tc168.hs:17:1: error:
    • Could not deduce (C a (a1, b0))
      from the context: C a (a1, b)
        bound by the inferred type for ‘g’:
                   forall a a1 b. C a (a1, b) => a -> a1
        at tc168.hs:17:1-16
      The type variable ‘b0’ is ambiguous
    • In the ambiguity check for the inferred type for ‘g’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the inferred type
        g :: forall a1 a2 b. C a1 (a2, b) => a1 -> a2