T8034.hs:6:3: error:
    • Couldn't match type ‘F a0’ with ‘F a’
      Expected type: F a -> F a
        Actual type: F a0 -> F a0
      NB: ‘F’ is a non-injective type family
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘foo’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method: foo :: forall a. C a => F a -> F a
      In the class declaration for ‘C’