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3546. common_iterator's postfix-proxy is not quite right

Section: 23.5.4.5 [common.iter.nav] Status: WP Submitter: Tim Song Opened: 2021-04-23 Last modified: 2021-06-12

Priority: Not Prioritized

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Discussion:

P2259R1 modeled common_iterator::operator++(int)'s postfix-proxy class on the existing proxy class used by common_iterator::operator->, but in doing so it overlooked two differences:

The proposed wording has been implemented and tested.

[2021-05-10; Reflector poll]

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

[2021-05-17; Reflector poll]

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

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

  1. Modify 23.5.4.5 [common.iter.nav] as indicated:

    decltype(auto) operator++(int);
    

    -4- Preconditions: holds_alternative<I>(v_) is true.

    -5- Effects: If I models forward_iterator, equivalent to:

    common_iterator tmp = *this;
    ++*this;
    return tmp;
    

    Otherwise, if requires (I& i) { { *i++ } -> can-reference; } is true or constructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>> is false, equivalent to:

    return get<I>(v_)++;
    

    Otherwise, equivalent to:

    postfix-proxy p(**this);
    ++*this;
    return p;
    

    where postfix-proxy is the exposition-only class:

    class postfix-proxy {
      iter_value_t<I> keep_;
      postfix-proxy(iter_reference_t<I>&& x)
        : keep_(std::moveforward<iter_reference_t<I>>(x)) {}
    public:
      const iter_value_t<I>& operator*() const {
        return keep_;
      }
    };