type families GHC "impossible happened"

the code: http://hpaste.org/2860?#a0

produces the following error:

$ ghci Foldable.hs
GHCi, version 6.8.0.20070921: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Data.Collections.Foldable ( Foldable.hs, interpreted )
ghc-6.8.0.20070921: panic! (the 'impossible' happened)
  (GHC version 6.8.0.20070921 for i386-unknown-linux):
        check_tau_type
    main:Data.Collections.Foldable.FoldElem{tc rE4} t{tv aFj} [tv]

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