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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. […]