[libcxx-commits] [libcxx] [libc++][tuple][utility] P2968R2: Make `std::ignore` a first-class object (PR #97401)

Hristo Hristov via libcxx-commits libcxx-commits at lists.llvm.org
Tue Jul 9 11:40:54 PDT 2024 

https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/97401

>From bd82810eb04cbdfaebeed6ad254831de1b687885 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Tue, 2 Jul 2024 10:40:15 +0300
Subject: [PATCH 01/13] [libc++][tuple][utility] P2968R2 Make `std::ignore` a
 first-class object

Implements as DR11:  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
- https://github.com/cplusplus/papers/issues/1640
- https://cplusplus.github.io/LWG/issue2933
- https://cplusplus.github.io/LWG/issue3978
---
 libcxx/docs/ReleaseNotes/19.rst               |  1 +
 libcxx/include/CMakeLists.txt                 |  1 +
 libcxx/include/__tuple/ignore.h               | 32 ++++++++++
 libcxx/include/module.modulemap               |  1 +
 libcxx/include/tuple                          | 24 +++-----
 libcxx/include/utility                        |  4 ++
 .../tuple/tuple.general/ignore.pass.cpp       | 61 +++++++++----------
 7 files changed, 74 insertions(+), 50 deletions(-)
 create mode 100644 libcxx/include/__tuple/ignore.h

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index d30021b7eb234..5c3737d79876e 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -46,6 +46,7 @@ Implemented Papers
 - P2872R3 - Remove ``wstring_convert`` From C++26
 - P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
 - P2944R3 - Comparisons for ``reference_wrapper`` (comparison operators for ``reference_wrapper`` only)
+- P2968R2 - Make ``std::ignore`` a first-class object (as DR against C++11)
 - P2302R4 - ``std::ranges::contains``
 - P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
 - P3029R1 - Better ``mdspan``'s CTAD
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 8d0ffd6ed725b..07dd25604a9c7 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -710,6 +710,7 @@ set(files
   __thread/timed_backoff_policy.h
   __tree
   __tuple/find_index.h
+  __tuple/ignore.h
   __tuple/make_tuple_types.h
   __tuple/sfinae_helpers.h
   __tuple/tuple_element.h
diff --git a/libcxx/include/__tuple/ignore.h b/libcxx/include/__tuple/ignore.h
new file mode 100644
index 0000000000000..2ddc850efccb3
--- /dev/null
+++ b/libcxx/include/__tuple/ignore.h
@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___TUPLE_IGNORE_H
+#define _LIBCPP___TUPLE_IGNORE_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifndef _LIBCPP_CXX03_LANG
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __ignore_type {
+  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const auto&) const noexcept { return *this; }
+};
+
+inline constexpr __ignore_type ignore;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_CXX03_LANG
+
+#endif // _LIBCPP___TUPLE_IGNORE_H
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 9ffccf66ff094..4ad506781c489 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -1840,6 +1840,7 @@ module std_private_thread_thread               [system] {
 module std_private_thread_timed_backoff_policy [system] { header "__thread/timed_backoff_policy.h" }
 
 module std_private_tuple_find_index             [system] { header "__tuple/find_index.h" }
+module std_private_tuple_ignore                 [system] { header "__tuple/ignore.h" }
 module std_private_tuple_make_tuple_types       [system] { header "__tuple/make_tuple_types.h" }
 module std_private_tuple_tuple_like_no_subrange [system] {
   header "__tuple/tuple_like_no_subrange.h"
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index 26652ffe81e9f..d2f6e19ede68d 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -132,7 +132,12 @@ tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;       // since C++
 template <class Alloc, class ...T>
 tuple(allocator_arg_t, Alloc, tuple<T...>) -> tuple<T...>;          // since C++17
 
