This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of WP status.

3541. indirectly_readable_traits should be SFINAE-friendly for all types

Section: 23.3.2.2 [readable.traits] Status: WP Submitter: Christopher Di Bella Opened: 2021-04-08 Last modified: 2021-06-12

Priority: Not Prioritized

View all other issues in [readable.traits].

View all issues with WP status.

Discussion:

Following the outcome of LWG 3446, a strict implementation of std::indirectly_readable_traits isn't SFINAE-friendly for types that have different value_type and element_type members due to the ambiguity between the has-member-value-type and has-member-element-type specialisations. Other traits types are empty when requirements aren't met; we should follow suit here.

[2021-04-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

  1. Modify 23.3.2.2 [readable.traits] p1 as indicated:

    […]
    template<class T>
      concept has-member-value-type = requires { typename T::value_type; }; // exposition only
    
    template<class T>
      concept has-member-element-type = requires { typename T::element_type; }; // exposition only
    
    template<class> struct indirectly_readable_traits { };
    
    […]
    
    template<has-member-value-type T>
    struct indirectly_readable_traits<T>
      : cond-value-type<typename T::value_type> { };
    
    template<has-member-element-type T>
    struct indirectly_readable_traits<T>
      : cond-value-type<typename T::element_type> { };
    
    template<has-member-value-type T>
      requires has-member-element-type<T>
    struct indirectly_readable_traits<T> { };
      
    template<has-member-value-type T>
      requires has-member-element-type<T> &&
               same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>
    struct indirectly_readable_traits<T>
      : cond-value-type<typename T::value_type> { };
    […]