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darcs 2.8 fails to compile with ghc 7.6

Ganesh Sittampalam reports the following failure when building darcs 2.8 with ghc 7.6 RC1:

ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.6.0.20120822 for i386-unknown-mingw32):
        ds_ev_term
    cobox{v a6Aho} [lid] `cast`
(<main:Darcs.Test.Patch.WithState.WithState{tc r1oms}
                                    model{tv t4H} [tv]
                                    (main:Darcs.Witnesses.Ordered.FL{tc
r1iNn} prim{tv t4D} [tv]
                                     main:Darcs.Witnesses.Ordered.:>{tc
r1iNy} main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv])>
                                 ghc-prim:GHC.Types.~{(w) tc 31Q}
main:Darcs.Test.Patch.WithState.WithState{tc r1oms}
                                                                    (Sym
cobox{v a6AgE} [lid])

<main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv]

main:Darcs.Witnesses.Ordered.:>{tc r1iNy}
main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv]>)
    (let {EvBinds{cobox{v a6zY5} [lid]
                    = cobox{v a6Aho} [lid] `cast`
(<main:Darcs.Test.Patch.WithState.WithState{tc r1oms}
                                                      model{tv t4H} [tv]

(main:Darcs.Witnesses.Ordered.FL{tc r1iNn}
                                                         prim{tv t4D} [tv]

main:Darcs.Witnesses.Ordered.:>{tc r1iNy}
main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv])>

ghc-prim:GHC.Types.~{(w) tc 31Q}
main:Darcs.Test.Patch.WithState.WithState{tc r1oms}

(Sym

cobox{v a6AgE} [lid])

<main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv]

main:Darcs.Witnesses.Ordered.:>{tc r1iNy}
main:Darcs.Witnesses.Ordered.FL{tc r1iNn}

prim{tv t4D} [tv]>)
                  cobox{v a6zY6} [lid] = CO <t{tv a6zY1} [tv]>}}
     cobox{v a6zY5} [lid] <xx{tv a6zY3} [sk]> <yy{tv a6zY4} [sk]>
     -> <t{tv a6zY1} [tv]>)

Project + dependencies can be found at http://urchin.earth.li/~ganesh/temp/tcproblem.tar.bz2 (run build.sh).

Trac metadata
Trac field Value
Version 7.6.1-rc1
Type Bug
TypeOfFailure OtherFailure
Priority high
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
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