-inline constexpr unspecified ignore;
++struct ignore-type { // exposition only
++  constexpr const ignore-type&
++    operator=(const auto &) const noexcept
++      { return *this; }
++};
+inline constexpr ignore-type ignore;
 
 template <class... T> tuple<V...>  make_tuple(T&&...); // constexpr in C++14
 template <class... T> tuple<ATypes...> forward_as_tuple(T&&...) noexcept; // constexpr in C++14
@@ -215,6 +220,7 @@ template <class... Types>
 #include <__memory/allocator_arg_t.h>
 #include <__memory/uses_allocator.h>
 #include <__tuple/find_index.h>
+#include <__tuple/ignore.h>
 #include <__tuple/make_tuple_types.h>
 #include <__tuple/sfinae_helpers.h>
 #include <__tuple/tuple_element.h>
@@ -1112,22 +1118,6 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<_Tp&...> tie(_T
   return tuple<_Tp&...>(__t...);
 }
 
-template <class _Up>
-struct __ignore_t {
-  template <class _Tp>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const __ignore_t& operator=(_Tp&&) const {
-    return *this;
-  }
-};
-
-#  if _LIBCPP_STD_VER >= 17
-inline constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
-#  else
-namespace {
-constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
-} // namespace
-#  endif
-
 template <class... _Tp>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 tuple<typename __unwrap_ref_decay<_Tp>::type...>
 make_tuple(_Tp&&... __t) {
diff --git a/libcxx/include/utility b/libcxx/include/utility
index 90713da621c5d..f2f0052df2755 100644
--- a/libcxx/include/utility
+++ b/libcxx/include/utility
@@ -274,6 +274,10 @@ template <class T>
 #include <compare>
 #include <initializer_list>
 
+// [tuple.creation]
+
+#include <__tuple/ignore.h>
+
 // [tuple.helper]
 #include <__tuple/tuple_element.h>
 #include <__tuple/tuple_size.h>
diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index 769c55e10fc43..a3e62e83bcdc9 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -6,11 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-// <tuple>
+// UNSUPPORTED: c++03
 
-// constexpr unspecified ignore;
+// <tuple>
 
-// UNSUPPORTED: c++03
+// inline constexpr ignore-type ignore;
 
 #include <cassert>
 #include <tuple>
@@ -18,39 +18,34 @@
 
 #include "test_macros.h"
 
-constexpr bool test_ignore_constexpr()
-{
-#if TEST_STD_VER > 11
-    { // Test that std::ignore provides constexpr converting assignment.
-        auto& res = (std::ignore = 42);
-        assert(&res == &std::ignore);
-    }
-    { // Test that std::ignore provides constexpr copy/move constructors
-        auto copy = std::ignore;
-        auto moved = std::move(copy);
-        ((void)moved);
-    }
-    { // Test that std::ignore provides constexpr copy/move assignment
-        auto copy = std::ignore;
-        copy = std::ignore;
-        auto moved = std::ignore;
-        moved = std::move(copy);
-    }
-#endif
-    return true;
+constexpr bool test() {
+  {
+    constexpr auto& ignore_v = std::ignore;
+    ((void)ignore_v);
+  }
+  { // Test that std::ignore provides constexpr converting assignment.
+    auto& res = (std::ignore = 42);
+    assert(&res == &std::ignore);
+  }
+  { // Test that std::ignore provides constexpr copy/move constructors
+    auto copy  = (std::ignore = 42);
+    auto moved = std::move(copy);
+    ((void)moved);
+  }
+  { // Test that std::ignore provides constexpr copy/move assignment
+    auto copy  = (std::ignore = 82);
+    copy       = std::ignore;
+    auto moved = (std::ignore = 94);
+    moved      = std::move(copy);
+  }
+
+  return true;
 }
 
 int main(int, char**) {
-    {
-        constexpr auto& ignore_v = std::ignore;
-        ((void)ignore_v);
-    }
-    {
-        static_assert(test_ignore_constexpr(), "");
-    }
-    {
-        LIBCPP_STATIC_ASSERT(std::is_trivial<decltype(std::ignore)>::value, "");
-    }
+  test();
+  static_assert(test(), "");
+  LIBCPP_STATIC_ASSERT(std::is_trivial<decltype(std::ignore)>::value, "");
 
   return 0;
 }

>From 1aa5e01df162a3cae796cceffb45ca5aa178f3d5 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Wed, 3 Jul 2024 07:27:21 +0300
Subject: [PATCH 02/13] Fixed C++11

