Merge 2023-02 LWG Motion 16

P2652R2 Disallow User Specialization of allocator_traits

Author: tkoeppe

source/compatibility.tex

@@ -189,6 +189,20 @@
 }
 \end{codeblock}
 
+\rSec2[diff.cpp20.memory]{\ref{mem}: memory management library}
+
+\diffref{allocator.traits.general}
+\change
+Forbid partial and explicit program-defined specializations
+of \tcode{allocator_traits}.
+\rationale
+Allow addition of \tcode{allocate_at_least} to \tcode{allocator_traits},
+and potentially other members in the future.
+\effect
+Valid \CppXX{} code
+that partially or explicitly specializes \tcode{allocator_traits}
+is ill-formed with no diagnostic required in this revision of \Cpp{}.
+
 \rSec2[diff.cpp20.utilities]{\ref{utilities}: general utilities library}
 
 \diffref{format}

source/lib-intro.tex

@@ -2009,9 +2009,10 @@
 given type or expression is specified.
 Within the standard library \tcode{allocator_traits}
 template, an optional requirement that is not supplied by an allocator is
-replaced by the specified default type or expression. A user specialization of
-\tcode{allocator_traits} may provide different defaults and may provide
-defaults for different requirements than the primary template.
+replaced by the specified default type or expression.
+\begin{note}
+There are no program-defined specializations of \tcode{allocator_traits}.
+\end{note}
 
 \begin{itemdecl}
 typename X::pointer
@@ -2318,11 +2319,11 @@
 \begin{itemdescr}
 \pnum
 \result
-\tcode{allocation_result<XX::pointer>}
+\tcode{allocation_result<XX::pointer, XX::size_type>}
 
 \pnum
 \returns
-\tcode{allocation_result<XX::pointer>\{ptr, count\}}
+\tcode{allocation_result<XX::pointer, XX::size_type>\{ptr, count\}}
 where \tcode{ptr} is memory allocated for an array of \tcode{count} \tcode{T}
 and such an object is created but array elements are not constructed,
 such that $\tcode{count} \geq \tcode{n}$.
@@ -2334,10 +2335,7 @@
 
 \pnum
 \remarks
-An allocator need not support \tcode{allocate_at_least},
-but no default is provided in \tcode{allocator_traits}.
-If an allocator has an \tcode{allocate_at_least} member,
-it shall satisfy the requirements.
+Default: \tcode{\{a.allocate(n), n\}}.
 \end{itemdescr}
 
 \begin{itemdecl}

source/memory.tex

@@ -156,16 +156,12 @@
   // \ref{allocator.traits}, allocator traits
   template<class Alloc> struct allocator_traits;                                    // freestanding
 
-  template<class Pointer>
+  template<class Pointer, class SizeType = size_t>
   struct allocation_result {                                                        // freestanding
     Pointer ptr;
-    size_t count;
+    SizeType count;
   };
 
-  template<class Allocator>
-    [[nodiscard]] constexpr allocation_result<typename allocator_traits<Allocator>::pointer>
-      allocate_at_least(Allocator& a, size_t n);                                    // freestanding
-
   // \ref{default.allocator}, the default allocator
   template<class T> class allocator;
   template<class T, class U>
@@ -1351,6 +1347,9 @@
 Thus, it is always possible to create
 a derived class from an allocator.
 \end{note}
+If a program declares
+an explicit or partial specialization of \tcode{allocator_traits},
+the program is ill-formed, no diagnostic required.
 
 \indexlibraryglobal{allocator_traits}%
 \begin{codeblock}
@@ -1379,6 +1378,8 @@
     [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n);
     [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n,
                                                     const_void_pointer hint);
+    [[nodiscard]] static constexpr allocation_result<pointer, size_type>
+      allocate_at_least(Alloc& a, size_type n);
 
     static constexpr void deallocate(Alloc& a, pointer p, size_type n);
 
@@ -1565,6 +1566,19 @@
 \tcode{a.allocate(n, hint)} if that expression is well-formed; otherwise, \tcode{a.allocate(n)}.
 \end{itemdescr}
 
+\indexlibrarymember{allocate_at_least}{allocator_traits}%
+\begin{itemdecl}
+[[nodiscard]] static constexpr allocation_result<pointer, size_type>
+  allocate_at_least(Alloc& a, size_type n);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{a.allocate_at_least(n)} if that expression is well-formed;
+otherwise, \tcode{\{a.allocate(n), n\}}.
+\end{itemdescr}
+
 \indexlibrarymember{deallocate}{allocator_traits}%
 \begin{itemdecl}
 static constexpr void deallocate(Alloc& a, pointer p, size_type n);
@@ -1638,19 +1652,6 @@
 the template parameters, data members, and special members specified above.
 It has no base classes or members other than those specified.
 
-\begin{itemdecl}
-template<class Allocator>
-[[nodiscard]] constexpr allocation_result<typename allocator_traits<Allocator>::pointer>
-  allocate_at_least(Allocator& a, size_t n);
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\returns
-\tcode{a.allocate_at_least(n)} if that expression is well-formed;
-otherwise, \tcode{\{a.allocate(n), n\}}.
-\end{itemdescr}
-
 \rSec2[default.allocator]{The default allocator}
 
 \rSec3[default.allocator.general]{General}