Type instance unused variable warnings

I noticed (while reading !3364 (closed)) that in testsuite/tests/indexed-types/should_compile/T16632.stderr we have the error warning:

Defined but not used on the right hand side: type variable ‘b’

I think this message predates the explicit forall, when the LHS was the closest thing we had to a definition. I wonder if we should be doing instead symmetrical like

Used on one side of the equation but not the other: type variable ‘b’

Also, I assume the intended resolution is to use _ for the LHS, but I don't actually know whether we allow that on the RHS.

CC @RyanGlScott