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3919. enumerate_view may invoke UB for sized common non-forward underlying ranges

Section: 25.7.24 [range.enumerate] Status: WP Submitter: Patrick Palka Opened: 2023-04-07 Last modified: 2024-04-02

Priority: 3

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Discussion:

For a sized common range, enumerate_view::end() is specified to call ranges::distance. But ranges::distance is not necessarily well-defined for a sized non-forward range after calling ranges::begin (according to 25.4.3 [range.sized]).

So for a sized common non-forward underlying range, it seems calling enumerate_view::begin() followed by enumerate_view::end() may invoke UB and thus make enumerate_view potentially unusable for such ranges.

I suppose we might need to instead call and cache the result of ranges::distance from enumerate_view::begin() for such ranges.

[2022-04-12; Patrick Palka provides wording]

The proposed wording follows the suggestion provided by Tim Song, to simply make enumerate non-common for this case.

[2023-05-24; Reflector poll]

Set priority to 3 after reflector poll.

[Kona 2023-11-10; move to Ready]

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4944.

1. Modify 25.7.24.2 [range.enumerate.view], class template class enumerate_view synopsis, as indicated:

   […]
   constexpr auto end() requires (!simple-view<V>) {
     if constexpr (forward_range<V> && common_range<V> && sized_range<V>)
       return iterator<false>(ranges::end(base_), ranges::distance(base_));
     else
       return sentinel<false>(ranges::end(base_));
   }
   constexpr auto end() const requires range-with-movable-references<const V> {
     if constexpr (forward_range<const V> && common_range<const V> && sized_range<const V>)
       return iterator<true>(ranges::end(base_), ranges::distance(base_));
     else
       return sentinel<true>(ranges::end(base_));
   }
   […]