[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).

libcxx/include/utility

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

AmrDeveloper · 2025-03-04T07:24:34Z

Nice CI is green 😅

philnik777 · 2025-03-04T09:22:08Z

libcxx/test/libcxx/transitive_includes/cxx17.csv

@@ -640,6 +640,7 @@ experimental/utility compare
 experimental/utility cstddef
 experimental/utility cstdint
 experimental/utility cstdlib
+experimental/utility cstring


That this adds new dependencies is a bit unexpected to me. I thought that we already had std::hash in <utility>, but that we didn't makes me think we shouldn't back-port this after all.

Yes i think it's not that problem to support from Cxx17

I thought that we already had std::hash in <utility>, but that we didn't makes me think we shouldn't back-port this after all.

Oh... libc++'s <utility> used to provide std::hash, but such extension (?) was removed by 69d5a66 (https://reviews.llvm.org/D104002).

Should we keep the dependencies or just support from C++26 🤔 @frederick-vs-ja @philnik777

Let's only add it in C++26 for now. It's much easier to add features than to remove them.

philnik777

LGTM assuming the CI is green.

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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AmrDeveloper force-pushed the put_monostate_in_utility branch 3 times, most recently from 495c7e7 to 05d91ee Compare March 1, 2025 11:03

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AmrDeveloper added 12 commits March 3, 2025 19:24

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

56d0405

Add utility/monostate.h to cmakelist

4217fd7

Fix hyperlink in release note

95d12dc

Replace doublicate monostate by include it in utility

1f4fb8c

Remove building files

0255bb1

Format the macro endif

6ffa308

Format the code

381d9a6

Add utility cstring to cxx26.csv

2ec2815

Add experimental/utility cstring to cxx26.csv

7136158

Enable moving monostate from C++17

a526ba6

Add tuple cstring to Cxx 17 and 20

4201028

typeindex cstring for cxx 17, 20

1f90241

AmrDeveloper force-pushed the put_monostate_in_utility branch from 05d91ee to 1f90241 Compare March 3, 2025 18:24

philnik777 reviewed Mar 4, 2025

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AmrDeveloper requested a review from frederick-vs-ja March 6, 2025 19:32

Put std::monostate in C++26 and above

7ee3ea6

AmrDeveloper added 2 commits March 25, 2025 13:03

Merge branch 'main' into put_monostate_in_utility

d6db25a

Update the test requirements to target C++26

9a24af3

philnik777 approved these changes Mar 25, 2025

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frederick-vs-ja approved these changes Mar 25, 2025

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AmrDeveloper merged commit 3573065 into llvm:main Mar 25, 2025
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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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