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### 3363. `drop_while_view` should opt-out of `sized_range`

Section: 26.7.13.2 [range.drop.while.view] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2020-01-07 Last modified: 2021-02-25

Priority: 1

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Discussion:

If `drop_while_view`'s `iterator_t` and `sentinel_t` model `forward_iterator` and `sized_sentinel_for`, it will incorrectly satisfy `sized_range` thanks to the size member function inherited from `view_interface`.

Because it has to compute its `begin()`, it can never model `sized_range` due to not meeting its non-amortized O(1) requirement.

[2020-01-16 Priority set to 1 after discussion on the reflector.]

[2020-02-10; Prague 2020; Casey comments and provides alternative wording]

The fundamental issue here is that both `ranges::size` and `view_interface::size` (it should be unsurprising that many of the "default" implementations of member `meow` in `view_interface` look just like fallback cases of the `ranges::meow` CPO) have a case that returns the difference of `ranges::end` and `ranges::begin`. If `begin` and `end` are amortized* O(1) but not "true" O(1), then the resulting `size` operation is amortized O(1) and not "true" O(1) as required by the `sized_range` concept. I don't believe we can or should fix this on a case by case basis, but we should instead relax the complexity requirement for `sized_range` to be handwavy-amortized O(1).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Add the following specialization to 26.2 [ranges.syn]:

>    >     // [drop.while.view], drop while view
>    >       template<view V, class Pred>
>    >         requires input_range<V> && is_object_v<Pred> &&
>    >           indirect_unary_predicate<const Pred, iterator_t<V>>
>    >         class drop_while_view;

>    >       template<view V, class Pred>
>    >         inline constexpr bool disable_sized_range<drop_while_view<V, Pred>> = true;

>    >       namespace views { inline constexpr unspecified drop_while = unspecified; }

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

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

   > -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(t)`, and

   > 2. […]

   > -3- [Note: The complexity requirement for the evaluation of `ranges::size` is non-amortized, unlike the case for the complexity of the evaluations of `ranges::begin` and `ranges::end` in the `range` concept. — end note]