T1897b.hs:16:1: error:
    • Couldn't match type ‘Depend a’ with ‘Depend a0’
      Expected type: t (Depend a) -> Bool
        Actual type: t (Depend a0) -> Bool
      NB: ‘Depend’ is a non-injective type family
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for the inferred type for ‘isValid’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the inferred type
        isValid :: forall (t :: * -> *) a.
                   (Foldable t, Bug a) =>
                   t (Depend a) -> Bool