Issue 3398: tuple_element_t is also wrong for const subrange

Title: tuple_element_t is also wrong for const subrange
Status: c++20
Section: [ranges.syn]
Submitter: Casey Carter

Created on 2019-02-13.00:00:00 last changed 58 months ago

Messages

Date: 2020-02-14.16:06:37

Proposed resolution:

This wording is relative to N4849.

Modify [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;
  };
}

msg11086 (view)

Date: 2020-02-15.00:00:00

[ 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.

msg11085 (view)

Date: 2020-02-14.16:06:37

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Previous resolution [SUPERSEDED]: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.

Modify [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;
  };
}

Add the following wording to Annex D:

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

1 The header <ranges> ([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>> {};
}

History
Date	User	Action	Args
2021-02-25 10:48:01	admin	set	status: wp -> c++20
2020-02-24 16:02:59	admin	set	status: immediate -> wp
2020-02-14 16:06:37	admin	set	messages: + msg11131
2020-02-14 16:06:37	admin	set	status: new -> immediate
2020-02-13 21:09:02	admin	set	messages: + msg11086
2019-02-13 00:00:00	admin	create