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3490. ranges::drop_while_view::begin() is missing a precondition

Section: 26.7.13.2 [range.drop.while.view] Status: WP Submitter: Michael Schellenberger Costa Opened: 2020-10-13 Last modified: 2021-02-26

Priority: 0

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Discussion:

Similar to ranges::filter_view 26.7.8.2 [range.filter.view] p3, ranges::drop_while_view should have a precondition on its begin() method that the predicate is set.

I propose to add as 26.7.13.2 [range.drop.while.view] p3:

Preconditions: pred_.has_value().  

[2020-11-07; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4868.

1. Modify 26.7.13.2 [range.drop.while.view] as indicated:

   Since we usually don't rely on implicit bool conversion in Preconditions: elements an explicit "is true" has been added. Editorial fixes of the referenced paragraph 26.7.13.2 [range.drop.while.view] p3 and similar places have been requested separately.

   constexpr auto begin();
   

   -?- Preconditions: pred_.has_value() is true.

   -3- Returns: ranges::find_if_not(base_, cref(*pred_)).

   -4- […]