---
 libcxx/include/__tuple/ignore.h               |  3 +-
 .../tuple/tuple.general/ignore.pass.cpp       | 33 ++++++++++++++++---
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/__tuple/ignore.h b/libcxx/include/__tuple/ignore.h
index 2ddc850efccb3..a8261de7d73b4 100644
--- a/libcxx/include/__tuple/ignore.h
+++ b/libcxx/include/__tuple/ignore.h
@@ -20,7 +20,8 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct __ignore_type {
-  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const auto&) const noexcept { return *this; }
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const _Tp&) const noexcept { return *this; }
 };
 
 inline constexpr __ignore_type ignore;
diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index a3e62e83bcdc9..34a7607b82afc 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -18,13 +18,38 @@
 
 #include "test_macros.h"
 
-constexpr bool test() {
+static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
+
+void test() {
+  {
+    [[maybe_unused]] constexpr auto& ignore_v = std::ignore;
+  }
+  { // Test that std::ignore provides constexpr converting assignment.
+    constexpr auto& res = (std::ignore = 42);
+    static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
+    assert(&res == &std::ignore);
+  }
+  { // Test that std::ignore provides constexpr copy/move constructors
+    constexpr auto copy  = std::ignore;
+    [[maybe_unused]] constexpr auto moved = std::move(copy);
+  }
+  { // Test that std::ignore provides constexpr copy/move assignment
+    constexpr auto copy  = std::ignore;
+    copy                 = std::ignore;
+    constexpr auto moved = std::ignore;
+    moved                = std::move(copy);
+  }
+}
+
+constexpr bool test_constexpr() {
+#if TEST_STD_VER >= 14
   {
-    constexpr auto& ignore_v = std::ignore;
+    auto& ignore_v = std::ignore;
     ((void)ignore_v);
   }
   { // Test that std::ignore provides constexpr converting assignment.
     auto& res = (std::ignore = 42);
+    static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
     assert(&res == &std::ignore);
   }
   { // Test that std::ignore provides constexpr copy/move constructors
@@ -38,14 +63,14 @@ constexpr bool test() {
     auto moved = (std::ignore = 94);
     moved      = std::move(copy);
   }
+#endif
 
   return true;
 }
 
 int main(int, char**) {
   test();
-  static_assert(test(), "");
-  LIBCPP_STATIC_ASSERT(std::is_trivial<decltype(std::ignore)>::value, "");
+  static_assert(test_constexpr(), "");
 
   return 0;
 }

>From cf0e266a02ab7cba91caeb9377eae7d454ca28ce Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Wed, 3 Jul 2024 07:29:38 +0300
Subject: [PATCH 03/13] Updated test

---
 .../tuple/tuple.general/ignore.pass.cpp       | 42 ++-----------------
 1 file changed, 4 insertions(+), 38 deletions(-)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index 34a7607b82afc..22b7b2fdebe41 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -18,19 +18,17 @@
 
 #include "test_macros.h"
 
-static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
+int main(int, char**) {
+  static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
 
-void test() {
-  {
-    [[maybe_unused]] constexpr auto& ignore_v = std::ignore;
-  }
+  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
   { // Test that std::ignore provides constexpr converting assignment.
     constexpr auto& res = (std::ignore = 42);
     static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
     assert(&res == &std::ignore);
   }
   { // Test that std::ignore provides constexpr copy/move constructors
-    constexpr auto copy  = std::ignore;
+    constexpr auto copy                   = std::ignore;
     [[maybe_unused]] constexpr auto moved = std::move(copy);
   }
   { // Test that std::ignore provides constexpr copy/move assignment
@@ -39,38 +37,6 @@ void test() {
     constexpr auto moved = std::ignore;
     moved                = std::move(copy);
   }
-}
-
-constexpr bool test_constexpr() {
-#if TEST_STD_VER >= 14
-  {
-    auto& ignore_v = std::ignore;
-    ((void)ignore_v);
-  }
-  { // Test that std::ignore provides constexpr converting assignment.
-    auto& res = (std::ignore = 42);
-    static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
-    assert(&res == &std::ignore);
-  }
-  { // Test that std::ignore provides constexpr copy/move constructors
-    auto copy  = (std::ignore = 42);
-    auto moved = std::move(copy);
-    ((void)moved);
-  }
-  { // Test that std::ignore provides constexpr copy/move assignment
-    auto copy  = (std::ignore = 82);
-    copy       = std::ignore;
-    auto moved = (std::ignore = 94);
-    moved      = std::move(copy);
-  }
-#endif
-
-  return true;
-}
-
-int main(int, char**) {
-  test();
-  static_assert(test_constexpr(), "");
 
