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4074.compatible-joinable-ranges is underconstrained

Section: 25.7.15.2[range.join.with.view]Status:WPSubmitter: Hewill KangOpened: 2024-04-21Last 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 throughcompatible-joinable-ranges.

However, unlike whatconcat_view andgenerator do, this concept only requires that these three types be valid and does not further check the relationship between them to be compatible with theindirectly_readable requirement forinput_iterator.This results in a validly-constructedjoin_with_view that may not modelinput_range, which seems unintended.

The proposed resolution aliasescompatible-joinable-ranges toconcatable i.e. specialization for two ranges to fully constrain, and I believe this could also be a better fit for LWG3971⁽ⁱ⁾.

Previous resolution [SUPERSEDED]:

This wording is relative toN4981.

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

   namespace std::ranges {  template<class R, class P>    conceptcompatible-joinable-ranges =concatable<R, P>;  //exposition onlycommon_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 toN4981.

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

   #include <compare>              //see 17.12.1[compare.syn]#include <initializer_list>     //see 17.11.2[initializer.list.syn]#include <iterator>             //see 24.2[iterator.synopsis]namespace std::ranges {  […]  //25.7.15[range.join.with],join with viewtemplate<class R, class P>    conceptcompatible-joinable-ranges =see below; //exposition only  template<input_range V, forward_range Pattern>    requiresview<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>    conceptcompatible-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> {      returniterator<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 {    […]  };}