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

AmrDeveloper · 2025-02-22T22:14:16Z

Fixes: #127874

llvmbot · 2025-02-22T22:18:51Z

@llvm/pr-subscribers-libcxx

Author: Amr Hesham (AmrDeveloper)

Changes

Fixes: #127874

Full diff: https://github.com/llvm/llvm-project/pull/128373.diff

6 Files Affected:

(modified) libcxx/docs/ReleaseNotes/21.rst (+1)
(modified) libcxx/docs/Status/Cxx2cPapers.csv (+1-1)
(added) libcxx/include/__cxx03/__utility/monostate.h (+65)
(added) libcxx/include/__utility/monostate.h (+66)
(added) libcxx/test/std/utilities/utility/utility.monostate.relpos/relops.pass.cpp (+47)
(added) libcxx/test/std/utilities/utility/utility.monostate/monostate.pass.cpp (+31)

diff --git a/libcxx/docs/ReleaseNotes/21.rst b/libcxx/docs/ReleaseNotes/21.rst
index 30c6cc5eaf0c2..d1935c8af02e0 100644
--- a/libcxx/docs/ReleaseNotes/21.rst
+++ b/libcxx/docs/ReleaseNotes/21.rst
@@ -40,6 +40,7 @@ Implemented Papers
 
 - N4258: Cleaning-up noexcept in the Library (`Github <https://github.com/llvm/llvm-project/issues/99937>`__)
 - P1361R2: Integration of chrono with text formatting (`Github <https://github.com/llvm/llvm-project/issues/100014>`__)
+- P0472R3: Put std::monostate in <utility> (`Github <https://github.com/llvm/llvm-project/issues/127874`__)
 
 Improvements and New Features
 -----------------------------
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 1436db6cf2b45..a47cc193382da 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/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)","","",""
diff --git a/libcxx/include/__cxx03/__utility/monostate.h b/libcxx/include/__cxx03/__utility/monostate.h
new file mode 100644
index 0000000000000..486a18bfa54ca
--- /dev/null
+++ b/libcxx/include/__cxx03/__utility/monostate.h
@@ -0,0 +1,65 @@
+
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___CXX03___UTILITY_MONOSTATE_H
+#define _LIBCPP___CXX03___UTILITY_MONOSTATE_H
+
+#include <__cxx03/__compare/ordering.h>
+#include <__cxx03/__config>
+#include <__cxx03/__functional/hash.h>
+#include <__cxx03/cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 17
+
+struct _LIBCPP_TEMPLATE_VIS monostate {};
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
+
+#  if _LIBCPP_STD_VER >= 20
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
+  return strong_ordering::equal;
+}
+
+#  else // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+
+#  endif // _LIBCPP_STD_VER >= 20
+
+template <>
+struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
+  using argument_type = monostate;
+  using result_type   = size_t;
+
+  inline _LIBCPP_HIDE_FROM_ABI result_type operator()(const argument_type&) const _NOEXCEPT {
+    return 66740831; // return a fundamentally attractive random value.
+  }
+};
+
+#endif // _LIBCPP_STD_VER >= 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CXX03___UTILITY_MONOSTATE_H
diff --git a/libcxx/include/__utility/monostate.h b/libcxx/include/__utility/monostate.h
new file mode 100644
index 0000000000000..3a2a659228e1c
--- /dev/null
+++ b/libcxx/include/__utility/monostate.h
@@ -0,0 +1,66 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___UTILITY_MONOSTATE_H
+#define _LIBCPP___UTILITY_MONOSTATE_H
+
+#include <__compare/ordering.h>
+#include <__configUTILITY>
+#include <__cstddef/size_t.h>
+#include <__functional/hash.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 17
+
+struct _LIBCPP_TEMPLATE_VIS monostate {};
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(monostate, monostate) noexcept { return true; }
+
+#  if _LIBCPP_STD_VER >= 20
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(monostate, monostate) noexcept {
+  return strong_ordering::equal;
+}
+
+#  else // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>(monostate, monostate) noexcept { return false; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+
+#  endif // _LIBCPP_STD_VER >= 20
+
+template <>
+struct _LIBCPP_TEMPLATE_VIS hash<monostate> {
+#  if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_BINDER_TYPEDEFS)
+  using argument_type _LIBCPP_DEPRECATED_IN_CXX17 = monostate;
+  using result_type _LIBCPP_DEPRECATED_IN_CXX17   = size_t;
+#  endif
+
+  inline _LIBCPP_HIDE_FROM_ABI size_t operator()(const monostate&) const noexcept {
+    return 66740831; // return a fundamentally attractive random value.
+  }
+};
+
+#endif // _LIBCPP_STD_VER >= 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___UTILITY_MONOSTATE_H
diff --git a/libcxx/test/std/utilities/utility/utility.monostate.relpos/relops.pass.cpp b/libcxx/test/std/utilities/utility/utility.monostate.relpos/relops.pass.cpp
new file mode 100644
index 0000000000000..f29d89acdc739
--- /dev/null
+++ b/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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <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;
+}
diff --git a/libcxx/test/std/utilities/utility/utility.monostate/monostate.pass.cpp b/libcxx/test/std/utilities/utility/utility.monostate/monostate.pass.cpp
new file mode 100644
index 0000000000000..c78ce5a5c2f18
--- /dev/null
+++ b/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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <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;
+}

