ghc: panic! (the 'impossible' happened)

I downloaded the beta of the Haskell Platform installer from http://hackage.haskell.org/platform/2009.2.0.1/haskell-platform-2009.2.0.1-beta1-i386.dmg

and when I tried to cabal install the HEAD version of darcs, I got this error message:

[ 57 of 134] Compiling Darcs.Repository.Prefs ( src/Darcs/Repository/Prefs.lhs, dist/build/Darcs/Repository/Prefs.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.3 for i386-apple-darwin):
	cat_evals
    base:GHC.Arr.Array{d ras}
    [ww{v a2ZC6} [lid], ww1{v a2ZC7} [lid], ww2{v a2ZC8} [lid]]
    [!, !, _, _]

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

Darcs repo version (last patch; see attached for context file):

Mon Jun  1 10:19:28 BST 2009  Petr Rockai <me@mornfall.net>
  * Remove tentativelyMerge from Gorsvet, as it's unused and confusing.