tcfail097.hs:5:6:
    Could not deduce (Eq a0) arising from the ambiguity check for ‘f’
    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: forall a. Eq a => Int -> Int
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature for ‘f’: f :: Eq a => Int -> Int