EmptyDataDecls + DeriveFunctor == Panic!

I get "ghc: panic! (the 'impossible' happened)" from GHC 6.12.1 with a simple two-line program

    {-# LANGUAGE EmptyDataDecls, DeriveFunctor #-}
    data Void a deriving Functor

Also on hpaste.

GHC says

    ghc: panic! (the 'impossible' happened)
      (GHC version 6.12.1 for i386-apple-darwin):
            TcPat.checkArgs

I asked on the #ghc IRC room and learned that

!ClaudiusMaximus: conal: ghc-6.12.3 here, same error. (GHC version 6.12.3 for x86_64-unknown-linux)

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	conal@conal.net
Operating system
Architecture