T9109.hs:8:13: error:
    • Couldn't match expected type ‘p’ with actual type ‘Bool’
        ‘p’ is untouchable
          inside the constraints: a ~ Bool
          bound by a pattern with constructor: GBool :: G Bool,
                   in an equation for ‘foo’
          at T9109.hs:8:5-9
      ‘p’ is a rigid type variable bound by
        the inferred type of foo :: G a -> p at T9109.hs:8:1-16
      Possible fix: add a type signature for ‘foo’
    • In the expression: True
      In an equation for ‘foo’: foo GBool = True
    • Relevant bindings include foo :: G a -> p (bound at T9109.hs:8:1)