4108. lazy_split_view should be sized_range when pattern is empty tiny-range

Section: 25.7.16.2 [range.lazy.split.view], 25.7.17.2 [range.split.view] Status: SG9 Submitter: Hewill Kang Opened: 2024-05-23 Last modified: 2024-08-02

Priority: 4

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Discussion:

When the pattern range is empty, lazy_split_view will split each element into individual subranges, which means its size is equal to the size of the underlying range.

Since we already have a tiny-range that can determine whether the range is empty by its type, it seems valuable to provide a size for lazy_split_view in this case, given that we already specifically checked for it by Pattern::size() == 0 in inner-iterator::operator++().

[2024-08-02; Reflector poll]

Set priority to 4 after reflector poll. Set Status to SG9. "Design change". "tiny-range should accept span<T, 0|1> or ref_view<array<T, 0|1>>, see related paper P1419."

Proposed resolution:

This wording is relative to N4981.

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>> {
       […]
       constexpr auto size()
         requires sized_range<V> &&
                  tiny-range<Pattern> && (Pattern::size() == 0) {
         return ranges::size(base_);
       }
   
       constexpr auto size() const
         requires sized_range<const V> &&
                  tiny-range<Pattern> && (Pattern::size() == 0) {
         return ranges::size(base_);
       }
     };
     […]
   }
   

2. Modify 25.7.17.2 [range.split.view] as indicated:

   namespace std::ranges {
     template<forward_range V, forward_range Pattern>
       requires view<V> && view<Pattern> &&
                indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
     class split_view : public view_interface<split_view<V, Pattern>> {
       […]
       constexpr auto size()
         requires sized_range<V> &&
                  tiny-range<Pattern> && (Pattern::size() == 0) {
         return ranges::size(base_);
       }
   
       constexpr auto size() const
         requires sized_range<const V> &&
                  tiny-range<Pattern> && (Pattern::size() == 0) {
         return ranges::size(base_);
       }
     };
     […]
   }