[libc++][memory] P2652R2: Disallow Specialization of allocator_traits (#79978)

Implements P2652R2 <https://wg21.link/P2652R2>:
- https://eel.is/c++draft/allocator.requirements.general
- https://eel.is/c++draft/memory.syn
- https://eel.is/c++draft/allocator.traits.general
- https://eel.is/c++draft/allocator.traits.members
- https://eel.is/c++draft/diff.cpp20.concepts
- https://eel.is/c++draft/diff.cpp20.utilities

---------

Co-authored-by: Zingam <[email protected]>

Author: H-G-Hristov

libcxx/docs/FeatureTestMacroTable.rst

@@ -304,7 +304,7 @@ Status
     ---------------------------------------------------------------------
     ``__cpp_lib_adaptor_iterator_pair_constructor``     ``202106L``
     --------------------------------------------------- -----------------
-    ``__cpp_lib_allocate_at_least``                     ``202106L``
+    ``__cpp_lib_allocate_at_least``                     ``202302L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_associative_heterogeneous_erasure``     *unimplemented*
     --------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes/19.rst

@@ -37,7 +37,9 @@ What's New in Libc++ 19.0.0?
 
 Implemented Papers
 ------------------
+
 - P2637R3 - Member ``visit``
+- P2652R2 - Disallow User Specialization of ``allocator_traits``
 
 
 Improvements and New Features

libcxx/docs/Status/Cxx23Papers.csv

@@ -115,7 +115,7 @@
 "`P2679R2 <https://wg21.link/P2679R2>`__","LWG", "Fixing ``std::start_lifetime_as`` for arrays","February 2023","","",""
 "`P2674R1 <https://wg21.link/P2674R1>`__","LWG", "A trait for implicit lifetime types","February 2023","","",""
 "`P2655R3 <https://wg21.link/P2655R3>`__","LWG", "``common_reference_t`` of ``reference_wrapper`` Should Be a Reference Type","February 2023","","",""
-"`P2652R2 <https://wg21.link/P2652R2>`__","LWG", "Disallow User Specialization of ``allocator_traits``","February 2023","","",""
+"`P2652R2 <https://wg21.link/P2652R2>`__","LWG", "Disallow User Specialization of ``allocator_traits``","February 2023","|Complete|","19.0",""
 "`P2787R1 <https://wg21.link/P2787R1>`__","LWG", "``pmr::generator`` - Promise Types are not Values","February 2023","","",""
 "`P2614R2 <https://wg21.link/P2614R2>`__","LWG", "Deprecate ``numeric_limits::has_denorm``","February 2023","|Complete|","18.0",""
 "`P2588R3 <https://wg21.link/P2588R3>`__","LWG", "``barrier``’s phase completion guarantees","February 2023","","",""

libcxx/docs/Status/Cxx2cIssues.csv

@@ -2,7 +2,7 @@
 "`2994 <https://wg21.link/LWG2994>`__","Needless UB for ``basic_string`` and ``basic_string_view``","Varna June 2023","|Complete|","5.0",""
 "`3884 <https://wg21.link/LWG3884>`__","``flat_foo`` is missing allocator-extended copy/move constructors","Varna June 2023","","","|flat_containers|"
 "`3885 <https://wg21.link/LWG3885>`__","``op`` should be in [zombie.names]","Varna June 2023","|Nothing To Do|","",""
-"`3887 <https://wg21.link/LWG3887>`__","Version macro for ``allocate_at_least``","Varna June 2023","","",""
+"`3887 <https://wg21.link/LWG3887>`__","Version macro for ``allocate_at_least``","Varna June 2023","|Complete|","19.0",""
 "`3893 <https://wg21.link/LWG3893>`__","LWG 3661 broke ``atomic<shared_ptr<T>> a; a = nullptr;``","Varna June 2023","","",""
 "`3894 <https://wg21.link/LWG3894>`__","``generator::promise_type::yield_value(ranges::elements_of<Rng, Alloc>)`` should not be ``noexcept``","Varna June 2023","","",""
 "`3903 <https://wg21.link/LWG3903>`__","span destructor is redundantly noexcept","Varna June 2023","|Complete|","7.0",""

libcxx/include/__memory/allocate_at_least.h

@@ -20,28 +20,14 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 23
-template <class _Pointer>
-struct allocation_result {
-  _Pointer ptr;
-  size_t count;
-};
-_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(allocation_result);
-
-template <class _Alloc>
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr allocation_result<typename allocator_traits<_Alloc>::pointer>
-allocate_at_least(_Alloc& __alloc, size_t __n) {
-  if constexpr (requires { __alloc.allocate_at_least(__n); }) {
-    return __alloc.allocate_at_least(__n);
-  } else {
-    return {__alloc.allocate(__n), __n};
-  }
-}
 
 template <class _Alloc>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto __allocate_at_least(_Alloc& __alloc, size_t __n) {
-  return std::allocate_at_least(__alloc, __n);
+  return std::allocator_traits<_Alloc>::allocate_at_least(__alloc, __n);
 }
+
 #else
+
 template <class _Pointer>
 struct __allocation_result {
   _Pointer ptr;

libcxx/include/__memory/allocator_traits.h

@@ -22,6 +22,7 @@
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
+#include <cstddef>
 #include <limits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -231,6 +232,17 @@ struct __has_select_on_container_copy_construction<
 
