I tried this out on the panic we're currently getting for #3103:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.3.20111128 for x86_64-unknown-linux):
        tcIfaceGlobal (local): not found:
    base:GHC.Word.W#{d 6w}
    [(32R, Type constructor `base:GHC.Word.Word{tc 32R}'),
     (r6O, Identifier `base:GHC.Word.$fNumWord{v r6O}'),
     (r6P, Identifier `base:GHC.Word.$fEqWord{v r6P}'),
     (r6Q, Identifier `base:GHC.Word.$fNumWord1{v r6Q}'),
     (r6R, Identifier `base:GHC.Word.$fNumWord2{v r6R}'),
     (r6S, Data constructor `base:GHC.Word.W#{d r6S}'),
     (r6U, Identifier `base:GHC.Word.W#{v r6U}'),
     (r75, Identifier `base:GHC.Word.$fNumWord_$csignum{v r75}'),
     (r76, Identifier `base:GHC.Word.$fEqWord_$c/={v r76}'),
     (r77, Identifier `base:GHC.Word.$fEqWord_$c=={v r77}')]
{ Main.main
   GHC.defaultErrorHandler
    GHC.runGhc
     GhcMonad.>>=
      GhcMonad.>>=.\
       Main.main'
        Main.doMake
         GhcMake.load
          GhcMake.load2
           GhcMake.upsweep
            GhcMake.upsweep.upsweep'
             GhcMake.reTypecheckLoop
              GhcMake.typecheckLoop
               GhcMake.typecheckLoop.\
                TcRnMonad.initIfaceCheck
                 TcRnMonad.initTcRnIf
                  IOEnv.runIOEnv
                   IOEnv.thenM
                    IOEnv.thenM.\
                     TcIface.typecheckIface
                      TcIface.typecheckIface.\
                       LoadIface.loadDecls
                        LoadIface.loadDecl
                         TcIface.tcIfaceDecl
                          TcIface.tc_iface_decl
                           TcIface.tcIdInfo
                            MonadUtils.foldlM
                             TcIface.tcIdInfo.tcPrag
                              TcIface.tcUnfolding
                               TcIface.tcPragExpr
                                TcIface.tcIfaceExpr
                                 TcIface.tcIfaceAlt
                                  TcIface.tcIfaceDataCon }