comment added

Just for the record, it isn't just type signatures in patterns:

[rl@rl-lap stuff]$ cat R.hs
$(id [d| f :: forall a . a -> a
         f x = (x :: a) |])
[rl@rl-lap stuff]$ ghc -fglasgow-exts -c R.hs
ghc-6.4.2: panic! (the `impossible' happened, GHC version 6.4.2):
        Failed binder lookup: a{tv a189}