id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4524,Template variable unbound in rewrite rule,guest,,"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.",bug,closed,normal,,Compiler,7.0.1,fixed,,dagitj@… ganesh,Linux,x86,Compile-time crash,,typecheck/should_compile/T4524,,,