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3398. tuple_element_t is also wrong for const subrange

Section: 25.2 [ranges.syn] Status: C++20 Submitter: Casey Carter Opened: 2019-02-13 Last modified: 2021-02-25

Priority: 0

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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 N4849.

1. Modify 25.2 [ranges.syn], header <ranges> synopsis, as indicated:

   […]
   namespace std {
     namespace views = ranges::views;
   
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_size<ranges::subrange<I, S, K>>
       : integral_constant<size_t, 2> {};
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<0, ranges::subrange<I, S, K>> {
       using type = I;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<1, ranges::subrange<I, S, K>> {
       using type = S;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<0, const ranges::subrange<I, S, K>> {
       using type = I;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<1, const ranges::subrange<I, S, K>> {
       using type = S;
     };
   }
   

2. Add the following wording to Annex D:

   D.? Deprecated subrange tuple interface [depr.ranges.syn]

   1 The header <ranges> (25.2 [ranges.syn]) has the following additions:

   namespace std {
     template<size_t X, class I, class S, ranges::subrange_kind K>
     struct tuple_element<X, volatile ranges::subrange<I, S, K>> {};
     template<size_t X, class I, class S, ranges::subrange_kind K>
     struct tuple_element<X, const volatile ranges::subrange<I, S, K>> {};
   }
   

[2020-02-14, Prague]

LWG decided to remove the volatile support, we shouldn't give the impression to support an idiom that wouldn't work.

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.2 [ranges.syn], header <ranges> synopsis, as indicated:

   […]
   namespace std {
     namespace views = ranges::views;
   
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_size<ranges::subrange<I, S, K>>
       : integral_constant<size_t, 2> {};
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<0, ranges::subrange<I, S, K>> {
       using type = I;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<1, ranges::subrange<I, S, K>> {
       using type = S;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<0, const ranges::subrange<I, S, K>> {
       using type = I;
     };
     template<class I, class S, ranges::subrange_kind K>
     struct tuple_element<1, const ranges::subrange<I, S, K>> {
       using type = S;
     };
   }