   return 0;
 }

>From dd7e8385dc03bea603a3f8bec41cc76165f04a4e Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Wed, 3 Jul 2024 07:52:08 +0300
Subject: [PATCH 04/13] Formatting

---
 libcxx/include/__tuple/ignore.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/__tuple/ignore.h b/libcxx/include/__tuple/ignore.h
index a8261de7d73b4..a00fc7e7d5b85 100644
--- a/libcxx/include/__tuple/ignore.h
+++ b/libcxx/include/__tuple/ignore.h
@@ -21,7 +21,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct __ignore_type {
   template <class _Tp>
-  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const _Tp&) const noexcept { return *this; }
+  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const _Tp&) const noexcept {
+    return *this;
+  }
 };
 
 inline constexpr __ignore_type ignore;

>From 99900ba54551053177daefb8def4e1b74f321915 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Fri, 5 Jul 2024 14:36:40 +0300
Subject: [PATCH 05/13] Updated tests

---
 .../tuple/tuple.general/ignore.pass.cpp       | 26 ++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index 22b7b2fdebe41..49f3249a4e413 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -13,13 +13,29 @@
 // inline constexpr ignore-type ignore;
 
 #include <cassert>
+#include <cstdint>
 #include <tuple>
 #include <type_traits>
 
 #include "test_macros.h"
 
+static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
+
+#if TEST_STD_VER >= 17
+[[nodiscard]] constexpr int test_nodiscard() { return 8294; }
+#endif
+
+constexpr bool test() {
+#if TEST_STD_VER >= 17
+  { std::ignore = test_nodiscard(); }
+#endif
+
+  return true;
+}
+
 int main(int, char**) {
-  static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
+  test();
+  static_assert(test(), "");
 
   { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
   { // Test that std::ignore provides constexpr converting assignment.
@@ -27,6 +43,14 @@ int main(int, char**) {
     static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
     assert(&res == &std::ignore);
   }
+  { // Test bit-field binding.
+    struct S {
+      unsigned int bf : 3;
+    };
+    constexpr S s{0b010};
+    constexpr auto& res = (std::ignore = s.bf);
+    assert(&res == &std::ignore);
+  }
   { // Test that std::ignore provides constexpr copy/move constructors
     constexpr auto copy                   = std::ignore;
     [[maybe_unused]] constexpr auto moved = std::move(copy);

>From be73bf4c9644f786fd644f5ee62871029450e7bf Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sat, 6 Jul 2024 17:08:09 +0300
Subject: [PATCH 06/13] Added `include<>` tests

---
 .../ignore.include.compile.pass.cpp           | 19 +++++++++++++++++++
 .../ignore/ignore.include.compile.pass.cpp    | 19 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
 create mode 100644 libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
new file mode 100644
index 0000000000000..049d484408b11
--- /dev/null
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <tuple>
+
+// inline constexpr ignore-type ignore;
+
+#include <tuple>
+
+int main(int, char**) {
+  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
+}
diff --git a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
new file mode 100644
index 0000000000000..be6d61506af01
--- /dev/null
+++ b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <utility>
+
+// inline constexpr ignore-type ignore;
+
+#include <utility>
+
+int main(int, char**) {
+  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
+}

>From 4a124c9517828e703693fd470735c9b4482256a4 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 14:13:08 +0300
Subject: [PATCH 07/13] Addressed comments

