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I was trying to compile the latest darcs source (darcs get --lazy http://darcs.net) using ghc7. I had to modify darcs.cabal to add !NoMonoLocalBinds to 3 places (just search for extensions, and add it to the END of the lists).

When I do a build I get this:

[ 65 of 154] Compiling Darcs.Patch.V1.Commute ( src/Darcs/Patch/V1/Commute.lhs, dist/build/Darcs/Patch/V1/Commute.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.0.1 for i386-unknown-linux):
	Template variable unbound in rewrite rule
    sg_s2vkQ{tv} [co]
    [x{tv a2uFt} [sk], y{tv a2uFu} [sk], sg_s2vkQ{tv} [co],
     sc_s2vkR{v} [lid], sc_s2vkS{v} [lid]]
    [x{tv X2uJc} [sk], y{tv X2uJe} [sk], sg_s2vkQ{tv} [co],
     sc_s2vkR{v} [lid], sc_s2vkS{v} [lid]]
    [TYPE y{tv a2uFu} [sk], TYPE y{tv a2uFu} [sk],
     TYPE x{tv a2uFt} [sk], TYPE x{tv a2uFt} [sk],
     TYPE x{tv a2uFt} [sk], sc_s2vkS{v} [lid], sc_s2vkR{v} [lid],
     darcs-2.5:Darcs.Witnesses.Ordered.NilRL{v ruI5} [gid[DataCon]]
       @ darcs-2.5:Darcs.Patch.V1.Core.Patch{tc r1EYF}
       @ x{tv a2uFt} [sk]
       @ x{tv a2uFt} [sk]
       @ x{tv a2uFt} [sk]]
    [TYPE y{tv a2uFu} [sk], TYPE y{tv a2uFu} [sk],
     TYPE x{tv a2uFt} [sk], TYPE x{tv a2uFt} [sk],
     TYPE x{tv a2uFt} [sk], wild_s2vhT{v} [lid], sc_s2vke{v} [lid],
     darcs-2.5:Darcs.Witnesses.Ordered.NilRL{v ruI5} [gid[DataCon]]
       @ darcs-2.5:Darcs.Patch.V1.Core.Patch{tc r1EYF}
       @ x{tv a2uFt} [sk]
       @ x{tv a2uFt} [sk]
       @ x{tv a2uFt} [sk]]

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

As an additional data point, this module builds correctly without !NoMonoLocalBinds. The reason I was adding !NoMonoLocalBinds to the .cabal file is because I can't use #if __GLASGOW_HASKELL >= 700 to conditionally add in this language flag. The reason is because ghc6 will not correctly understand the language flag appearing inside a CPP #if/#endif block and I want this code to compile on ghc6 and 7.

Please let me know if I can provide additional information to help reproduce this bug. My email address is dagitj on gmail, also on CC of this ticket.

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	dagitj@gmail.com
Operating system	Unknown/Multiple
Architecture	Unknown/Multiple