libcxx/include/__cxx03/__utility/monostate.h

libcxx/include/__utility/monostate.h

github-actions · 2025-02-23T12:46:36Z

✅ With the latest revision this PR passed the C/C++ code formatter.

AmrDeveloper · 2025-02-23T18:47:23Z

@frederick-vs-ja

4 unreleased tests fail from 77, I will rebase and rerun it after addressing your new code review comments, if there is a new comments :D

frederick-vs-ja

The test failure seem due to unrelated reasons. Let's wait for the fix.

I guess some maintainers want to implement this in C++17 (as a conforming extension).

frederick-vs-ja · 2025-02-24T01:56:27Z

libcxx/include/utility

@@ -279,6 +279,10 @@ template <class T>
 #    include <__utility/unreachable.h>
 #  endif

+#  if _LIBCPP_STD_VER >= 26


I guess some maintainers want this to be in the # if _LIBCPP_STD_VER >= 17 region, although I have no strong preference.

Okey, i will make it from C++17

I don't have a strong preference. Making it available from 17 as an extension seems fine. It's a really small header with few dependencies.

frederick-vs-ja · 2025-02-24T01:57:33Z

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

+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23


Let's use // REQUIRES: std-at-least-c++26 in both test files, but see the comment above.

@frederick-vs-ja Is that something new? When was this introduced?

Yes. This was introduced in #89499.

AmrDeveloper requested a review from a team as a code owner February 22, 2025 22:14

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Feb 22, 2025

AmrDeveloper added c++26 wg21 paper Issues representing the implementation of a WG21 paper labels Feb 22, 2025

frederick-vs-ja requested changes Feb 23, 2025

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frederick-vs-ja reviewed Feb 24, 2025

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AmrDeveloper force-pushed the put_monostate_in_utility branch from b373a40 to b0a68ea Compare February 24, 2025 17:50

AmrDeveloper added 12 commits February 27, 2025 21:07

[libcxx] Put std::monostate in <utility>

c6bd66c

Add utility/monostate.h to cmakelist

a397070

Fix hyperlink in release note

a3555b0

Replace doublicate monostate by include it in utility

09d14e9

Remove building files

27715a5

Format the macro endif

0cb3b70

Format the code

cc95d11

Add utility cstring to cxx26.csv

f13a7d6

Add experimental/utility cstring to cxx26.csv

2d4b99c

Enable moving monostate from C++17

9ff9e69

Add tuple cstring to Cxx 17 and 20

20009c9

typeindex cstring for cxx 17, 20

495c7e7

AmrDeveloper force-pushed the put_monostate_in_utility branch from 282cfac to 495c7e7 Compare February 27, 2025 20:07

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

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

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

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

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

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

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