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3736. move_iterator missing disable_sized_sentinel_for specialization

Section: 25.2 [iterator.synopsis] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-14 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.synopsis].

View all issues with C++23 status.

Discussion:

Since reverse_iterator::operator- is not constrained, the standard adds a disable_sized_sentinel_for specialization for it to avoid situations where the underlying iterator can be subtracted making reverse_iterator accidentally model sized_sentinel_for.

However, given that move_iterator::operator- is also unconstrained and the standard does not have the disable_sized_sentinel_for specialization for it, this makes subrange<move_iterator<I>, move_iterator<I>> unexpectedly satisfy sized_range and incorrectly use I::operator- to get size when I can be subtracted but not modeled sized_sentinel_for<I>.

In addition, since P2520 makes move_iterator no longer just input_iterator, ranges::size can get the size of the range by subtracting two move_iterator pairs, this also makes r | views::as_rvalue may satisfy sized_range when neither r nor r | views::reverse is sized_range.

We should add a move_iterator version of the disable_sized_sentinel_for specialization to the standard to avoid the above situation.

[2022-08-23; Reflector poll]

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

"but I don't think the issue text is quite right - both move_iterator and reverse_iterator's operator- are constrained on x.base() - y.base() being valid."

Does anyone remember why we did this for reverse_iterator and not move_iterator?

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.2 [iterator.synopsis], header <iterator> synopsis, as indicated:

   namespace std::ranges {
     […]
     template<class Iterator>
       constexpr move_iterator<Iterator> make_move_iterator(Iterator i);
   
     template<class Iterator1, class Iterator2>
         requires (!sized_sentinel_for<Iterator1, Iterator2>)
       inline constexpr bool disable_sized_sentinel_for<move_iterator<Iterator1>,
                                                        move_iterator<Iterator2>> = true;
   
     […]
   }