If I say this

{-# LANGUAGE TypeFamilies, PolyKinds, DataKinds #-}

module Bug where

class C a where
  type F (x :: Maybe a)

then GHCi says this

*Bug> :k C
C :: k -> Constraint

That's silly. C should have kind Type -> Constraint, because the usage of a in the kind of the associated type constraints a's kind.

Will fix.