tcfail097.hs:5:6: error:
    Could not deduce (Eq a0)
    from the context: Eq a
      bound by the type signature for:
                 f :: Eq a => Int -> Int
      at tcfail097.hs:5:6-23
    The type variable ‘a0’ is ambiguous
    In the ambiguity check for ‘f’
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature: f :: Eq a => Int -> Int