This program using the derive package:

{-# LANGUAGE TemplateHaskell, EmptyDataDecls #-}
import Data.DeriveTH
import Data.Derive.Enum
data Void
$(derive makeEnum ''Void)

produces a compiler panic:

rwbarton@functor:~/code/haskell$ ghc --make panic
[1 of 1] Compiling Main             ( panic.hs, panic.o )
Loading package base ... linking ... done.
Loading package array-0.1.0.0 ... linking ... done.
Loading package packedstring-0.1.0.0 ... linking ... done.
Loading package containers-0.1.0.1 ... linking ... done.
Loading package pretty-1.0.0.0 ... linking ... done.
Loading package template-haskell ... linking ... done.
Loading package old-locale-1.0.0.0 ... linking ... done.
Loading package old-time-1.0.0.0 ... linking ... done.
Loading package filepath-1.1.0.0 ... linking ... done.
Loading package directory-1.0.0.0 ... linking ... done.
Loading package mtl-1.1.0.1 ... linking ... done.
Loading package unix-2.3.0.0 ... linking ... done.
Loading package process-1.0.0.0 ... linking ... done.
Loading package random-1.0.0.0 ... linking ... done.
Loading package derive-0.1.1 ... linking ... done.
ghc-6.8.2: panic! (the 'impossible' happened)
  (GHC version 6.8.2 for i386-unknown-linux):
	TcPat.checkArgs

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

Matt Morrow has reproduced this in a ghc 6.10 prelease.

I don't particularly need a derived Enum instance for Void, but the error message told me to report a bug.