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### 3559. Semantic requirements of `sized_range` is circular

Section: 26.4.3 [range.sized] Status: WP Submitter: Tim Song Opened: 2021-06-03 Last modified: 2021-10-14

Priority: Not Prioritized

View all other issues in [range.sized].

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Discussion:

26.4.3 [range.sized] p2.1 requires that for a `sized_range t`, `ranges::size(t)` is equal to `ranges::distance(t)`. But for a `sized_range`, `ranges::distance(t)` is simply `ranges::size(t)` cast to the difference type.

Presumably what we meant is that `distance(begin, end)` — the actual distance you get from traversing the range — is equal to what `ranges::size` produces, and not merely that casting from the size type to difference type is value-preserving. Otherwise, `sized_range` would be more or less meaningless.

[2021-06-14; Reflector poll]

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

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 26.4.3 [range.sized] as indicated:

   >     template<class T>
   >       concept sized_range =
   >         range<T> &&
   >         requires(T& t) { ranges::size(t); };

   > > -2- Given an lvalue `t` of type `remove_reference_t<T>`, `T` models `sized_range` only if

   > > 1. (2.1) — `ranges::size(t)` is amortized 𝒪(1), does not modify `t`, and is equal to `ranges::distance(ranges::begin(t), ranges::end(t)t)`, and

   > > 2. (2.2) — if `iterator_t<T>` models `forward_iterator`, `ranges::size(t)` is well-defined regardless of the evaluation of `ranges::begin(t)`.