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3736.move_iterator missingdisable_sized_sentinel_for specialization

Section: 24.2[iterator.synopsis]Status:C++23Submitter: Hewill KangOpened: 2022-07-14Last modified: 2023-11-22

Priority:Not Prioritized

View all otherissues in [iterator.synopsis].

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Discussion:

Sincereverse_iterator::operator- is not constrained, the standard adds adisable_sized_sentinel_for specialization for it to avoid situations where the underlying iterator can be subtracted makingreverse_iterator accidentally modelsized_sentinel_for.

However, given thatmove_iterator::operator- is also unconstrained and the standard does not have thedisable_sized_sentinel_for specialization for it, this makessubrange<move_iterator<I>, move_iterator<I>> unexpectedly satisfysized_range and incorrectly useI::operator- to get size whenI can be subtracted but not modeledsized_sentinel_for<I>.

In addition, sinceP2520 makesmove_iterator no longer justinput_iterator,ranges::size can get the size of the range by subtracting twomove_iterator pairs,this also makesr | views::as_rvalue may satisfysized_range when neitherr norr | views::reverse issized_range.

We should add amove_iterator version of thedisable_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 - bothmove_iterator andreverse_iterator'soperator- are constrained onx.base() - y.base()being valid."

Does anyone remember why we did this forreverse_iteratorand notmove_iterator?

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

Proposed resolution:

This wording is relative toN4910.

1. Modify 24.2[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;  […]}