---
 libcxx/docs/ReleaseNotes/19.rst               |  5 +-
 libcxx/include/__tuple/ignore.h               |  5 +-
 .../tuple/tuple.general/ignore.pass.cpp       | 50 ++++++++++---------
 3 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index 5c3737d79876e..f72e13cf6d9af 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -46,7 +46,7 @@ Implemented Papers
 - P2872R3 - Remove ``wstring_convert`` From C++26
 - P3142R0 - Printing Blank Lines with ``println`` (as DR against C++23)
 - P2944R3 - Comparisons for ``reference_wrapper`` (comparison operators for ``reference_wrapper`` only)
-- P2968R2 - Make ``std::ignore`` a first-class object (as DR against C++11)
+- P2968R2 - Make ``std::ignore`` a first-class object
 - P2302R4 - ``std::ranges::contains``
 - P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
 - P3029R1 - Better ``mdspan``'s CTAD
@@ -75,6 +75,9 @@ Improvements and New Features
 
 - The formatting library is updated to Unicode 15.1.0.
 
+- ``std::ignore``\s ``const __ignore_t& operator=(_Tp&&) const`` was changed to
+  ``const __ignore_type& operator=(const _Tp&) const noexcept`` for all language versions.
+
 Deprecations and Removals
 -------------------------
 
diff --git a/libcxx/include/__tuple/ignore.h b/libcxx/include/__tuple/ignore.h
index a00fc7e7d5b85..d7c3a18c14775 100644
--- a/libcxx/include/__tuple/ignore.h
+++ b/libcxx/include/__tuple/ignore.h
@@ -26,7 +26,10 @@ struct __ignore_type {
   }
 };
 
-inline constexpr __ignore_type ignore;
+#  if _LIBCPP_STD_VER >= 17
+inline
+#  endif
+    _LIBCPP_CONSTEXPR_SINCE_CXX14 __ignore_type ignore;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index 49f3249a4e413..120a41423eceb 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -25,21 +25,12 @@ static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
 [[nodiscard]] constexpr int test_nodiscard() { return 8294; }
 #endif
 
-constexpr bool test() {
-#if TEST_STD_VER >= 17
-  { std::ignore = test_nodiscard(); }
-#endif
 
-  return true;
-}
+TEST_CONSTEXPR_CXX14 bool test() {
+  { [[maybe_unused]] auto& ignore_v = std::ignore; }
 
-int main(int, char**) {
-  test();
-  static_assert(test(), "");
-
-  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
-  { // Test that std::ignore provides constexpr converting assignment.
-    constexpr auto& res = (std::ignore = 42);
+  { // Test that std::ignore provides converting assignment.
+    auto& res = (std::ignore = 42);
     static_assert(noexcept(res = (std::ignore = 42)), "Must be noexcept");
     assert(&res == &std::ignore);
   }
@@ -47,20 +38,33 @@ int main(int, char**) {
     struct S {
       unsigned int bf : 3;
     };
-    constexpr S s{0b010};
-    constexpr auto& res = (std::ignore = s.bf);
+    S s{0b010};
+    auto& res = (std::ignore = s.bf);
     assert(&res == &std::ignore);
   }
-  { // Test that std::ignore provides constexpr copy/move constructors
-    constexpr auto copy                   = std::ignore;
-    [[maybe_unused]] constexpr auto moved = std::move(copy);
+  { // Test that std::ignore provides copy/move constructors
+    auto copy                   = std::ignore;
+    [[maybe_unused]] auto moved = std::move(copy);
   }
-  { // Test that std::ignore provides constexpr copy/move assignment
-    constexpr auto copy  = std::ignore;
-    copy                 = std::ignore;
-    constexpr auto moved = std::ignore;
-    moved                = std::move(copy);
+  { // Test that std::ignore provides copy/move assignment
+    auto copy  = std::ignore;
+    copy       = std::ignore;
+    auto moved = std::ignore;
+    moved      = std::move(copy);
   }
 
+#if TEST_STD_VER >= 17
+  { std::ignore = test_nodiscard(); }
+#endif
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 14
+  static_assert(test(), "");
+#endif
+
   return 0;
 }

>From f79cd96f4da863d8c90ec3c4457b1955a40c314e Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 14:19:44 +0300
Subject: [PATCH 08/13] Fixes

