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3277. Pre-increment on prvalues is not a requirement of weakly_incrementable

Section: 25.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2021-02-25

Priority: 0

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Discussion:

See 25.3.4.13 [iterator.concept.random.access]/2.6, which shows ++ being applied to a prvalue iterator.

A similar change has already been made to 26.6.4.2 [range.iota.view]/4.6.

Suggest priority P0 or P1 because it effects the definition of a concept.

[2019-09-24 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify 25.3.4.13 [iterator.concept.random.access] as indicated:

   -2- Let a and b be valid iterators of type I such that b is reachable from a after n applications of ++a, let D be iter_difference_t<I>, and let n denote a value of type D. I models random_access_iterator only if

   1. (2.1) — (a += n) is equal to b.

   2. […]

   3. (2.6) — If (a + D(n - 1)) is valid, then (a + n) is equal to ++[](I c){ return ++c; }(a + D(n - 1)).

   4. […]