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
+#if _LIBCPP_STD_VER >= 23
+
+template <class _Pointer, class _SizeType = size_t>
+struct allocation_result {
+  _Pointer ptr;
+  _SizeType count;
+};
+_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(allocation_result);
+
+#endif // _LIBCPP_STD_VER
+
 template <class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS allocator_traits {
   using allocator_type     = _Alloc;
@@ -284,6 +296,18 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits {
     return __a.allocate(__n);
   }
 
+#if _LIBCPP_STD_VER >= 23
+  template <class _Ap = _Alloc>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr allocation_result<pointer, size_type>
+  allocate_at_least(_Ap& __alloc, size_type __n) {
+    if constexpr (requires { __alloc.allocate_at_least(__n); }) {
+      return __alloc.allocate_at_least(__n);
+    } else {
+      return {__alloc.allocate(__n), __n};
+    }
+  }
+#endif
+
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static void
   deallocate(allocator_type& __a, pointer __p, size_type __n) _NOEXCEPT {
     __a.deallocate(__p, __n);

libcxx/include/memory

@@ -88,6 +88,9 @@ struct allocator_traits
     static pointer allocate(allocator_type& a, size_type n);                          // constexpr and [[nodiscard]] in C++20
     static pointer allocate(allocator_type& a, size_type n, const_void_pointer hint); // constexpr and [[nodiscard]] in C++20
 
+    [[nodiscard]] static constexpr allocation_result<pointer, size_type>
+      allocate_at_least(Alloc& a, size_type n);                                 // Since C++23
+
     static void deallocate(allocator_type& a, pointer p, size_type n) noexcept; // constexpr in C++20
 
     template <class T, class... Args>
@@ -100,15 +103,11 @@ struct allocator_traits
     static allocator_type select_on_container_copy_construction(const allocator_type& a); // constexpr in C++20
 };
 
-template<class Pointer>
+template<class Pointer, class SizeType = size_t>
 struct allocation_result {
     Pointer ptr;
-    size_t count;
-}; // since C++23
-
-template<class Allocator>
-[[nodiscard]] constexpr allocation_result<typename allocator_traits<Allocator>::pointer>
-    allocate_at_least(Allocator& a, size_t n); // since C++23
+    SizeType count;
+}; // Since C++23
 
 template <>
 class allocator<void> // removed in C++20

libcxx/include/version

@@ -16,7 +16,7 @@
 Macro name                                              Value   Headers
 __cpp_lib_adaptor_iterator_pair_constructor             202106L <queue> <stack>
 __cpp_lib_addressof_constexpr                           201603L <memory>
-__cpp_lib_allocate_at_least                             202106L <memory>
+__cpp_lib_allocate_at_least                             202302L <memory>
 __cpp_lib_allocator_traits_is_always_equal              201411L <deque> <forward_list> <list>
                                                                 <map> <memory> <scoped_allocator>
                                                                 <set> <string> <unordered_map>
@@ -433,7 +433,7 @@ __cpp_lib_within_lifetime                               202306L <type_traits>
 
 #if _LIBCPP_STD_VER >= 23
 # define __cpp_lib_adaptor_iterator_pair_constructor    202106L
-# define __cpp_lib_allocate_at_least                    202106L
+# define __cpp_lib_allocate_at_least                    202302L
 // # define __cpp_lib_associative_heterogeneous_erasure    202110L
 // # define __cpp_lib_bind_back                            202202L
 # define __cpp_lib_byteswap                             202110L

libcxx/modules/std/memory.inc

@@ -43,8 +43,6 @@ export namespace std {
 
 #if _LIBCPP_STD_VER >= 23
   using std::allocation_result;
-
-  using std::allocate_at_least;
 #endif
 
   // [default.allocator], the default allocator

libcxx/test/std/language.support/support.limits/support.limits.general/memory.version.compile.pass.cpp

@@ -17,7 +17,7 @@
 
 /*  Constant                                      Value
     __cpp_lib_addressof_constexpr                 201603L [C++17]
-    __cpp_lib_allocate_at_least                   202106L [C++23]
+    __cpp_lib_allocate_at_least                   202302L [C++23]
     __cpp_lib_allocator_traits_is_always_equal    201411L [C++17]
     __cpp_lib_assume_aligned                      201811L [C++20]
     __cpp_lib_atomic_value_initialization         201911L [C++20]
@@ -432,8 +432,8 @@
 # ifndef __cpp_lib_allocate_at_least
 #   error "__cpp_lib_allocate_at_least should be defined in c++23"
 # endif
-# if __cpp_lib_allocate_at_least != 202106L
-#   error "__cpp_lib_allocate_at_least should have the value 202106L in c++23"
+# if __cpp_lib_allocate_at_least != 202302L
+#   error "__cpp_lib_allocate_at_least should have the value 202302L in c++23"
 # endif
 
 # ifndef __cpp_lib_allocator_traits_is_always_equal
@@ -569,8 +569,8 @@
 # ifndef __cpp_lib_allocate_at_least
 #   error "__cpp_lib_allocate_at_least should be defined in c++26"
 # endif
-# if __cpp_lib_allocate_at_least != 202106L
-#   error "__cpp_lib_allocate_at_least should have the value 202106L in c++26"
+# if __cpp_lib_allocate_at_least != 202302L
+#   error "__cpp_lib_allocate_at_least should have the value 202302L in c++26"
 # endif
 
 # ifndef __cpp_lib_allocator_traits_is_always_equal