Proposed resolution:

This wording is relative to N4910.

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

   [Drafting note: s and count_ usually have different types, but I think it's safe to perform comparison and subtraction, as count_ is non-negative as long as the behavior is well-defined.]

   […]
   constexpr auto size() requires sized_range<V> {
     const auto s = ranges::size(base_);
     const auto c = static_cast<decltype(s)>(count_);
     return static_cast<decltype(s)>(s < ccount_ ? 0 : s - ccount_);
   }
   
   constexpr auto size() const requires sized_range<const V> {
     const auto s = ranges::size(base_);
     const auto c = static_cast<decltype(s)>(count_);
     return static_cast<decltype(s)>(s < ccount_ ? 0 : s - ccount_);
   }
   […]