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3446. indirectly_readable_traits ambiguity for types with both value_type and element_type

Section: 25.3.2.2 [readable.traits] Status: WP Submitter: Casey Carter Opened: 2020-05-15 Last modified: 2020-11-09

Priority: 2

View all other issues in [readable.traits].

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Discussion:

Per 25.3.2.2 [readable.traits], indirectly_readable_traits<T>::value_type is the same type as remove_cv_t<T::value_type> if it denotes an object type, or remove_cv_t<T::element_type> if it denotes an object type. If both T::value_type and T::element_type denote types, indirectly_readable_traits<T>::value_type is ill-formed. This was perhaps not the best design, given that there are iterators in the wild (Boost's unordered containers) that define both nested types. indirectly_readable_traits should tolerate iterators that define both nested types consistently.

[2020-07-17; Priority set to 2 in telecon]

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.

  1. Modify 25.3.2.2 [readable.traits] as indicated:

    […]
template<class> struct cond-value-type { }; // exposition only

template<class T>
  requires is_object_v<T>
struct cond-value-type<T> {
  using value_type = remove_cv_t<T>;
};

template<class> struct indirectly_readable_traits { };

[…]

template<class T>
  requires requires { typename T::value_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::value_type> { };

template<class T>
  requires requires { typename T::element_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::element_type> { };

template<class T>
  requires requires {
    typename T::element_type;
    typename T::value_type;
    requires 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> { };

[…]

[2020-07-23; Casey improves wording per reflector discussion]

[2020-08-21; moved to Tentatively Ready after five votes in favour in reflector poll]

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

  1. Modify 25.3.2.2 [readable.traits] as indicated:

    […]
template<class> struct cond-value-type { }; // exposition only

template<class T>
  requires is_object_v<T>
struct cond-value-type<T> {
  using value_type = remove_cv_t<T>;
};

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<classhas-member-value-type T>
  requires requires { typename T::value_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::value_type> { };

template<classhas-member-element-type T>
  requires requires { typename T::element_type; }
struct indirectly_readable_traits<T>
  : cond-value-type<typename T::element_type> { };

template<classhas-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> { };

[…]