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3541.indirectly_readable_traits should be SFINAE-friendly for all types

Section: 24.3.2.2[readable.traits]Status:C++23Submitter: Christopher Di BellaOpened: 2021-04-08Last modified: 2023-11-22

Priority:Not Prioritized

View all otherissues in [readable.traits].

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Discussion:

Following the outcome of LWG3446⁽ⁱ⁾, a strict implementation ofstd::indirectly_readable_traits isn't SFINAE-friendly for types that have differentvalue_type andelement_type members due to the ambiguity between thehas-member-value-type andhas-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 toN4885.

1. Modify 24.3.2.2[readable.traits] p1 as indicated:

   […]template<class T>  concepthas-member-value-type = requires { typename T::value_type; };// exposition onlytemplate<class T>  concepthas-member-element-type = requires { typename T::element_type; };// exposition onlytemplate<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>  requireshas-member-element-type<T>struct indirectly_readable_traits<T> { };  template<has-member-value-type T>  requireshas-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> { };[…]