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3850. views::as_const on empty_view<T> should return empty_view<const T>

Section: 26.7.22.1 [range.as.const.overview] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

Currently, applying views::as_const to an empty_view<int> will result in an as_const_view<empty_view<int>>, and its iterator type will be basic_const_iterator<int*>.

This amount of instantiation is not desirable for such a simple view, in which case simply returning empty_view<const int> should be enough.

[2023-02-01; Reflector poll]

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

[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 26.7.22.1 [range.as.const.overview] as indicated:

   -2- The name views::as_const denotes a range adaptor object (26.7.2 [range.adaptor.object]). Let E be an expression, let T be decltype((E)), and let U be remove_cvref_t<T>. The expression views::as_const(E) is expression-equivalent to:

   1. (2.1) — If views::all_t<T> models constant_range, then views::all(E).

   2. (2.?) — Otherwise, if U denotes empty_view<X> for some type X, then auto(views::empty<const X>).

   3. (2.2) — Otherwise, if U denotes span<X, Extent> for some type X and some extent Extent, then span<const X, Extent>(E).

   4. (2.3) — Otherwise, if E is an lvalue, const U models constant_range, and U does not model view, then ref_view(static_cast<const U&>(E)).

   5. (2.4) — Otherwise, as_const_view(E).