7.6.1 RC1 panic "coVarsOfTcCo:Bind"

[Ganesh Sittampalam]

Hi,

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.

Cheers,

Ganesh

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]
                                       
(main:Darcs.Witnesses.Ordered.FL{tc r1Dy1} prim{tv ty} [tv]
                                        
main:Darcs.Witnesses.Ordered.:>{tc r1Dyc}
main:Darcs.Witnesses.Ordered.FL{tc r1Dy1}
                                                                                    
prim{tv ty} [tv])>
                                     ghc-prim:GHC.Types.~{(w) tc 31Q}
main:Darcs.Test.Patch.WithState.WithState{tc r1LL8}
                                                                       
(Sym cobox{v a6CSH} [lid])
                                                                       
<main:Darcs.Witnesses.Ordered.FL{tc r1Dy1}
                                                                          
prim{tv ty} [tv]
                                                                        
main:Darcs.Witnesses.Ordered.:>{tc r1Dyc} main:Darcs.Witnesses.Order
ed.FL{tc r1Dy1}
                                                                                                                    
prim{tv ty} [tv]>)

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