Issue 3700: The const begin of the join_view family does not require InnerRng to be a range

Title: The const begin of the join_view family does not require InnerRng to be a range
Status: resolved
Section: [range.join.view][range.join.with.view]
Submitter: Hewill Kang

Created on 2022-05-17.00:00:00 last changed 20 months ago

Messages

msg12511 (view)

Date: 2023-03-22.00:00:00

[ 2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved. ]

msg12472 (view)

Date: 2022-06-15.00:00:00

[ 2022-06-21; Reflector poll ]

Set priority to 3 after reflector poll.

This wording is relative to N4910.

Modify [range.join.view], class template join_view synopsis, as indicated:

namespace std::ranges {

  […]

  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>>
  class join_view : public view_interface<join_view<V>> {
  private:
    […]
  public:
    […]

    constexpr auto begin() {
      […]
    }

    constexpr auto begin() const
      requires input_range<const V> &&
                input_range<range_reference_t<const V>> &&
                is_reference_v<range_reference_t<const V>>
    { return iterator<true>{*this, ranges::begin(base_)}; }

    constexpr auto end() {
      […]
    }

    constexpr auto end() const
      requires input_range<const V> &&
                input_range<range_reference_t<const V>> &&
                is_reference_v<range_reference_t<const V>> {
      if constexpr (forward_range<const V> &&
                    forward_range<range_reference_t<const V>> &&
                    common_range<const V> &&
                    common_range<range_reference_t<const V>>)
        return iterator<true>{*this, ranges::end(base_)};
      else
        return sentinel<true>{*this};
    }
  };

  […]

}

Modify [range.join.with.view], class template join_with_view synopsis, as indicated:

namespace std::ranges {

  […]

  template<input_range V, forward_range Pattern>
  requires view<V> && input_range<range_reference_t<V>>
        && view<Pattern>
        && compatible-joinable-ranges<range_reference_t<V>, Pattern>
  class join_with_view : public view_interface<join_with_view<V, Pattern>> {
  private:
    […]
  public:
    […]

    constexpr auto begin() {
      […]
    }
    constexpr auto begin() const
      requires input_range<const V> &&
                forward_range<const Pattern> &&
                input_range<range_reference_t<const V>> &&
                is_reference_v<range_reference_t<const V>> {
      return iterator<true>{*this, ranges::begin(base_)};
    }

    constexpr auto end() {
      […]
    }
    constexpr auto end() const
      requires input_range<const V> && forward_range<const Pattern> &&
                input_range<range_reference_t<const V>> &&
                is_reference_v<range_reference_t<const V>> {
      using InnerConstRng = range_reference_t<const V>;
      if constexpr (forward_range<const V> && forward_range<InnerConstRng> &&
                    common_range<const V> && common_range<InnerConstRng>)
        return iterator<true>{*this, ranges::end(base_)};
      else
        return sentinel<true>{*this};
    }
  };

  […]

}

msg12471 (view)

Date: 2022-05-17.00:00:00

This is a follow-up of LWG 3599.

Currently, the const version of join_view::begin has the following constraints

constexpr auto begin() const
  requires input_range<const V> &&
           is_reference_v<range_reference_t<const V>>
{ return iterator<true>{*this, ranges::begin(base_)}; }

which only requires InnerRng to be a reference type, but in some cases, InnerRng may not be a range. Consider

#include <ranges>

int main() {
  auto r = std::views::iota(0, 5)
         | std::views::split(1);
  auto s = std::views::single(r);
  auto j = s | std::views::join;
  auto f = j.front();
}

The reference type of single_view is const split_view&, which only has the non-const version of begin, which will cause view_interface's const front to be incorrectly instantiated, making r.front() unnecessarily ill-formed.

We should add this check for join_view's const begin, as well as join_with_view.

History
Date	User	Action	Args
2023-03-23 11:42:08	admin	set	status: new -> resolved
2022-06-21 11:46:11	admin	set	messages: + msg12511
2022-05-21 11:42:57	admin	set	messages: + msg12472
2022-05-17 00:00:00	admin	create