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3855. tiny-range is not quite right

Section: 25.7.16.2 [range.lazy.split.view] Status: New Submitter: Hewill Kang Opened: 2023-01-07 Last modified: 2023-02-01

Priority: 4

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Discussion:

Currently, lazy_split_view supports input range when the element of the pattern is less than or equal to 1. In order to ensure this condition at compile time, tiny-range constrains the type R to model sized_range and requires (remove_reference_t<R>::size() <= 1) to be a constant expression.

However, modeling a sized_range does not guarantee that ranges::size will be evaluated by R::size(). For example, when disable_sized_range<R> is specialized to true or R::size() returns a non-integer-like type, ranges::size can still compute the size by subtracting the iterator-sentinel pair when both satisfy sized_sentinel_for.

Since the lazy_split_view's iterator uses R::size() to get the constant value of the pattern, we must ensure that this is indeed how ranges::size is calculated. Also, I think we can simplify tiny-range with bool_constant in a way similar to LWG 3150⁽ⁱ⁾, which removes the introduction of require-constant.

[2023-02-01; Reflector poll]

Set priority to 4 after reflector poll. Only matters for pathological types. Maybe use requires bool_constant<ranges::size(r) <= 1>.

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.16.2 [range.lazy.split.view] as indicated:

   namespace std::ranges {
     template<auto> struct require-constant; // exposition only
   
     template<class R>
     concept tiny-range = // exposition only
        sized_range<R> &&
        requires { typename require-constant<remove_reference_t<R>::size()>; } &&
        (remove_reference_t<R>::size() <= 1);
      
     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>> {
       […]
     };
     […]
   }
   

   
   template<class R>
   concept tiny-range = // exposition only
      sized_range<R> &&
      requires { requires bool_constant<(remove_reference_t<R>::size() <= 1)>::value; };
   

   -?- Given an lvalue r of type remove_reference_t<R>, R models tiny-range only if ranges::size(r) is evaluated by remove_reference_t<R>::size().

   constexpr lazy_split_view(V base, Pattern pattern);
   

   -1- Effects: : Initializes base_ with std::move(base), and pattern_ with std::move(pattern).