Ghc panics (variable not found)

When attempting to compile a module with ghc-7.8-RC2, using the flags --ghc-options=-j8 -O2 -Werror, I encountered this error:

    ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.0.20140228 for x86_64-unknown-linux):
  StgCmmEnv: variable not found
    foldlM'_loop{v i1iSV} [lid]
    local binds for:

followed by about 1100 bindings (none of are the binding in question). Omitting the -j flag makes no difference. Building -O0 succeeds.

I don't have a standalone test case, and it's not clear to me how to make one as I have no idea what's causing this. I'll try to narrow it down, but if anyone could suggest some flags to twiddle or some other factor to adjust I'd appreciate it. I'm suspecting that it's a function referenced from inlining something vector-related.

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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