---
 .../tuple/tuple.general/ignore.include.compile.pass.cpp         | 2 +-
 libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp   | 1 -
 .../utilities/utility/ignore/ignore.include.compile.pass.cpp    | 2 +-
 3 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
index 049d484408b11..dfa0272ed1248 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
@@ -15,5 +15,5 @@
 #include <tuple>
 
 int main(int, char**) {
-  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
+  { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
 }
diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
index 120a41423eceb..525ffde5cf177 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.pass.cpp
@@ -25,7 +25,6 @@ static_assert(std::is_trivial<decltype(std::ignore)>::value, "");
 [[nodiscard]] constexpr int test_nodiscard() { return 8294; }
 #endif
 
-
 TEST_CONSTEXPR_CXX14 bool test() {
   { [[maybe_unused]] auto& ignore_v = std::ignore; }
 
diff --git a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
index be6d61506af01..1e925adf9cccd 100644
--- a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
@@ -15,5 +15,5 @@
 #include <utility>
 
 int main(int, char**) {
-  { [[maybe_unused]] constexpr auto& ignore_v = std::ignore; }
+  { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
 }

>From 6ceb5c7ae6378c4937485641db85f2c0923d84ed Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 14:20:20 +0300
Subject: [PATCH 09/13] More fixes

---
 .../tuple/tuple.general/ignore.include.compile.pass.cpp         | 2 ++
 .../utilities/utility/ignore/ignore.include.compile.pass.cpp    | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
index dfa0272ed1248..ae550ee1cd578 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
@@ -14,6 +14,8 @@
 
 #include <tuple>
 
+#include "test_macros.h"
+
 int main(int, char**) {
   { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
 }
diff --git a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
index 1e925adf9cccd..64ebb051ea05d 100644
--- a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
@@ -14,6 +14,8 @@
 
 #include <utility>
 
+#include "test_macros.h"
+
 int main(int, char**) {
   { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
 }

>From 48132c91cd5387998ab5001cf59cd89b299a236f Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 14:55:41 +0300
Subject: [PATCH 10/13] Fixes

---
 libcxx/include/__tuple/ignore.h |  7 ++++---
 libcxx/include/tuple            | 10 +++++-----
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/libcxx/include/__tuple/ignore.h b/libcxx/include/__tuple/ignore.h
index d7c3a18c14775..43cce5387411b 100644
--- a/libcxx/include/__tuple/ignore.h
+++ b/libcxx/include/__tuple/ignore.h
@@ -21,15 +21,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct __ignore_type {
   template <class _Tp>
-  _LIBCPP_HIDE_FROM_ABI constexpr const __ignore_type& operator=(const _Tp&) const noexcept {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const __ignore_type& operator=(const _Tp&) const noexcept {
     return *this;
   }
 };
 
 #  if _LIBCPP_STD_VER >= 17
-inline
+inline constexpr __ignore_type ignore;
+#  else
+constexpr __ignore_type ignore;
 #  endif
-    _LIBCPP_CONSTEXPR_SINCE_CXX14 __ignore_type ignore;
 
 _LIBCPP_END_NAMESPACE_STD
 
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index d2f6e19ede68d..081b90c7bbec5 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -132,11 +132,11 @@ tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;       // since C++
 template <class Alloc, class ...T>
 tuple(allocator_arg_t, Alloc, tuple<T...>) -> tuple<T...>;          // since C++17
 
-+struct ignore-type { // exposition only
-+  constexpr const ignore-type&
-+    operator=(const auto &) const noexcept
-+      { return *this; }
-+};
+struct ignore-type { // exposition only                             // Since C++26
+  constexpr const ignore-type&
+    operator=(const auto &) const noexcept
+      { return *this; }
+};
 inline constexpr ignore-type ignore;
 
 template <class... T> tuple<V...>  make_tuple(T&&...); // constexpr in C++14

>From 1cacd7119e5298f08249749af49e09a79370bcf8 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 15:03:02 +0300
Subject: [PATCH 11/13] Addressed review comments

---
 .../tuple/tuple.general/ignore.include.compile.pass.cpp  | 9 ++++-----
 .../utility/ignore/ignore.include.compile.pass.cpp       | 9 ++++-----
 2 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
index ae550ee1cd578..8960066058a1f 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
@@ -12,10 +12,9 @@
 
 // inline constexpr ignore-type ignore;
 
-#include <tuple>
+// std::ignore should be provided by the headers <tuple> and <utility>.
+// This tests validates its presence in <tuple>.
 
