tcfail098.hs:12:10: error:
    Could not deduce (Bar a0)
    from the context: Bar a
      bound by an instance declaration: Bar a => Bar Bool
      at tcfail098.hs:12:10-26
    The type variable ‘a0’ is ambiguous
    In the ambiguity check for an instance declaration
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the instance declaration for ‘Bar Bool’