[libc++][memory] P2868R1: Removing deprecated typedef std::allocator::is_always_equal (#78562)

Implements:
- https://wg21.link/P2868R1
- https://wg21.link/LWG3170

---------

Co-authored-by: Zingam <zingam@outlook.com>

libcxx/.clang-format

@@ -28,6 +28,7 @@ AttributeMacros: [
                   '_LIBCPP_DEPRECATED_IN_CXX14',
                   '_LIBCPP_DEPRECATED_IN_CXX17',
                   '_LIBCPP_DEPRECATED_IN_CXX20',
+                  '_LIBCPP_DEPRECATED_IN_CXX23',
                   '_LIBCPP_DEPRECATED',
                   '_LIBCPP_DISABLE_EXTENTSION_WARNING',
                   '_LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION',

libcxx/docs/ReleaseNotes/18.rst

@@ -59,6 +59,7 @@ Implemented Papers
 - P2821R5 - ``span.at()``
 - P0521R0 - Proposed Resolution for CA 14 (``shared_ptr`` ``use_count/unique``)
 - P1759R6 - Native handles and file streams
+- P2868R3 - Remove Deprecated ``std::allocator`` Typedef From C++26
 - P2517R1 - Add a conditional ``noexcept`` specification to ``std::apply``
 
 
@@ -86,6 +87,9 @@ Improvements and New Features
 - The ``_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE`` macro has been added to make
   the function ``std::basic_string<...>::reserve()`` available.
 
+- The ``_LIBCPP_ENABLE_CXX26_REMOVED_ALLOCATOR_MEMBERS`` macro has been added to make
+  the function ``allocator<T>::is_always_equal`` available.
+
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE`` macro has been added to make
   the function ``std::shared_ptr<...>::unique()`` available.
 

libcxx/docs/Status/Cxx23Issues.csv

@@ -15,7 +15,7 @@
 "`2743 <https://wg21.link/LWG2743>`__","P0083R3 ``node_handle`` private members missing ""exposition only"" comment","November 2020","|Nothing To Do|",""
 "`2820 <https://wg21.link/LWG2820>`__","Clarify ``<cstdint>`` macros","November 2020","|Nothing To Do|",""
 "`3120 <https://wg21.link/LWG3120>`__","Unclear behavior of ``monotonic_buffer_resource::release()``","November 2020","",""
-"`3170 <https://wg21.link/LWG3170>`__","``is_always_equal`` added to ``std::allocator`` makes the standard library treat derived types as always equal","November 2020","",""
+"`3170 <https://wg21.link/LWG3170>`__","``is_always_equal`` added to ``std::allocator`` makes the standard library treat derived types as always equal","November 2020","|Complete|","18.0"
 "`3036 <https://wg21.link/LWG3036>`__","``polymorphic_allocator::destroy`` is extraneous","November 2020","",""
 "`3171 <https://wg21.link/LWG3171>`__","LWG2989 breaks ``directory_entry`` stream insertion","November 2020","|Complete|","14.0"
 "`3306 <https://wg21.link/LWG3306>`__","``ranges::advance`` violates its preconditions","November 2020","|Complete|","14.0","|ranges|"

libcxx/docs/Status/Cxx2cPapers.csv

@@ -36,7 +36,7 @@
 "`P0952R2 <https://wg21.link/P0952R2>`__","LWG","A new specification for ``std::generate_canonical``","Kona November 2023","","",""
 "`P2447R6 <https://wg21.link/P2447R6>`__","LWG","``std::span`` over an initializer list","Kona November 2023","","",""
 "`P2821R5 <https://wg21.link/P2821R5>`__","LWG","``span.at()``","Kona November 2023","|Complete|","18.0",""
-"`P2868R3 <https://wg21.link/P2868R3>`__","LWG","Remove Deprecated ``std::allocator`` Typedef From C++26","Kona November 2023","","",""
+"`P2868R3 <https://wg21.link/P2868R3>`__","LWG","Remove Deprecated ``std::allocator`` Typedef From C++26","Kona November 2023","|Complete|","18.0",""
 "`P2870R3 <https://wg21.link/P2870R3>`__","LWG","Remove ``basic_string::reserve()`` From C++26","Kona November 2023","|Complete|","18.0",""
 "`P2871R3 <https://wg21.link/P2871R3>`__","LWG","Remove Deprecated Unicode Conversion Facets from C++26","Kona November 2023","|Complete|","18.0",""
 "`P2819R2 <https://wg21.link/P2819R2>`__","LWG","Add tuple protocol to complex","Kona November 2023","","",""

libcxx/docs/UsingLibcxx.rst

@@ -283,6 +283,9 @@ C++26 Specific Configuration Macros
   This macro is used to re-enable the function
   ``std::basic_string<...>::reserve()``.
 
