GHC 6.12.1 panic
GHC 6.12.1 reports the following when compiling the attached module:
$ ghc --make Trampoline.hs -O[1 of 1] Compiling Control.Concurrent.SCC.Trampoline ( Trampoline.hs, Trampoline.o ) ghc: panic! (the 'impossible' happened) (GHC version 6.12.1 for x86_64-unknown-linux): expectJust chooseExternalIds: ds_d1HQ Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
So I'm dutifully reporting it. The module in question could probably be easily trimmed down, but I'm too sleepy at the moment. Let me know if you need that.