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[libcxx] Put std::monostate in <utility> 
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‎libcxx/docs/ReleaseNotes/21.rst‎

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- N4258: Cleaning-up noexcept in the Library (`Github <https://github.com/llvm/llvm-project/issues/99937>`__)
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- P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
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- P0472R3: Put std::monostate in <utility> (`Github <https://github.com/llvm/llvm-project/issues/127874`__)
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Improvements and New Features
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-----------------------------

‎libcxx/docs/Status/Cxx2cPapers.csv‎

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"`P3475R2 <https://wg21.link/P3475R2>`__","Defang and deprecate ``memory_order::consume``","2025-02 (Hagenberg)","","",""
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"`P2786R13 <https://wg21.link/P2786R13>`__","Trivial Relocatability For C++26","2025-02 (Hagenberg)","","",""
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"`P3137R3 <https://wg21.link/P3137R3>`__","``views::to_input``","2025-02 (Hagenberg)","","",""
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"`P0472R3 <https://wg21.link/P0472R3>`__","Put ``std::monostate`` in ``<utility>``","2025-02 (Hagenberg)","","",""
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"`P0472R3 <https://wg21.link/P0472R3>`__","Put ``std::monostate`` in ``<utility>``","2025-02 (Hagenberg)","|Complete|","21",""
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"`P3349R1 <https://wg21.link/P3349R1>`__","Converting contiguous iterators to pointers","2025-02 (Hagenberg)","","",""
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"`P3372R3 <https://wg21.link/P3372R3>`__","constexpr containers and adaptors","2025-02 (Hagenberg)","","",""
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"`P3378R2 <https://wg21.link/P3378R2>`__","constexpr exception types","2025-02 (Hagenberg)","","",""

‎libcxx/include/__cxx03/__utility/monostate.h‎

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// -*- C++ -*-
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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___CXX03___UTILITY_MONOSTATE_H
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#define _LIBCPP___CXX03___UTILITY_MONOSTATE_H
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#include <__cxx03/__compare/ordering.h>
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#include <__cxx03/__config>
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#include <__cxx03/__functional/hash.h>
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#include <__cxx03/cstddef>
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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 >= 17
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struct _LIBCPP_TEMPLATE_VIS monostate {};
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
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# if _LIBCPP_STD_VER >= 20
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_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
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return strong_ordering::equal;
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}
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# else // _LIBCPP_STD_VER >= 20
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
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# endif // _LIBCPP_STD_VER >= 20
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template <>
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struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
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using argument_type = monostate;
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using result_type = size_t;
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inline _LIBCPP_HIDE_FROM_ABI result_type operator()(const argument_type&) const _NOEXCEPT {
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return 66740831; // return a fundamentally attractive random value.
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}
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};
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#endif // _LIBCPP_STD_VER >= 17
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___CXX03___UTILITY_MONOSTATE_H

‎libcxx/include/__utility/monostate.h‎

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// -*- C++ -*-
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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___UTILITY_MONOSTATE_H
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#define _LIBCPP___UTILITY_MONOSTATE_H
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#include <__compare/ordering.h>
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#include <__configUTILITY>
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#include <__cstddef/size_t.h>
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#include <__functional/hash.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 >= 17
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struct _LIBCPP_TEMPLATE_VIS monostate {};
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
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# if _LIBCPP_STD_VER >= 20
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_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
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return strong_ordering::equal;
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}
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# else // _LIBCPP_STD_VER >= 20
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
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_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
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# endif // _LIBCPP_STD_VER >= 20
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template <>
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struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
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# if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_BINDER_TYPEDEFS)
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using argument_type _LIBCPP_DEPRECATED_IN_CXX17 = monostate;
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using result_type _LIBCPP_DEPRECATED_IN_CXX17 = size_t;
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# endif
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inline _LIBCPP_HIDE_FROM_ABI size_t operator()(const monostate&) const noexcept {
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return 66740831; // return a fundamentally attractive random value.
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}
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};
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#endif // _LIBCPP_STD_VER >= 17
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___UTILITY_MONOSTATE_H

‎libcxx/test/std/utilities/utility/utility.monostate.relpos/relops.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, c++11, c++14, c++17, c++20, c++23
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// <utility>
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// constexpr bool operator<(monostate, monostate) noexcept { return false; }
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// constexpr bool operator>(monostate, monostate) noexcept { return false; }
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// constexpr bool operator<=(monostate, monostate) noexcept { return true; }
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// constexpr bool operator>=(monostate, monostate) noexcept { return true; }
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// constexpr bool operator==(monostate, monostate) noexcept { return true; }
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// constexpr bool operator!=(monostate, monostate) noexcept { return false; }
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// constexpr strong_ordering operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; } // since C++20
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#include <cassert>
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#include <utility>
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#include "test_comparisons.h"
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#include "test_macros.h"
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constexpr bool test() {
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using M = std::monostate;
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constexpr M m1{};
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constexpr M m2{};
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assert(testComparisons(m1, m2, /*isEqual*/ true, /*isLess*/ false));
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AssertComparisonsAreNoexcept<M>();
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#if TEST_STD_VER > 17
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assert(testOrder(m1, m2, std::strong_ordering::equal));
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AssertOrderAreNoexcept<M>();
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#endif // TEST_STD_VER > 17
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return true;
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}
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int main(int, char**) {
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test();
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static_assert(test());
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return 0;
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}

‎libcxx/test/std/utilities/utility/utility.monostate/monostate.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, c++11, c++14, c++17, c++20, c++23
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// <utility>
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// struct monostate {};
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#include <type_traits>
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#include <utility>
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#include "test_macros.h"
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int main(int, char**) {
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using M = std::monostate;
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static_assert(std::is_trivially_default_constructible<M>::value, "");
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static_assert(std::is_trivially_copy_constructible<M>::value, "");
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static_assert(std::is_trivially_copy_assignable<M>::value, "");
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static_assert(std::is_trivially_destructible<M>::value, "");
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constexpr M m{};
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((void)m);
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return 0;
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}

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