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3768. possibly-const-range is overconstrained

Section: 26.2 [ranges.syn] Status: NAD Submitter: Hewill Kang Opened: 2022-09-08 Last modified: 2022-11-30

Priority: Not Prioritized

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Discussion:

Due to the possibly-const-range constraint that the template parameter R must model input_range, this makes the ranges::cend that tries using it for meaningful const casting never be applied to an output_range, even though const_sentinel does not require the template parameter S to model input_iterator.

This is unnecessary, we should relax its constraint.

[2022-09-23; Reflector poll]

Tentatively NAD. "const output ranges don’t seem to be useful."

[2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 26.2 [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 25.2 [iterator.synopsis]
   
   namespace std::ranges {
     […]
   
     // 26.7.21 [range.as.const], as const view
     template<input_range R>
       constexpr auto& possibly-const-range(R& r) {          // exposition only
         if constexpr (constant_range<const R> && !constant_range<R>) {
           return const_cast<const R&>(r);
         } else {
           return r;
         }
       } 
     
     […]
   }