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3670. Cpp17InputIterators don't have integer-class difference types

Section: 25.6.4.3 [range.iota.iterator] Status: C++23 Submitter: Casey Carter Opened: 2022-02-04 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

25.6.4.3 [range.iota.iterator] defines 

using iterator_category = input_iterator_tag; // present only if W models incrementable

but when difference_type is an integer-class type the iterator does not meet the Cpp17InputIterator requirements.

[2022-02-07; Daniel comments]

As requested by LWG 3376(i) and wording implemented by P2393R1, integer-class types are no longer required to have class type.

Previous resolution [SUPERSEDED]:

This wording is relative to N4901.

  1. Modify 25.6.4.3 [range.iota.iterator], class iota_view::iterator synopsis, as indicated:

    namespace std::ranges {
  template<weakly_incrementable W, semiregular Bound>
    requires weakly-equality-comparable-with<W, Bound> && copyable<W>
  struct iota_view<W, Bound>::iterator {
    […]
    using iterator_category = input_iterator_tag; // present only if W models incrementable and 
                                                  // IOTA-DIFF-T(W) is not a class type
    using value_type = W;
    using difference_type = IOTA-DIFF-T(W);
    […]
  };
}

[2022-02-07; Casey Carter provides improved wording]

[2022-03-04; Reflector poll]

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

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

  1. Modify 25.6.4.3 [range.iota.iterator], class iota_view::iterator synopsis, as indicated:

    namespace std::ranges {
  template<weakly_incrementable W, semiregular Bound>
    requires weakly-equality-comparable-with<W, Bound> && copyable<W>
  struct iota_view<W, Bound>::iterator {
    […]
    using iterator_category = input_iterator_tag; // present only if W models incrementable and 
                                                  // IOTA-DIFF-T(W) is an integral type
    using value_type = W;
    using difference_type = IOTA-DIFF-T(W);
    […]
  };
}