Proposed resolution:

This wording is relative to N4830.

1. Modify [range.take.while.view], class template take_while_view synopsis, as indicated:

   namespace std::ranges {
     […]
     template<classinput_range R, class Pred>
       requires viewable_range<R> && 
           indirect_unary_predicate<const Pred, iterator_t<R>>
         take_while_view(R&&, Pred) -> take_while_view<all_view<R>, Pred>;
   }
   

2. Modify [range.drop.view], class template drop_view synopsis, as indicated:

   namespace std::ranges {
     […]
     template<classinput_range R>
       requires viewable_range<R>
         drop_view(R&&, range_difference_t<R>) -> drop_view<all_view<R>>;
   }
   

3. Modify [range.drop.while.view], class template drop_while_view synopsis, as indicated:

   namespace std::ranges {
     […]
     template<classinput_range R, class Pred>
       requires viewable_range<R> && 
           indirect_unary_predicate<const Pred, iterator_t<R>>
         drop_while_view(R&&, Pred) -> drop_while_view<all_view<R>, Pred>;
   }
   

[ 2020-02 Status to NAD on Thursday morning in Prague. ]

After adoption of P1035 range adaptor closure is described to take a viewable_range as input, and return a view as output. The deduction-guides for take_while_view, drop_view, and drop_while_view do not currently impose the viewable_range requirement.