T7194.hs:18:35:
    Couldn't match expected type ‛b0’ with actual type ‛F a’
      because type variable ‛a’ would escape its scope
    This (rigid, skolem) type variable is bound by
      the type signature for g :: C (F a) => a -> Int
      at T7194.hs:17:23-41
    Relevant bindings include
      y :: a (bound at T7194.hs:18:20)
      g :: a -> Int (bound at T7194.hs:18:18)
      x :: b0 (bound at T7194.hs:17:9)
    In the expression: foo y
    In the first argument of ‛length’, namely ‛[x, foo y]’