Issue 4303: std::decay_t in the specification of ranges::distance is problematic (original) (raw)

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4303. std::decay_t in the specification of ranges::distance is problematic

Section: 24.4.4.3 [range.iter.op.distance] Status: New Submitter: Jiang An Opened: 2025-07-24 Last modified: 2025-08-29

Priority: 4

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Discussion:

This is discovered when implementing the resolution LWG 4242(i). Per LWG 4242(i), it is intended to allow ranges::distance to handle volatile-qualified iterator values. However, the uses of decay_t (established per LWG 3664(i)) are still problematic, because when sized_sentinel_for<S, decay_t<I>> is modeled, there's no semantic or syntactic requirement that S shall work with volatile-qualified I.

If we implement the constraint as is, there will still be some underconstrained cases. E.g. When the operator== or operator- intendedly rejects volatile-qualified iterators. And even when they accept volatile-qualified iterators, the additional semantic requirements imposed by sized_sentinel_for<S, decay_t<I>> are still undesired.

I think we should only decay arrays and keep volatile for non-array arguments.

[2025-08-29; Reflector poll]

Set priority to 4 after reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 24.2 [iterator.synopsis], header <iterator> synopsis, as indicated:

   […]
   namespace std: {
   […]
   // 24.4.4.3 [range.iter.op.distance], ranges::distance

   template
   using distance-iterator-t = // exposition only
   conditional_t<is_array_v<remove_reference_t>,

   decay_t<T>, remove_const_t<remove_reference_t<T>>>;  

template<class I, sentinel_for S>
requires (!sized_sentinel_for<S, I>)
constexpr iter_difference_t distance(I first, S last); // freestanding
template<class I, sized_sentinel_for<decay_t_distance-iterator-t_> S>
constexpr iter_difference_t<decay_t_distance-iterator-t_> distance(I&& first, S last); // freestanding
template
constexpr range_difference_t distance(R&& r); // freestanding
[…]
} 2. Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<class I, sized_sentinel_for<decay_t_distance-iterator-t_> S>
constexpr iter_difference_t<decay_t_distance-iterator-t_> distance(I&& first, S last);

-3- Effects: Equivalent to:

if constexpr (!is_array_v<remove_reference_t>)
return last - first;
else
return last - static_cast<decay_t>(first);