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3398. tuple_element_t is also wrong for const subrange

Section: 26.2 [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.

Previous resolution [SUPERSEDED]:

This wording is relative to N4849.

  1. Modify 26.2 [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> (26.2 [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 26.2 [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;
  };
}