Proposed resolution:

This wording is relative to N4910.

1. Modify [range.take.while.sentinel] as indicated:

   constexpr sentinel(sentinel<!Const> s)
     requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
   

   -2- Effects: Initializes end_ with std::move(s.end_) and pred_ with s.pred_.