Panic in coVarsOfTcCo

Ganesh says: I'm getting the panic below when building darcs 2.8 with GHC 7.6. It'll take some effort to cut it down or give repro instructions for an uncut-down version (I needed to hack a lot of underlying packages to be able to even get as far as doing this build), so could someone confirm that it's worth it before I do so? I can't spot anything already reporting this in trac.

ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.6.0.20120810 for i386-unknown-mingw32):
        coVarsOfTcCo:Bind
    cobox{v a6Czs} [lid]
      = cobox{v a6CTr} [lid] `cast`
(<main:Darcs.Test.Patch.WithState.WithState{tc r1LL8}
                                        model{tv tC} [tv]
   ..blah blah..

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture