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>));
   

[ 2020-09-06; Reflector prioritization ]

Set priority to 2 during reflector discussions.

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.