tcfail037.hs:7:11: error: [GHC-87543]
    Ambiguous occurrence ‘+’.
    It could refer to
       either ‘Prelude.+’,
              imported from ‘Prelude’ at tcfail037.hs:3:8-17
              (and originally defined in ‘GHC.Num’),
           or ‘ShouldFail.+’, defined at tcfail037.hs:10:5.