-#include "test_macros.h"
+#include <tuple>
 
-int main(int, char**) {
-  { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
-}
+[[maybe_unused]] auto& ignore_v = std::ignore;
diff --git a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
index 64ebb051ea05d..5d180141f2a20 100644
--- a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
@@ -12,10 +12,9 @@
 
 // inline constexpr ignore-type ignore;
 
-#include <utility>
+// std::ignore should be provided by the headers <tuple> and <utility>.
+// This tests validates its presence in <utility>.
 
-#include "test_macros.h"
+#include <utility>
 
-int main(int, char**) {
-  { [[maybe_unused]] TEST_CONSTEXPR_CXX14 auto& ignore_v = std::ignore; }
-}
+[[maybe_unused]] auto& ignore_v = std::ignore;

>From 1689a305afbc22fd543bdcb574490630cfa64cdc Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 7 Jul 2024 15:04:21 +0300
Subject: [PATCH 12/13] Fixed a typo

---
 .../tuple/tuple.general/ignore.include.compile.pass.cpp         | 2 +-
 .../utilities/utility/ignore/ignore.include.compile.pass.cpp    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
index 8960066058a1f..0fdf0d0af52b9 100644
--- a/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.general/ignore.include.compile.pass.cpp
@@ -13,7 +13,7 @@
 // inline constexpr ignore-type ignore;
 
 // std::ignore should be provided by the headers <tuple> and <utility>.
-// This tests validates its presence in <tuple>.
+// This test validates its presence in <tuple>.
 
 #include <tuple>
 
diff --git a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
index 5d180141f2a20..3aa74397acf20 100644
--- a/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
+++ b/libcxx/test/std/utilities/utility/ignore/ignore.include.compile.pass.cpp
@@ -13,7 +13,7 @@
 // inline constexpr ignore-type ignore;
 
 // std::ignore should be provided by the headers <tuple> and <utility>.
-// This tests validates its presence in <utility>.
+// This test validates its presence in <utility>.
 
 #include <utility>
 

>From 18889658b541f9e5d8478eb62c28cc69dadb536e Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Tue, 9 Jul 2024 21:40:33 +0300
Subject: [PATCH 13/13] Updated status pages

---
 libcxx/docs/Status/Cxx2cPapers.csv | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 48d45e24a05b2..aea413dd33588 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -71,7 +71,7 @@
 "`P2985R0 <https://wg21.link/P2985R0>`__","LWG","A type trait for detecting virtual base classes","St. Louis June 2024","","",""
 "`P0843R14 <https://wg21.link/P0843R14>`__","LWG","``inplace_vector``","St. Louis June 2024","","",""
 "`P3235R3 <https://wg21.link/P3235R3>`__","LWG","``std::print`` more types faster with less memory","St. Louis June 2024","","","|format| |DR|"
-"`P2968R2 <https://wg21.link/P2968R2>`__","LWG","Make ``std::ignore`` a first-class object","St. Louis June 2024","","",""
+"`P2968R2 <https://wg21.link/P2968R2>`__","LWG","Make ``std::ignore`` a first-class object","St. Louis June 2024","|Complete|","19.0",""
 "`P2075R6 <https://wg21.link/P2075R6>`__","LWG","Philox as an extension of the C++ RNG engines","St. Louis June 2024","","",""
 "`P2422R1 <https://wg21.link/P2422R1>`__","LWG","Remove ``nodiscard`` annotations from the standard library specification","St. Louis June 2024","|Complete| [#note-P2422R1]_","19.0",""
 "`P2300R10 <https://wg21.link/P2300R10>`__","LWG","``std::execution``","St. Louis June 2024","","",""

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