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

Section: 26.7.14.2 [range.join.view], 26.7.15.2 [range.join.with.view] Status: New Submitter: Hewill Kang Opened: 2022-05-17 Last modified: 2022-06-21

Priority: 3

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Discussion:

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.

[2022-06-21; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4910.

1. Modify 26.7.14.2 [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};
       }
     };
     
     […]
     
   }
   

2. Modify 26.7.15.2 [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};
       }
     };
     
     […]
     
   }