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3599. The const overload of lazy_split_view::begin should be constrained by const Pattern

Section: 26.7.16.2 [range.lazy.split.view] Status: New Submitter: Hewill Kang Opened: 2021-09-23 Last modified: 2021-10-14

Priority: 3

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Discussion:

Consider the following code snippet:

#include <ranges>

int main() {
  auto p = std::views::iota(0)
         | std::views::take(1)
         | std::views::reverse;
  auto r = std::views::single(42)
         | std::views::lazy_split(p);
  auto f = r.front();
}

r.front() is ill-formed even if r is a forward_range.

This is because the const overload of lazy_split_view::begin is not constrained by the const Pattern, which makes it still well-formed in such cases. When the const overload of view_interface<lazy_split_view<V, Pattern>>::front is instantiated, the subrange{parent_->pattern_} inside lazy_split_view::outer-iterator<true>::operator++() will cause a hard error since const Pattern is not a range.

[2021-09-24; Daniel comments]

This issue is related to LWG 3592.

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4892.

1. Modify 26.7.16.2 [range.lazy.split.view], class template lazy_split_view synopsis, as indicated:

   namespace std::ranges {
   
     […]
   
     template<input_range V, forward_range Pattern>
       requires view<V> && view<Pattern> &&
                indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
               (forward_range<V> || tiny-range<Pattern>)
     class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> {
     private:
       […]
     public:
       […]
   
       constexpr auto begin() {
         […]
       }
       
       constexpr auto begin() const requires forward_range<V> && forward_range<const V> &&
                                             forward_range<const Pattern>{
         return outer-iterator<true>{*this, ranges::begin(base_)};
       }
       
       constexpr auto end() requires forward_range<V> && common_range<V> {
         […]
       }
   
       constexpr auto end() const {
         if constexpr (forward_range<V> && forward_range<const V> && common_range<const V> &&
                       forward_range<const Pattern>)
           return outer-iterator<true>{*this, ranges::end(base_)};
         else
           return default_sentinel;
       }
     };
     
     […]
     
   }