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3601. common_iterator's postfix-proxy needs indirectly_readable

Section: [common.iter.nav] Status: C++23 Submitter: Casey Carter Opened: 2021-09-24 Last modified: 2023-11-22

Priority: Not Prioritized

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It would appear that when P2259R1 added postfix-proxy to common_iterator::operator++(int) LWG missed a crucial difference between operator++(int) and operator-> which uses a similar proxy: operator-> requires the wrapped type to be indirectly_readable, but operator++(int) does not. Consequently, operations that read from the wrapped type for the postfix-proxy case in operator++(int) are not properly constrained to be valid.

[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 [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;
    return tmp;

    Otherwise, if requires (I& i) { { *i++ } -> can-reference; } is true or indirectly_readable<I> && 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: […]