ghc-6.12.3 fails to compile xmonad-0.9.1 tests

Long story:

$ cabal unpack xmonad-0.9.1
$ cd xmonad-0.9.1
$ sed -i -e 's/Werror/Wall/g' xmonad.cabal # *shrug*
$ cabal configure -ftesting
$ cabal build

got:

[ 8 of 11] Compiling Properties       ( tests/Properties.hs, dist/build/xmonad/xmonad-tmp/Properties.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.3 for x86_64-unknown-linux):
        bind_args
    base:GHC.Real.D:Integral{d r3W}
    [tpl_B2{v} [lid], tpl_B3{v} [lid], tpl_B4{v} [lid],
     tpl_B5{v} [lid], tpl_B6{v} [lid], tpl_B7{v} [lid], tpl_B8{v} [lid],
     tpl_B9{v} [lid], tpl_Ba{v} [lid]]
    [TYPE ghc-prim:GHC.Types.Int{(w) tc 3J},
     base:GHC.Real.$fOrdRatio_$dEq{v reC} [gid]
     `cast` (ghc-prim:GHC.Prim.trans{(w) tc 34y}
               <pred>base:GHC.Classes.Eq{tc 23}
                       (ghc-prim:GHC.Prim.sym{(w) tc 34v}
                          (main:Properties.NTCo:NonNegative{tc r36K}
                             (base:GHC.Real.Ratio{tc 31y}
                                integer-gmp:GHC.Integer.Type.Integer{tc 314})))
               <pred>base:GHC.Classes.Eq{tc 23}
                       (main:Properties.NTCo:NonNegative{tc r36K}
                          ghc-prim:GHC.Types.Int{(w) tc 3J})
             :: <pred><pred>base:GHC.Classes.Eq{tc 23}
                              (base:GHC.Real.Ratio{tc 31y}
                                 integer-gmp:GHC.Integer.Type.Integer{tc 314})
                        ~
                      <pred>base:GHC.Classes.Eq{tc 23}
                              ghc-prim:GHC.Types.Int{(w) tc 3J}),
...

(whole error message is in whole-failure.txt)

As there is many packages involved i have tried to shrink error to package w/o external depends (instance_test.tar.gz). But it has simpler error, so it can be another bug:

$ tests:./make_bad.sh 
[1 of 3] Compiling Test_QuickCheck  ( Test_QuickCheck.hs, Test_QuickCheck.o )
[2 of 3] Compiling XMonad_StackSet  ( XMonad_StackSet.hs, XMonad_StackSet.o )
[3 of 3] Compiling Properties       ( Properties.hs, Properties.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.3 for x86_64-unknown-linux):
        expectJust chooseExternalIds: wild_B1

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

ghc-6.10.4 compiles even this stripped thing.

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case	yet
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