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3729. std::tuple_element_t<std::ranges::subrange<I, S, K>> should remove top-level cv-qualifiers

Section: 25.2 [ranges.syn] Status: New Submitter: Jiang An Opened: 2022-06-30 Last modified: 2022-07-08

Priority: 4

View all other issues in [ranges.syn].

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Discussion:

std::ranges::subrange<int * volatile>> is weird but valid. The return type (deduced type for auto) of std::ranges::get for this type is int*, because the replacing of that (cv-unqualified) auto drops top-level cv-qualifiers. I think get is doing the right thing, and std::tuple_element_t should be consistent with the return types.

[2022-07-08; Reflector poll]

Set priority to 4 after reflector poll. "This is just contrived, especially since lots of the iterator requirements on such a type involve deprecated operations."

Proposed resolution:

This wording is relative to N4910.

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

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