he list must have at least one @Match@ in it.

pprMatches :: (OutputableBndrId idR, Outputable body)
           => MatchGroup idR body -> SDoc
pprMatches MG { mg_alts = matches }
    = vcat (map pprMatch (map unLoc (unLoc matches)))
      -- Do
pprMatches :: (OutputableBndrId idR, Outputable body)
           => MatchGroup idR body -> SDoc
pprMatches MG { mg_alts = matches }
    = vcat (map pprMatch (map unLoc (unLoc matches)))
pprMatches MG { mg_alts = matches }
    = vcat (map pprMatch (map unLoc (unLoc matches)))