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3284. random_access_iterator semantic constraints accidentally promote difference type using unary negate

Section: 25.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25

Priority: 0

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Discussion:

25.3.4.13 [iterator.concept.random.access]/p2.7 says:

(b += -n) is equal to a

Unary minus can do integer promotion. That is not the intent here.

[2019-10-12 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five 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. (2.7) — (b += D(-n)) is equal to a.

   4. […]