZE_TRAIT(unwrap_ref_decay); // expected-error {{cannot be specialized}}
5454
# endif
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# if TEST_STD_VER >= 26
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template <>
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struct std::apply_result<S, S>; // expected-error {{cannot be specialized}}
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# endif
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# undef SPECIALIZE_TRAIT
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# define SPECIALIZE_UTT(Trait) \
5863
template <> \
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165170
# endif
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167172
# if TEST_STD_VER >= 26
168-
SPECIALIZE_BTT(is_virtual_base_of); // expected-error 2 {{cannot be specialized}}
173+
SPECIALIZE_BTT(is_applicable); // expected-error 2 {{cannot be specialized}}
174+
SPECIALIZE_BTT(is_nothrow_applicable); // expected-error 2 {{cannot be specialized}}
175+
SPECIALIZE_BTT(is_virtual_base_of); // expected-error 2 {{cannot be specialized}}
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# endif
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# undef SPECIALIZE_UTT

‎libcxx/test/std/utilities/meta/derived_from_integral_constant.compile.pass.cpp‎

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# if defined(__cpp_lib_is_virtual_base_of) && __cpp_lib_is_virtual_base_of >= 202406L
114114
static_assert(std::is_base_of<std::false_type, std::is_virtual_base_of<int, int>>::value, "");
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# endif
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static_assert(std::is_base_of<std::false_type, std::is_applicable<int, int>>::value, "");
117+
static_assert(std::is_base_of<std::false_type, std::is_nothrow_applicable<int, int>>::value, "");
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#endif

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