viewable_range mishandles lvalue move-only views
Casey Carter

Created on 2020-08-29.00:00:00 last changed 1 week ago


Date: 2020-09-13.15:00:23

Proposed resolution:

This wording is relative to N4861.

  1. Modify [range.refinements] as indicated:

    -5- The viewable_range concept specifies the requirements of a range type that can be converted to a view safely.

    template<class T>
      concept viewable_range =
        range<T> && (borrowed_range<T> || view<remove_cvref_t<T>>);
        ((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>) ||
        (!view<remove_cvref_t<T>> && borrowed_range<T>));
Date: 2020-09-15.00:00:00

[ 2020-09-06; Reflector prioritization ]

Set priority to 2 during reflector discussions.

Date: 2020-08-29.00:00:00

The viewable_range concept ([range.refinements]) and the views:all range adaptor ([range.all]) are duals: viewable_range is intended to admit exactly types T for which views::all(declval<T>()) is well-formed. (Recall that views::all(meow) is a prvalue whose type models view when it is well-formed.) Before the addition of move-only view types to the design, this relationship was in place (modulo an incredibly pathological case: a volatile value of a view type with volatile-qualified begin and end models viewable_range but is rejected by views::all unless it also has a volatile-qualified copy constructor and copy assignment operator). Adding move-only views to the design punches a bigger hole, however: viewable_range admits lvalues of move-only view types for which views::all is ill-formed because these lvalues cannot be decay-copied.

It behooves us to restore the correspondence between viewable_range and views::all so that instantiations of components constrained with viewable_range (which often appears indirectly as views::all_t<R> in deduction guides) continue to be well-formed when the constraints are satisfied.

Date User Action Args
2020-09-13 15:00:23adminsetmessages: + msg11482
2020-08-29 17:05:04adminsetmessages: + msg11467
2020-08-29 00:00:00admincreate