This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++23 status.

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

View all other issues in [readable.traits].

View all issues with C++23 status.

Discussion:

Per 24.3.2.2 [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]

This wording is relative to N4861.

1. Modify 24.3.2.2 [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 24.3.2.2 [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> { };
   
   […]
   

Pfad - The Proxy pFad of © 2024 Garber Painting. All rights reserved.

Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.

Alternative Proxies:

Alternative Proxy

pFad Proxy

pFad v3 Proxy

pFad v4 Proxy