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3610. iota_view::size sometimes rejects integer-class types

Section: 26.6.4.2 [range.iota.view] Status: WP Submitter: Jiang An Opened: 2021-09-29 Last modified: 2022-02-10

Priority: Not Prioritized

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Discussion:

It seems that the iota_view tends to accept integer-class types as its value types, by using is-integer-like or is-signed-integer-like through the specification, although it's unspecified whether any of them satisfies weakly_incrementable. However, the requires-clause of iota_view::size (26.6.4.2 [range.iota.view] p16) uses (integral<W> && integral<Bound>), which sometimes rejects integer-class types.

Should we relax the restrictions by changing this part to (is-integer-like<W> && is-integer-like<Bound>)?

[2021-10-14; Reflector poll]

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

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 26.6.4.2 [range.iota.view] as indicated:

   constexpr auto size() const requires see below;
   

   -15- Effects: Equivalent to:

   if constexpr (is-integer-like<W> && is-integer-like<Bound>)
     return (value_ < 0)
       ? ((bound_ < 0)
         ? to-unsigned-like(-value_) - to-unsigned-like(-bound_)
         : to-unsigned-like(bound_) + to-unsigned-like(-value_))
       : to-unsigned-like(bound_) - to-unsigned-like(value_);
   else
     return to-unsigned-like(bound_ - value_);
   

   -16- Remarks: The expression in the requires-clause is equivalent to:

   (same_as<W, Bound> && advanceable<W>) || (integralis-integer-like<W> && integralis-integer-like<Bound>) ||
     sized_sentinel_for<Bound, W>