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4074. compatible-joinable-ranges is underconstrained

Section: 25.7.15.2 [range.join.with.view] Status: WP Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2024-07-08

Priority: Not Prioritized

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Discussion:

join_with_view requires the value type, reference and rvalue reference of the inner range and pattern range to share common (reference) types through compatible-joinable-ranges.

However, unlike what concat_view and generator do, this concept only requires that these three types be valid and does not further check the relationship between them to be compatible with the indirectly_readable requirement for input_iterator. This results in a validly-constructed join_with_view that may not model input_range, which seems unintended.

The proposed resolution aliases compatible-joinable-ranges to concatable i.e. specialization for two ranges to fully constrain, and I believe this could also be a better fit for LWG 3971(i).

Previous resolution [SUPERSEDED]:

This wording is relative to N4981.

  1. Modify 25.7.15.2 [range.join.with.view] as indicated:

    namespace std::ranges {
  template<class R, class P>
    concept compatible-joinable-ranges = concatable<R, P>;  // exposition only
        common_with<range_value_t<R>, range_value_t<P>> &&
        common_reference_with<range_reference_t<R>, range_reference_t<P>> &&
        common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;
  
  […]
}

[2024-04-24; Hewill Kang provides improved wording]

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

  1. Modify 25.2 [ranges.syn] as indicated:

    #include <compare>              // see 17.11.1 [compare.syn]
#include <initializer_list>     // see 17.10.2 [initializer.list.syn]
#include <iterator>             // see 24.2 [iterator.synopsis]

namespace std::ranges {
  […]
  // 25.7.15 [range.join.with], join with view
  template<class R, class P>
    concept compatible-joinable-ranges = see below; // exposition only

  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>
          see below
  class join_with_view;                                                             // freestanding
  […]
}

  2. Modify 25.7.15.2 [range.join.with.view] as indicated:

    namespace std::ranges {
  template<class R, class P>
    concept compatible-joinable-ranges =            // exposition only
        common_with<range_value_t<R>, range_value_t<P>> &&
        common_reference_with<range_reference_t<R>, range_reference_t<P>> &&
        common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;
  
  […]

  template<input_range V, forward_range Pattern>
    requires view<V> && input_range<range_reference_t<V>>
          && view<Pattern>
          && compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern>
  class join_with_view : public view_interface<join_with_view<V, Pattern>> {
    […]
    constexpr auto begin() const
      requires forward_range<const V> &&
               forward_range<const Pattern> &&
               is_reference_v<range_reference_t<const V>> &&
               input_range<range_reference_t<const V>> &&
               concatable<range_reference_t<const V>, const Pattern> {
      return iterator<true>{*this, ranges::begin(base_)};
    }
    […]
    constexpr auto end() const
      requires forward_range<const V> && forward_range<const Pattern> &&
               is_reference_v<range_reference_t<const V>> &&
               input_range<range_reference_t<const V>> &&
               concatable<range_reference_t<const V>, const Pattern> {
      […]
    }
  };
}

  3. Modify 25.7.15.3 [range.join.with.iterator] 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-rangesconcatable<range_reference_t<V>, Pattern>
  template<bool Const>
  class join_with_view<V, Pattern>::iterator {
    […]
  };
}

  4. Modify 25.7.15.4 [range.join.with.sentinel] 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-rangesconcatable<range_reference_t<V>, Pattern>
  template<bool Const>
  class join_with_view<V, Pattern>::sentinel {
    […]
  };
}