newtype/deriving/fundep bug in GHC

This crashes GHC (on MacOS X, haven't tried others):

module DerivingBug where
    {
    class C a b | b -> a;
    instance C Int Bool;
    newtype T a = MkT Bool deriving C;
    }

$ ghc -fglasgow-exts -c DerivingBug.hs
ghc-5.04.3: panic! (the `impossible' happened, GHC version 5.04.3):
        basicTypes/VarEnv.lhs:173: Non-exhaustive patterns in case


Please report it as a compiler bug to glasgow-haskell-bugs@xxxxxxxxxxx,
or http://sourceforge.net/projects/ghc/.

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 5.04.3
$ uname -a
Darwin Avebury.local. 6.4 Darwin Kernel Version 6.4: Wed Jan 29 18:50:42 
PST 2003; root:xnu/xnu-344.26.obj~1/RELEASE_PPC  Power Macintosh powerpc

-- 
Ashley Yakeley, Seattle WA