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H-G-HristovZingam
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Zingam
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[libc++][type_traits] Implements "A type trait to detect reference binding to temporary" (#128649)
Implements partially: [P2255R2: A type trait to detect reference binding to temporary](https://wg21.link/P2255R2) Issue: #105180 https://eel.is/c++draft/meta.type.synop https://eel.is/c++draft/meta.unary.prop Implented type traits: - [x] `reference_constructs_from_temporary` - [x] `reference_converts_from_temporary` Closes #129049 Minor drive-by tweak to `std::is_implicit_lifetime` tests. --------- Co-authored-by: Hristo Hristov <[email protected]>
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‎libcxx/docs/ReleaseNotes/21.rst

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@@ -41,6 +41,7 @@ Implemented Papers
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- N4258: Cleaning-up noexcept in the Library (`Github <https://github.com/llvm/llvm-project/issues/99937>`__)
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- P0767R1: Deprecate POD (`Github <https://github.com/llvm/llvm-project/issues/104013>`__)
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- P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
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- P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`)
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Improvements and New Features
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-----------------------------

‎libcxx/docs/Status/Cxx23Papers.csv

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@@ -43,7 +43,7 @@
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"`P0627R6 <https://wg21.link/P0627R6>`__","Function to mark unreachable code","2022-02 (Virtual)","|Complete|","15",""
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"`P1206R7 <https://wg21.link/P1206R7>`__","``ranges::to``: A function to convert any range to a container","2022-02 (Virtual)","|Complete|","17",""
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"`P1413R3 <https://wg21.link/P1413R3>`__","Deprecate ``std::aligned_storage`` and ``std::aligned_union``","2022-02 (Virtual)","|Complete|","","``std::aligned_storage_t`` and ``std::aligned_union_t`` are marked deprecated, but clang doesn't issue a diagnostic for deprecated using template declarations."
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"`P2255R2 <https://wg21.link/P2255R2>`__","A type trait to detect reference binding to temporary","2022-02 (Virtual)","","",""
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"`P2255R2 <https://wg21.link/P2255R2>`__","A type trait to detect reference binding to temporary","2022-02 (Virtual)","|Partial|","","Implemented the type traits only."
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"`P2273R3 <https://wg21.link/P2273R3>`__","Making ``std::unique_ptr`` constexpr","2022-02 (Virtual)","|Complete|","16",""
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"`P2387R3 <https://wg21.link/P2387R3>`__","Pipe support for user-defined range adaptors","2022-02 (Virtual)","|Complete|","19",""
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"`P2440R1 <https://wg21.link/P2440R1>`__","``ranges::iota``, ``ranges::shift_left`` and ``ranges::shift_right``","2022-02 (Virtual)","","",""

‎libcxx/include/CMakeLists.txt

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@@ -867,6 +867,8 @@ set(files
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__type_traits/negation.h
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__type_traits/promote.h
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__type_traits/rank.h
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__type_traits/reference_constructs_from_temporary.h
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__type_traits/reference_converts_from_temporary.h
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__type_traits/remove_all_extents.h
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__type_traits/remove_const.h
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__type_traits/remove_const_ref.h

‎libcxx/include/__type_traits/reference_constructs_from_temporary.h

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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_REFERENCE_CONSTRUCTS_FROM_TEMPORARY_H
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#define _LIBCPP___TYPE_TRAITS_REFERENCE_CONSTRUCTS_FROM_TEMPORARY_H
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#include <__config>
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#include <__type_traits/integral_constant.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 >= 23 && __has_builtin(__reference_constructs_from_temporary)
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template <class _Tp, class _Up>
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struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS reference_constructs_from_temporary
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: public bool_constant<__reference_constructs_from_temporary(_Tp, _Up)> {};
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template <class _Tp, class _Up>
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_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool reference_constructs_from_temporary_v =
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__reference_constructs_from_temporary(_Tp, _Up);
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#endif
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___TYPE_TRAITS_REFERENCE_CONSTRUCTS_FROM_TEMPORARY_H

‎libcxx/include/__type_traits/reference_converts_from_temporary.h

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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_REFERENCE_CONVERTS_FROM_TEMPORARY_H
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#define _LIBCPP___TYPE_TRAITS_REFERENCE_CONVERTS_FROM_TEMPORARY_H
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#include <__config>
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#include <__type_traits/integral_constant.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 >= 23 && __has_builtin(__reference_converts_from_temporary)
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template <class _Tp, class _Up>
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struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS reference_converts_from_temporary
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: public bool_constant<__reference_converts_from_temporary(_Tp, _Up)> {};
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template <class _Tp, class _Up>
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_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool reference_converts_from_temporary_v =
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__reference_converts_from_temporary(_Tp, _Up);
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#endif
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___TYPE_TRAITS_REFERENCE_CONVERTS_FROM_TEMPORARY_H

