Steps to reproduce:

1. Run "ghci -fglasgow-exts"
2. Enter: "let const :: (forall a. (forall b. a->b)) = \x::a -> (\y -> x) :: (forall b. b -> a)".

Result:

GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> let const :: (forall a. (forall b. a->b)) = \x::a -> (\y -> x) :: (forall b. b -> a)
ghc-6.8.2: panic! (the 'impossible' happened)
 (GHC version 6.8.2 for i386-unknown-linux):
	nameModule a{tv anJ}

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

Let me know if any more information would be helpful. This is all on a Ubuntu 8.04. I have included below the output of running the above with "ghci -v -fglasgow-exts":

ghci -v

GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Glasgow Haskell Compiler, Version 6.8.2, for Haskell 98, stage 2 booted by GHC version 6.8.2
Using package config file: /usr/lib/ghc-6.8.2/package.conf
wired-in package base mapped to base-3.0.1.0
wired-in package rts mapped to rts-1.0
wired-in package haskell98 mapped to haskell98-1.0.1.0
wired-in package template-haskell mapped to template-haskell-2.2.0.0
wired-in package ndp not found.
Hsc static flags: -static
*** Parser:
*** Desugar:
*** Simplify:
*** CorePrep:
*** ByteCodeGen:
Loading package base ... linking ... done.
*** Parser:
*** Desugar:
*** Simplify: 
*** CorePrep:
*** ByteCodeGen:
*** Parser:
*** Desugar:
*** Simplify:
*** CorePrep:
*** ByteCodeGen:
Prelude> let const :: (forall a. (forall b. a->b)) = \x::a -> (\y -> x) :: (forall b. b -> a)
*** Parser:
ghc-6.8.2: panic! (the 'impossible' happened)
 (GHC version 6.8.2 for i386-unknown-linux):
	nameModule a{tv anJ}

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