GHC forbids kind signatures in data family instances
If I say
class C (a :: k) instance (C :: Type -> Constraint) a
GHC is happy. Note that this is actually useful to do.
But if I say
data family D (a :: k) data instance (D :: Type -> Type) a
GHC is unhappy:
Malformed head of type or class declaration: (D :: Type -> Type) a
Fixing this will require a change to the
FamEqn type, which doesn't have enough flexibility to deal with a kind signature there.