Proposed resolution:

This wording is relative to N5014.

1. Modify [span.cons] as indicated:

   template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);
   

   -?- Mandates: If `extent` is not equal to `dynamic_extent` and `ranges::size(r)` is a constant expression, then `ranges::size(r) == extent` is `true`.

   -16- Constraints: Let `U` be remove_reference_t<ranges::range_reference_t<R>>. […]

It is preferable to reject span<int, 42>(views::empty<int>) at compile-time after P2280R4, since applying `ranges::size` on those ranges is a constant expression now.