Use kind equality in instance context in type family instance

With TypeInType enabled, I can use a closed type family to compute the return kind of a type family from its arguments. But an equivalent open type family used in the same way triggers a compilation failure, as if the relevant instance of that open type family were not considered. (Of course, there may be some subtle prohibition of which I am not aware at the present time.) I will attach a test case.

EDIT: This ticket got repurposed at ticket:12432#comment:122754 and below. The original problem described above is fixed, but the commentary is useful to keep around and on this ticket.