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4105. ranges::ends_with's Returns misses difference casting

Section: 27.6.17 [alg.ends.with] Status: WP Submitter: Hewill Kang Opened: 2024-05-17 Last modified: 2024-07-08

Priority: Not Prioritized

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Discussion:

The Returns of the ranges version of ranges::ends_with are specified as ranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - N2), r2, ...) which is not quite right when N2 is an integer-class type and N1 is an integer type, because in this case N1 - N2 will be an integer-class type which cannot be implicitly converted to the difference_type of r1 leading to the construction of drop_view being ill-formed.

[2024-06-24; Reflector poll]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 27.6.17 [alg.ends.with] as indicated:

   template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity,
            class Proj2 = identity>
     requires (forward_range<R1> || sized_range<R1>) &&
              (forward_range<R2> || sized_range<R2>) &&
              indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
     constexpr bool ranges::ends_with(R1&& r1, R2&& r2, Pred pred = {},
                                      Proj1 proj1 = {}, Proj2 proj2 = {});
   

   -3- Let N1 be ranges::distance(r1) and N2 be ranges::distance(r2).

   -4- Returns: false if N1 < N2, otherwise

   ranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - static_cast<decltype(N1)>(N2)), r2, pred, proj1, proj2)