Steps to reproduce:

1. Clone Cabal HEAD
2. cabal configure --with-ghc=/path/to/ghc-head
3. dist/setup/setup build

Actual results:

[ 8 of 78] Compiling Distribution.Text ( Distribution/Text.hs, dist/build/Distribution/Text.o )
[ 9 of 78] Compiling Distribution.Version ( Distribution/Version.hs, dist/build/Distribution/Version.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.9.20140911 for x86_64-unknown-linux):
        Simplifier ticks exhausted
    When trying UnfoldingDone GHC.Word.$fNumWord8_$c+
    To increase the limit, use -fsimpl-tick-factor=N (default 100)
    If you need to do this, let GHC HQ know, and what factor you needed
    To see detailed counts use -ddump-simpl-stats
    Total ticks: 813720

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

I know that it still fails with tick factor 4096; I haven't found a tick amount which makes it succeed.