+**_LIBCPP_ENABLE_CXX26_REMOVED_ALLOCATOR_MEMBERS**:
+  This macro is used to re-enable redundant member of ``allocator<T>::is_always_equal``
+
 Libc++ Extensions
 =================
 

libcxx/include/__memory/allocator.h

@@ -107,7 +107,9 @@ public:
   typedef ptrdiff_t difference_type;
   typedef _Tp value_type;
   typedef true_type propagate_on_container_move_assignment;
-  typedef true_type is_always_equal;
+#if _LIBCPP_STD_VER <= 23 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_ALLOCATOR_MEMBERS)
+  _LIBCPP_DEPRECATED_IN_CXX23 typedef true_type is_always_equal;
+#endif
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 allocator() _NOEXCEPT = default;
 
@@ -185,7 +187,9 @@ public:
   typedef ptrdiff_t difference_type;
   typedef const _Tp value_type;
   typedef true_type propagate_on_container_move_assignment;
-  typedef true_type is_always_equal;
+#if _LIBCPP_STD_VER <= 23 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_ALLOCATOR_MEMBERS)
+  _LIBCPP_DEPRECATED_IN_CXX23 typedef true_type is_always_equal;
+#endif
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 allocator() _NOEXCEPT = default;
 

libcxx/include/memory

@@ -139,7 +139,7 @@ public:
     template <class U> struct rebind {typedef allocator<U> other;}; // deprecated in C++17, removed in C++20
 
     typedef true_type propagate_on_container_move_assignment;
-    typedef true_type is_always_equal;
+    typedef true_type is_always_equal;                   // Deprecated in C++23, removed in C++26
 
     constexpr allocator() noexcept;                      // constexpr in C++20
     constexpr allocator(const allocator&) noexcept;      // constexpr in C++20

libcxx/test/std/utilities/memory/default.allocator/allocator_types.deprecated_in_cxx23.verify.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: c++23
+
+// <memory>
+
+// template <class T>
+// class allocator
+// {
+// ...
+//     typedef true_type is_always_equal; // Deprecated in C++23, removed in C++26
+// ...
+// };
+
+#include <memory>
+
+void test() {
+  {
+    typedef std::allocator<char>::is_always_equal IAE; // expected-warning {{'is_always_equal' is deprecated}}
+  }
+  {
+    typedef std::allocator<const char>::is_always_equal IAE; // expected-warning {{'is_always_equal' is deprecated}}
+  }
+  {
+    typedef std::allocator<void>::is_always_equal IAE; // expected-warning {{'is_always_equal' is deprecated}}
+  }
+}

libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp

@@ -46,7 +46,9 @@ void test() {
     static_assert((std::is_same<typename Alloc::difference_type, std::ptrdiff_t>::value), "");
     static_assert((std::is_same<typename Alloc::value_type, T>::value), "");
     static_assert((std::is_same<typename Alloc::propagate_on_container_move_assignment, std::true_type>::value), "");
+#if TEST_STD_VER <= 23
     static_assert((std::is_same<typename Alloc::is_always_equal, std::true_type>::value), "");
+#endif
 
 #if TEST_STD_VER <= 17
     static_assert((std::is_same<typename Alloc::pointer, T*>::value), "");

libcxx/test/std/utilities/memory/default.allocator/allocator_types.removed_in_cxx26.verify.cpp

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <memory>
+
+// Check that the following nested types are removed in C++26:
+
+// template <class T>
+// class allocator
+// {
+// ...
+//     typedef true_type is_always_equal; // Deprecated in C++23, removed in C++26
+// ...
+// };
+
+#include <memory>
+
+template <typename T>
+void check() {
+  using IAE = typename std::allocator<T>::is_always_equal; // expected-error 3 {{no type named 'is_always_equal'}}
+}
+
+void test() {
+  check<char>();
+  check<char const>();
+  check<void>();
+}