-1- A sequence Args of template arguments is said to model a concept C if Args satisfies C (13.5.2 [temp.constr.decl]) and meets all semantic requirements (if any) given in the specification of C.
-2- If the validity or meaning of a program depends on whether a sequence of template arguments models a concept, and the concept is satisfied but not modeled, the program is ill-formed, no diagnostic required.
-3- If the semantic requirements of a declaration's constraints ([structure.requirements]) are not modeled at the point of use, the program is ill-formed, no diagnostic required.

With this wording:

• Does random_access_range<span<int>> impose any semantic requirement at all? Because its specification is only code.
• How about viewable_range<span<int>>? Is it clear that we're only requiring transitively modeling concepts only in the first part of the disjunction which evaluates to true (so borrowed_range and not view)?
• Is it clear that viewable_range<drop_view<span<int>>> does it the other way around?