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indirectly_readable_traits should be SFINAE-friendly for all typesSection: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Christopher Di Bella Opened: 2021-04-08 Last modified: 2023-11-22
Priority: Not Prioritized
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Discussion:
Following the outcome of LWG 3446(i), 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.
Modify 24.3.2.2 [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> { };
[…]