Proposed resolution:

This wording is relative to N4917.

1. Modify [pointer.traits.general] as indicated:

   -1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.

   namespace std {
     template<class Ptr> struct pointer_traits {
       using pointer         = Ptr;
       using element_type    = see below;
       using difference_type = see below;
   
       template<class U> using rebind = see below;
       static pointer pointer_to(see below r);
   
       see below;
     };
   
     template<class T> struct pointer_traits<T*> {
       using pointer         = T*;
       using element_type    = T;
       using difference_type = ptrdiff_t;
   
       template<class U> using rebind = U*;
       static constexpr pointer pointer_to(see below r) noexcept;
     };
   }
   

2. Modify [pointer.traits.types] as indicated:

   -?- The definitions in this subclause make use of the following exposition-only class template and concept:

   
   template<class T>
   struct ptr-traits-elem // exposition only
   { };
   
   template<class T> requires requires { typename T::element_type; }
   struct ptr-traits-elem<T>
   { using type = typename T::element_type; };
   
   template<template<class...> class SomePointer, class T, class... Args>
   requires (!requires { typename SomePointer<T, Args...>::element_type; })
   struct ptr-traits-elem<SomePointer<T, Args...>>
   { using type = T; };
   
   template<class Ptr>
     concept has-elem-type = // exposition only
       requires { typename ptr-traits-elem<Ptr>::type; }
   

   -?- If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the following members as well as those described in [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.

   using pointer = see below;

   -?- Type: Ptr.

   using element_type = see below;

   -1- Type: typename ptr-traits-elem<Ptr>::type. Ptr::element_type if the qualified-id Ptr::element_type is valid and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.

   using difference_type = see below;

   -2- Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_t.

   template<class U> using rebind = see below;

   -3- Alias template: Ptr::rebind<U> if the qualified-id Ptr::rebind<U> is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

3. Modify [pointer.traits.optmem] as indicated:

   -1- Specializations of pointer_traits may define the member declared in this subclause to customize the behavior of the standard library. A specialization generated from the pointer_traits primary template has no member by this name.

   static element_type* to_address(pointer p) noexcept;

   -1- Returns: A pointer of type element_type* that references the same location as the argument p.

[ 2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP. ]

[ 2022-10-19; Reflector poll ]

Set status to "Tentatively Ready" after six votes in favour in reflector poll.

[ 2022-10-11; Jonathan provides improved wording ]

[ 2022-09-27; Jonathan provides new wording ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

1. Modify [pointer.traits.general] as indicated:

   -1- The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.

   namespace std {
     template<class Ptr> struct pointer_traits {
       using pointer         = Ptr;
       using element_type    = see below;
       using difference_type = see below;
   
       template<class U> using rebind = see below;
       static pointer pointer_to(see below r);
   
       see below;
     };
   
     template<class T> struct pointer_traits<T*> {
       using pointer         = T*;
       using element_type    = T;
       using difference_type = ptrdiff_t;
   
       template<class U> using rebind = U*;
       static constexpr pointer pointer_to(see below r) noexcept;
     };
   }
   

2. Modify [pointer.traits.types] as indicated:

   -?- The definitions in this subclause make use of the following exposition-only class template and concept:

   
   template<class T>
   struct ptr-traits-elem // exposition only
   { };
   
   template<class T> requires requires { typename T::element_type; }
   struct ptr-traits-elem<T>
   { using type = typename T::element_type; };
   
   template<template<class...> class SomePointer, class T, class... Args>
   requires (!requires { typename SomePointer<T, Args...>::element_type; })
   struct ptr-traits-elem<SomePointer<T, Args...>>
   { using type = T; };
   
   template<class Ptr>
     concept has-elem-type = // exposition only
       requires { typename ptr-traits-elem<Ptr>::type; }
   

   -?- If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the members described in [pointer.traits.types] and [pointer.traits.functions]; otherwise, such a specialization has no members by any of the names described in those subclauses or in [pointer.traits.optmem].

   using pointer = Ptr;
   

   using element_type = see below typename ptr-traits-elem<Ptr>::type;

   -1- Type: Ptr::element_type if the qualified-id Ptr::element_type is valid and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.

   using difference_type = see below;

   -2- Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_t.

   template<class U> using rebind = see below;

   -3- Alias template: Ptr::rebind<U> if the qualified-id Ptr::rebind<U> is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

[ 2022-02-22 LEWG telecon; Status changed: LEWG → Open ]

No objection to unanimous consent for Jonathan's suggestion to make pointer_traits an empty class when there is no element_type. Jonathan to provide a paper.

Previous resolution [SUPERSEDED]:

This wording is relative to N4885.

1. Modify [pointer.traits.types] as indicated:

   As additional drive-by fix the improper usage of the term "instantiation" has been corrected.

   using element_type = see below;
   

   -1- Type: Ptr::element_type if the qualified-id Ptr::element_type is valid and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template instantiationspecialization of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formedpointer_traits has no member element_type.

[ 2022-01-29; Daniel comments ]

This issue has some overlap with LWG 3665 in regard to the question how we should handle the rebind_alloc member template of the allocator_traits template as specified by [allocator.traits.types]/11. It would seem preferable to decide for the same approach in both cases.

[ 2021-05-10; Reflector poll ]

Priority set to 2. Send to LEWG. Daniel: "there is no similar treatment for the rebind member template and I think it should be clarified whether pointer_to's signature should exist and in which form in the offending case."

P1474R1 chose to use std::to_address (a mechanism of converting pointer-like types to raw pointers) for contiguous iterators. std::to_address provides an optional customization point via an optional member in std::pointer_traits. However all iterators are not pointers, and the primary template of std::pointer_traits<Ptr> requires that either Ptr::element_type is valid or Ptr is of the form template<T, Args...> or the pointer_traits specialization is ill-formed. This requires specializing pointer_traits for those contiguous iterator types which is inconvenient for users. P1474 should have also made pointer_traits SFINAE friendly.