{-# LANGUAGE TypeSynonymInstances #-}
unit p where
    signature H where
        data T a
    module M where
        import H
        instance Functor T

unit q where
    module H where
        -- No good!
        type T a = a

unit r where
    dependency p[H=q:H]

{-
If we get:

     The type synonym ‘T’ should have 1 argument, but has been given none 
     In the instance declaration for ‘Functor T’

that is too late! Need to catch this earlier.
-}