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[libc++][tuple][utility] P2968R2: Make std::ignore a first-class object (llvm#97401)
Implements: https://wg21.link/P2968R2 References: - https://eel.is/c++draft/tuple.general - https://eel.is/c++draft/tuple.syn - https://eel.is/c++draft/tuple.creation - https://github.com/cplusplus/draft/milestone/31 - cplusplus/draft#7109 - cplusplus/papers#1640 - https://cplusplus.github.io/LWG/issue2933 - https://cplusplus.github.io/LWG/issue3978 --------- Co-authored-by: Hristo Hristov <[email protected]>
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‎libcxx/docs/ReleaseNotes/19.rst‎

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@@ -46,6 +46,7 @@ Implemented Papers
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- P2872R3 - Remove ``wstring_convert`` From C++26
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- P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
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- P2944R3 - Comparisons for ``reference_wrapper`` (comparison operators for ``reference_wrapper`` only)
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- P2968R2 - Make ``std::ignore`` a first-class object
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- P2302R4 - ``std::ranges::contains``
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- P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
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- P3029R1 - Better ``mdspan``'s CTAD
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- The formatting library is updated to Unicode 15.1.0.
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- ``std::ignore``\s ``const __ignore_t& operator=(_Tp&&) const`` was changed to
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``const __ignore_type& operator=(const _Tp&) const noexcept`` for all language versions.
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Deprecations and Removals
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-------------------------
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‎libcxx/docs/Status/Cxx2cPapers.csv‎

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"`P2985R0 <https://wg21.link/P2985R0>`__","LWG","A type trait for detecting virtual base classes","St. Louis June 2024","","",""
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"`P0843R14 <https://wg21.link/P0843R14>`__","LWG","``inplace_vector``","St. Louis June 2024","","",""
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"`P3235R3 <https://wg21.link/P3235R3>`__","LWG","``std::print`` more types faster with less memory","St. Louis June 2024","","","|format| |DR|"
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"`P2968R2 <https://wg21.link/P2968R2>`__","LWG","Make ``std::ignore`` a first-class object","St. Louis June 2024","","",""
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"`P2968R2 <https://wg21.link/P2968R2>`__","LWG","Make ``std::ignore`` a first-class object","St. Louis June 2024","|Complete|","19.0",""
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"`P2075R6 <https://wg21.link/P2075R6>`__","LWG","Philox as an extension of the C++ RNG engines","St. Louis June 2024","","",""
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"`P2422R1 <https://wg21.link/P2422R1>`__","LWG","Remove ``nodiscard`` annotations from the standard library specification","St. Louis June 2024","|Complete| [#note-P2422R1]_","19.0",""
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"`P2300R10 <https://wg21.link/P2300R10>`__","LWG","``std::execution``","St. Louis June 2024","","",""

‎libcxx/include/CMakeLists.txt‎

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__thread/timed_backoff_policy.h
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__tree
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__tuple/find_index.h
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__tuple/ignore.h
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__tuple/make_tuple_types.h
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__tuple/sfinae_helpers.h
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__tuple/tuple_element.h

‎libcxx/include/__tuple/ignore.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___TUPLE_IGNORE_H
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#define _LIBCPP___TUPLE_IGNORE_H
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#include <__config>
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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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#ifndef _LIBCPP_CXX03_LANG
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_LIBCPP_BEGIN_NAMESPACE_STD
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struct __ignore_type {
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template <class _Tp>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const __ignore_type& operator=(const _Tp&) const noexcept {
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return *this;
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}
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};
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# if _LIBCPP_STD_VER >= 17
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inline constexpr __ignore_type ignore;
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# else
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constexpr __ignore_type ignore;
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# endif
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP_CXX03_LANG
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#endif // _LIBCPP___TUPLE_IGNORE_H

‎libcxx/include/module.modulemap‎

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@@ -1840,6 +1840,7 @@ module std_private_thread_thread [system] {
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module std_private_thread_timed_backoff_policy [system] { header "__thread/timed_backoff_policy.h" }
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module std_private_tuple_find_index [system] { header "__tuple/find_index.h" }
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module std_private_tuple_ignore [system] { header "__tuple/ignore.h" }
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module std_private_tuple_make_tuple_types [system] { header "__tuple/make_tuple_types.h" }
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module std_private_tuple_tuple_like_no_subrange [system] {
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header "__tuple/tuple_like_no_subrange.h"

