Issue 3446: indirectly_readable_traits ambiguity for types with both value_type and element_type (original) (raw)

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3446. indirectly_readable_traits ambiguity for types with both value_type and element_type

Section: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Casey Carter Opened: 2020-05-15 Last modified: 2023-11-22

Priority: 2

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Discussion:

Per 24.3.2.2 [readable.traits], indirectly_readable_traits<T>::value_type is the same type asremove_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Previous resolution [SUPERSEDED]: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 24.3.2.2 [readable.traits] as indicated:

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

   template
   requires is_object_v
   struct cond-value-type {
   using value_type = remove_cv_t;
   };

   template struct indirectly_readable_traits { };

   […]

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

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

   template
   requires requires {
   typename T::element_type;
   typename T::value_type;
   requires same_as<
   remove_cv_t,

 remove_cv_t<typename T::value_type>>;  

}
struct indirectly_readable_traits
: cond-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 24.3.2.2 [readable.traits] as indicated:

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

   template
   requires is_object_v
   struct cond-value-type {
   using value_type = remove_cv_t;
   };

   template
   concept has-member-value-type = requires { typename T::value_type; }; // exposition only

   template
   concept has-member-element-type = requires { typename T::element_type; }; // exposition only

   template struct indirectly_readable_traits { };

   […]

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

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

   template<class_has-member-value-type_ T>
   requires has-member-element-type &&
   same_as<remove_cv_t, remove_cv_t>

struct indirectly_readable_traits
: cond-value-type { };

[…]