panic compiling associated type synonyms

Trying to compile this:

{-# OPTIONS -fglasgow-exts #-}
data Cons a b
data Nil
data Foo a = Foo 
class (Appendt l0 l1 ~ l2) => Append l0 l1 l2 where
    type Appendt l0 l1 :: *
    append :: Foo l0 -> Foo l1 -> Foo l2
instance (Appendt b l1 ~ l2) => Append (Cons a b) l1 (Cons c l2) where
    type Appendt (Cons a b) l1 = Cons a (Appendt b l1)

gives this error:

ghc-6.8.1: panic! (the 'impossible' happened)
  (GHC version 6.8.1 for i386-unknown-linux):
        Coercion.splitCoercionKindOf
    base:GHC.Prim.trans{(w) tc 34y}
      (base:GHC.Prim.sym{(w) tc 34v}
         ((main:Main.:CoF:R1Appendt{tc roB})
            a{tv aph} [sk] b{tv ape} [sk] l1{tv apf} [sk]))
      t_apM{tv} [tau]
    <pred>(main:Main.:R1Appendt{tc ros})
            a{tv aph} [sk] b{tv ape} [sk] l1{tv apf} [sk]
            ~
          t_apM{tv} [tau]

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

i've tried to reduce the code to the minimum that will trigger the panic.

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	sanzhiyan@gmail.com
Operating system	Unknown
Architecture	Unknown