‎libcxx/include/module.modulemap

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@@ -369,6 +369,14 @@ module std_core [system] {
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module negation { header "__type_traits/negation.h" }
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module promote { header "__type_traits/promote.h" }
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module rank { header "__type_traits/rank.h" }
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module reference_constructs_from_temporary {
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header "__type_traits/reference_constructs_from_temporary.h"
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export std_core.type_traits.integral_constant
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}
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module reference_converts_from_temporary {
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header "__type_traits/reference_converts_from_temporary.h"
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export std_core.type_traits.integral_constant
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}
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module remove_all_extents { header "__type_traits/remove_all_extents.h" }
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module remove_const_ref { header "__type_traits/remove_const_ref.h" }
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module remove_const { header "__type_traits/remove_const.h" }

‎libcxx/include/type_traits

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template<class T> struct has_unique_object_representations; // C++17
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template<class T, class U> struct reference_constructs_from_temporary; // Since C++23
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template<class T, class U> struct reference_converts_from_temporary; // Since C++23
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// Relationships between types:
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template <class T, class U> struct is_same;
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template <class Base, class Derived> struct is_base_of;
@@ -382,6 +385,12 @@ namespace std
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= has_virtual_destructor<T>::value; // C++17
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template<class T> inline constexpr bool has_unique_object_representations_v // C++17
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= has_unique_object_representations<T>::value;
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template<class T, class U>
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constexpr bool reference_constructs_from_temporary_v
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= reference_constructs_from_temporary<T, U>::value; // Since C++23
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template<class T, class U>
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constexpr bool reference_converts_from_temporary_v
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= reference_converts_from_temporary<T, U>::value; // Since C++23
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// See C++14 20.10.5, type property queries
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template <class T> inline constexpr size_t alignment_of_v
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# if _LIBCPP_STD_VER >= 23
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# include <__type_traits/is_implicit_lifetime.h>
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# include <__type_traits/reference_constructs_from_temporary.h>
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# include <__type_traits/reference_converts_from_temporary.h>
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# endif
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# include <version>

‎libcxx/modules/std/type_traits.inc

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using std::has_unique_object_representations;
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109-
// using std::reference_constructs_from_temporary;
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// using std::reference_converts_from_temporary;
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#if _LIBCPP_STD_VER >= 23
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# if __has_builtin(__reference_constructs_from_temporary)
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using std::reference_constructs_from_temporary;
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# endif
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# if __has_builtin(__reference_converts_from_temporary)
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using std::reference_converts_from_temporary;
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# endif
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#endif
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// [meta.unary.prop.query], type property queries
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using std::alignment_of;
@@ -284,8 +290,14 @@ export namespace std {
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using std::is_unbounded_array_v;
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using std::is_unsigned_v;
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using std::is_volatile_v;
287-
// using std::reference_constructs_from_temporary_v;
288-
// using std::reference_converts_from_temporary_v;
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#if _LIBCPP_STD_VER >= 23
294+
# if __has_builtin(__reference_constructs_from_temporary)
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using std::reference_constructs_from_temporary_v;
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# endif
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# if __has_builtin(__reference_converts_from_temporary)
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using std::reference_converts_from_temporary_v;
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# endif
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#endif
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// [meta.unary.prop.query], type property queries
291303
using std::alignment_of_v;

‎libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/common.h

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A& operator=(const A&);
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};
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42-
class Abstract
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{
44-
virtual ~Abstract() = 0;
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class Abstract {
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virtual ~Abstract() = 0;
4544
};
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// Types for reference_{constructs/converts}_from_temporary
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#if TEST_STD_VER >= 23
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class NonPODClass {
51+
public:
52+
NonPODClass(int);
53+
};
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enum Enum { EV };
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struct Base {
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Enum e;
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int i;
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float f;
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NonPODClass* p;
60+
};
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// Not PODs
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struct Derived : Base {};
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template <class T, class RefType = T&>
65+
class ConvertsToRef {
66+
public:
67+
operator RefType() const { return static_cast<RefType>(obj); }
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mutable T obj = 42;
69+
};
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template <class T, class RefType = T&>
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class ConvertsToRefPrivate {
72+
operator RefType() const { return static_cast<RefType>(obj); }
73+
mutable T obj = 42;
74+
};
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class ExplicitConversionRvalueRef {
77+
public:
78+
operator int();
79+
explicit operator int&&();
80+
};
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class ExplicitConversionRef {
83+
public:
84+
operator int();
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explicit operator int&();
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};
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#endif
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#endif // TEST_META_UNARY_COMP_COMMON_H

‎libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_implicit_lifetime.pass.cpp

