[libcxx] Put std::monostate in <utility> (#128373)

Fixes: #127874

libcxx/docs/ReleaseNotes/21.rst

@@ -44,6 +44,7 @@ Implemented Papers
 - P2255R2: A type trait to detect reference binding to temporary (implemented the type traits only) (`Github <https://github.com/llvm/llvm-project/issues/105180>`__)
 - P2562R1: ``constexpr`` Stable Sorting (`Github <https://github.com/llvm/llvm-project/issues/105360>`__)
 - P1222R4: A Standard ``flat_set`` is partially implemented and ``flat_set`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105193>`__)
+- P0472R3: Put std::monostate in <utility> (`Github <https://github.com/llvm/llvm-project/issues/127874>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2cPapers.csv

@@ -102,7 +102,7 @@
 "`P3475R2 <https://wg21.link/P3475R2>`__","Defang and deprecate ``memory_order::consume``","2025-02 (Hagenberg)","","",""
 "`P2786R13 <https://wg21.link/P2786R13>`__","Trivial Relocatability For C++26","2025-02 (Hagenberg)","","",""
 "`P3137R3 <https://wg21.link/P3137R3>`__","``views::to_input``","2025-02 (Hagenberg)","","",""
-"`P0472R3 <https://wg21.link/P0472R3>`__","Put ``std::monostate`` in ``<utility>``","2025-02 (Hagenberg)","","",""
+"`P0472R3 <https://wg21.link/P0472R3>`__","Put ``std::monostate`` in ``<utility>``","2025-02 (Hagenberg)","|Complete|","21",""
 "`P3349R1 <https://wg21.link/P3349R1>`__","Converting contiguous iterators to pointers","2025-02 (Hagenberg)","","",""
 "`P3372R3 <https://wg21.link/P3372R3>`__","constexpr containers and adaptors","2025-02 (Hagenberg)","","",""
 "`P3378R2 <https://wg21.link/P3378R2>`__","constexpr exception types","2025-02 (Hagenberg)","","",""

libcxx/include/utility

@@ -279,6 +279,10 @@ template <class T>
 #    include <__utility/unreachable.h>
 #  endif
 
+#  if _LIBCPP_STD_VER >= 26
+#    include <__variant/monostate.h>
+#  endif
+
 #  include <version>
 
 // standard-mandated includes

libcxx/test/libcxx/transitive_includes/cxx26.csv

@@ -301,6 +301,7 @@ experimental/type_traits type_traits
 experimental/type_traits version
 experimental/utility compare
 experimental/utility cstdint
+experimental/utility cstring
 experimental/utility initializer_list
 experimental/utility limits
 experimental/utility utility
@@ -1127,6 +1128,7 @@ unordered_set tuple
 unordered_set version
 utility compare
 utility cstdint
+utility cstring
 utility initializer_list
 utility limits
 utility version

libcxx/test/std/utilities/utility/utility.monostate.relpos/relops.pass.cpp

@@ -0,0 +1,47 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <utility>
+
+// constexpr bool operator<(monostate, monostate) noexcept { return false; }
+// constexpr bool operator>(monostate, monostate) noexcept { return false; }
+// constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+// constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+// constexpr bool operator==(monostate, monostate) noexcept { return true; }
+// constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+// constexpr strong_ordering operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; } // since C++20
+
+#include <cassert>
+#include <utility>
+
+#include "test_comparisons.h"
+#include "test_macros.h"
+
+constexpr bool test() {
+  using M = std::monostate;
+  constexpr M m1{};
+  constexpr M m2{};
+  assert(testComparisons(m1, m2, /*isEqual*/ true, /*isLess*/ false));
+  AssertComparisonsAreNoexcept<M>();
+
+#if TEST_STD_VER > 17
+  assert(testOrder(m1, m2, std::strong_ordering::equal));
+  AssertOrderAreNoexcept<M>();
+#endif // TEST_STD_VER > 17
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}

libcxx/test/std/utilities/utility/utility.monostate/monostate.pass.cpp

@@ -0,0 +1,31 @@
+
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <utility>
+
+// struct monostate {};
+
+#include <type_traits>
+#include <utility>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  using M = std::monostate;
+  static_assert(std::is_trivially_default_constructible<M>::value, "");
+  static_assert(std::is_trivially_copy_constructible<M>::value, "");
+  static_assert(std::is_trivially_copy_assignable<M>::value, "");
+  static_assert(std::is_trivially_destructible<M>::value, "");
+  constexpr M m{};
+  ((void)m);
+
+  return 0;
+}