‎libcxx/include/tuple‎

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@@ -132,7 +132,12 @@ tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>; // since C++
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template <class Alloc, class ...T>
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tuple(allocator_arg_t, Alloc, tuple<T...>) -> tuple<T...>; // since C++17
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inline constexpr unspecified ignore;
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struct ignore-type { // exposition only // Since C++26
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constexpr const ignore-type&
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operator=(const auto &) const noexcept
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{ return *this; }
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};
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inline constexpr ignore-type ignore;
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template <class... T> tuple<V...> make_tuple(T&&...); // constexpr in C++14
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template <class... T> tuple<ATypes...> forward_as_tuple(T&&...) noexcept; // constexpr in C++14
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#include <__memory/allocator_arg_t.h>
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#include <__memory/uses_allocator.h>
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#include <__tuple/find_index.h>
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#include <__tuple/ignore.h>
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#include <__tuple/make_tuple_types.h>
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#include <__tuple/sfinae_helpers.h>
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#include <__tuple/tuple_element.h>
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return tuple<_Tp&...>(__t...);
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}
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template <class _Up>
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struct __ignore_t {
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template <class _Tp>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const __ignore_t& operator=(_Tp&&) const {
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return *this;
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}
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};
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# if _LIBCPP_STD_VER >= 17
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inline constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
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# else
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namespace {
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constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
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} // namespace
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# endif
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template <class... _Tp>
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inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<typename __unwrap_ref_decay<_Tp>::type...>
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make_tuple(_Tp&&... __t) {

‎libcxx/include/utility‎

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#include <compare>
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#include <initializer_list>
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// [tuple.creation]
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#include <__tuple/ignore.h>
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// [tuple.helper]
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#include <__tuple/tuple_element.h>
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#include <__tuple/tuple_size.h>

‎libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.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.
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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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// UNSUPPORTED: c++03
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// <tuple>
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// inline constexpr ignore-type ignore;
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// std::ignore should be provided by the headers <tuple> and <utility>.
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// This test validates its presence in <tuple>.
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#include <tuple>
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[[maybe_unused]] auto& ignore_v = std::ignore;

‎libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp‎

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//
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//===----------------------------------------------------------------------===//
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// <tuple>
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// UNSUPPORTED: c++03
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// constexpr unspecified ignore;
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// <tuple>
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// UNSUPPORTED: c++03
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// inline constexpr ignore-type ignore;
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#include <cassert>
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#include <cstdint>
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#include <tuple>
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#include <type_traits>
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#include "test_macros.h"
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constexpr bool test_ignore_constexpr()
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{
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#if TEST_STD_VER > 11
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{ // Test that std::ignore provides constexpr converting assignment.
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auto& res = (std::ignore = 42);
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assert(&res == &std::ignore);
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}
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{ // Test that std::ignore provides constexpr copy/move constructors
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auto copy = std::ignore;
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auto moved = std::move(copy);
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((void)moved);
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}
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{ // Test that std::ignore provides constexpr copy/move assignment
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auto copy = std::ignore;
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copy = std::ignore;
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auto moved = std::ignore;
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moved = std::move(copy);
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}
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static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
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#if TEST_STD_VER >= 17
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[[nodiscard]] constexpr int test_nodiscard() { return 8294; }
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#endif
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return true;
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TEST_CONSTEXPR_CXX14 bool test() {
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{ [[maybe_unused]] auto& ignore_v = std::ignore; }
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{ // Test that std::ignore provides converting assignment.
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auto& res = (std::ignore = 42);
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static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
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assert(&res == &std::ignore);
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}
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{ // Test bit-field binding.
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struct S {
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unsigned int bf : 3;
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};
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S s{0b010};
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auto& res = (std::ignore = s.bf);
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assert(&res == &std::ignore);
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}
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{ // Test that std::ignore provides copy/move constructors
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auto copy = std::ignore;
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[[maybe_unused]] auto moved = std::move(copy);
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}
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{ // Test that std::ignore provides copy/move assignment
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auto copy = std::ignore;
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copy = std::ignore;
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auto moved = std::ignore;
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moved = std::move(copy);
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}
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#if TEST_STD_VER >= 17
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{ std::ignore = test_nodiscard(); }
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#endif
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return true;
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}
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int main(int, char**) {
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{
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constexpr auto& ignore_v = std::ignore;
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((void)ignore_v);
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}
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{
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static_assert(test_ignore_constexpr(), "");
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}
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{
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LIBCPP_STATIC_ASSERT(std::is_trivial<decltype(std::ignore)>::value, "");
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}
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test();
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#if TEST_STD_VER >= 14
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static_assert(test(), "");
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#endif
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return 0;
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}

‎libcxx/test/std/utilities/utility/ignore/ignore.include.compile.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.
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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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// UNSUPPORTED: c++03
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// <utility>
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// inline constexpr ignore-type ignore;
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// std::ignore should be provided by the headers <tuple> and <utility>.
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// This test validates its presence in <utility>.
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#include <utility>
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[[maybe_unused]] auto& ignore_v = std::ignore;

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