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// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
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// These compilers don't support __builtin_is_implicit_lifetime yet.
12-
// UNSUPPORTED: clang-18, clang-19, gcc-14, apple-clang-15, apple-clang-16, apple-clang-17
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// UNSUPPORTED: clang-18, clang-19, gcc-14, apple-clang-15, apple-clang-16
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1414
// <type_traits>
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‎libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_implicit_lifetime.verify.cpp

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// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
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// These compilers don't support __builtin_is_implicit_lifetime yet.
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// UNSUPPORTED: clang-18, clang-19, gcc-14, apple-clang-15, apple-clang-16, apple-clang-17
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// UNSUPPORTED: clang-18, clang-19, gcc-14, apple-clang-15, apple-clang-16
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// <type_traits>
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‎libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp

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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.
4+
// See https://llvm.org/LICENSE.txt for license information.
5+
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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// REQUIRES: std-at-least-c++23
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// These compilers don't support std::reference_converts_from_temporary yet.
12+
// UNSUPPORTED: android, apple-clang-15, apple-clang-16, clang-19.1
13+
14+
// <type_traits>
15+
16+
// template<class T, class U> struct reference_constructs_from_temporary;
17+
18+
// template<class T, class U>
19+
// constexpr bool reference_constructs_from_temporary_v
20+
// = reference_constructs_from_temporary<T, U>::value;
21+
22+
#include <cassert>
23+
#include <type_traits>
24+
25+
#include "common.h"
26+
#include "test_macros.h"
27+
28+
template <typename T, typename U, bool Expected>
29+
constexpr void test_reference_constructs_from_temporary() {
30+
assert((std::reference_constructs_from_temporary<T, U>::value == Expected));
31+
assert((std::reference_constructs_from_temporary_v<T, U> == Expected));
32+
}
33+
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constexpr bool test() {
35+
test_reference_constructs_from_temporary<int&, int&, false>();
36+
test_reference_constructs_from_temporary<int&, int&, false>();
37+
test_reference_constructs_from_temporary<int&, int&&, false>();
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test_reference_constructs_from_temporary<const int&, int&, false>();
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test_reference_constructs_from_temporary<const int&, const int&, false>();
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test_reference_constructs_from_temporary<const int&, int&&, false>();
42+
43+
test_reference_constructs_from_temporary<int&, long&, false>(); // doesn't construct
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test_reference_constructs_from_temporary<const int&, long&, true>();
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test_reference_constructs_from_temporary<const int&, long&&, true>();
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test_reference_constructs_from_temporary<int&&, long&, true>();
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49+
assert((std::is_constructible_v<int&, ConvertsToRef<int, int&>>));
50+
test_reference_constructs_from_temporary<int&, ConvertsToRef<int, int&>, false>();
51+
52+
assert((std::is_constructible_v<int&&, ConvertsToRef<int, int&&>>));
53+
test_reference_constructs_from_temporary<int&&, ConvertsToRef<int, int&&>, false>();
54+
55+
assert((std::is_constructible_v<const int&, ConvertsToRef<int, const int&>>));
56+
test_reference_constructs_from_temporary<int&&, ConvertsToRef<int, const int&>, false>();
57+
58+
assert((std::is_constructible_v<const int&, ConvertsToRef<long, long&>>));
59+
test_reference_constructs_from_temporary<const int&, ConvertsToRef<long, long&>, true>();
60+
#ifndef TEST_COMPILER_GCC
61+
test_reference_constructs_from_temporary<const int&, ConvertsToRefPrivate<long, long&>, false>();
62+
#endif
63+
64+
// Test that it doesn't accept non-reference types as input.
65+
test_reference_constructs_from_temporary<int, long, false>();
66+
67+
test_reference_constructs_from_temporary<const int&, long, true>();
68+
69+
// Additional checks
70+
test_reference_constructs_from_temporary<const Base&, Derived, true>();
71+
test_reference_constructs_from_temporary<int&&, int, true>();
72+
test_reference_constructs_from_temporary<const int&, int, true>();
73+
test_reference_constructs_from_temporary<int&&, int&&, false>();
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test_reference_constructs_from_temporary<const int&, int&&, false>();
75+
test_reference_constructs_from_temporary<int&&, long&&, true>();
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test_reference_constructs_from_temporary<int&&, long, true>();
77+
78+
test_reference_constructs_from_temporary<int&, ExplicitConversionRef, false>();
79+
test_reference_constructs_from_temporary<const int&, ExplicitConversionRef, false>();
80+
test_reference_constructs_from_temporary<int&&, ExplicitConversionRvalueRef, false>();
81+
82+
return true;
83+
}
84+
85+
int main(int, char**) {
86+
test();
87+
static_assert(test());
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return 0;
90+
}

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