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3212. tuple_element_t<1, const span<int, 42>> is const int

Section: 99 [span.tuple] Status: Resolved Submitter: Tim Song Opened: 2019-05-31 Last modified: 2020-09-06

Priority: 2

View all issues with Resolved status.

Discussion:

As currently specified, it uses the cv-qualified partial specialization, which incorrectly adds cv-qualification to the element type.

This wording is relative to N4810.

1. Modify 24.7.2.1 [span.syn], header <span> synopsis, as indicated:

   […]
   // 99 [span.tuple], tuple interface
   template<class T> class tuple_size;
   template<size_t I, class T> class tuple_element;
   template<class ElementType, size_t Extent>
     struct tuple_size<span<ElementType, Extent>>;
   template<class ElementType>
     struct tuple_size<span<ElementType, dynamic_extent>>; // not defined
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, span<ElementType, Extent>>;
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const span<ElementType, Extent>>;
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, volatile span<ElementType, Extent>>;
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const volatile span<ElementType, Extent>>;
   […]
   

2. Modify 99 [span.tuple] before p1 as indicated:

   [Drafting note: The representation style for the newly added wording follows the new style for tuple_element<I, span<ElementType, Extent>>::type, which had recently been changed editorially.]

   template<class ElementType, size_t Extent>
     struct tuple_size<span<ElementType, Extent>>
       : integral_constant<size_t, Extent> { };
   
   tuple_element<I, span<ElementType, Extent>>::type
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const span<ElementType, Extent>> {
       using type = ElementType;
     };
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, volatile span<ElementType, Extent>> {
       using type = ElementType;
     };
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const volatile span<ElementType, Extent>> {
       using type = ElementType;
     };
   

   -1- Mandates: Extent != dynamic_extent && I < Extent is true.

[2020-02-13, Prague]

Wording needs to be adjusted, because we accepted P1831R1.

This wording is relative to N4849.

1. Modify 24.7.2.1 [span.syn], header <span> synopsis, as indicated:

   […]
   // 99 [span.tuple], tuple interface
   template<class T> class tuple_size;
   template<size_t I, class T> class tuple_element;
   template<class ElementType, size_t Extent>
     struct tuple_size<span<ElementType, Extent>>;
   template<class ElementType>
     struct tuple_size<span<ElementType, dynamic_extent>>; // not defined
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, span<ElementType, Extent>>;
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const span<ElementType, Extent>>;
   […]
   

2. Modify 99 [span.tuple] before p1 as indicated:

   template<class ElementType, size_t Extent>
     struct tuple_size<span<ElementType, Extent>>
       : integral_constant<size_t, Extent> { };
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, span<ElementType, Extent>> {
       using type = ElementType;
     };
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const span<ElementType, Extent>> {
       using type = ElementType;
     };
   

   -1- Mandates: Extent != dynamic_extent && I < Extent is true.

3. Add the following wording to Annex D:

   D.? Deprecated span tuple interface [depr.span.syn]

   1 The header <span> (24.7.2.1 [span.syn]) has the following additions:

   
   namespace std {
     template<size_t I, class ElementType, size_t Extent>
       struct tuple_element<I, volatile span<ElementType, Extent>>;
     template<size_t I, class ElementType, size_t Extent>
       struct tuple_element<I, const volatile span<ElementType, Extent>>;
   }
   

   D.1.? tuple interface [depr.span.tuple]

   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, volatile span<ElementType, Extent>> {
       using type = ElementType;
     };
   
   template<size_t I, class ElementType, size_t Extent>
     struct tuple_element<I, const volatile span<ElementType, Extent>> {
       using type = ElementType;
     };
   

   -?- Mandates: Extent != dynamic_extent && I < Extent is true.

[2020-02-14, Prague]

Lengthy discussion about the intended design of span's tuple interface. Has been send to LEWG, who made several polls and concluded to remove the tuple interface for span for C++20.

Will be resolved by P2116R0.

[2020-05-01; reflector discussions]

Resolved by P2116R0 voted in in Prague.

Proposed resolution:

Resolved by P2116R0.