tcfail116.hs:6:5: error:
    • Could not deduce (Foo a0)
      from the context: Foo a
        bound by the type signature for:
                   bug :: Foo a => ()
        at tcfail116.hs:6:5-13
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘bug’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method: bug :: forall a. Foo a => ()
      In the class declaration for ‘Foo’