Note [Improvement from Ground Wanteds]

Suppose class C b a => D a b
and consider
  [W] D Int Bool
Is there any point in emitting [W] C Bool Int?  No!  The only point of
emitting superclass constraints for W constraints is to get
improvement, extra unifications that result from functional
dependencies.  See Note [Why adding superclasses can help] above.

But no variables means no improvement; case closed.