simplCore/Simplify.lhs:1442: Non-exhaustive patterns in function cat_evals

Compiling a program with ghc built from CVS shortly after the 6.0 release 
produced this error message:

ghc -fglasgow-exts -O2 -cpp -DBUILD_DATE="\\\"`date`\\\"" -package-conf 
../../mk/package.conf -package flux_utils -package flux_clang -package 
haskell98  -c FlattenCore.hs -o FlattenCore.o -hisuf hi
ghc-6.1: panic! (the `impossible' happened, GHC version 6.1):
        simplCore/Simplify.lhs:1442: Non-exhaustive patterns in function 
cat_evals


Please report it as a compiler bug to glasgow-haskell-bugs@xxxxxxxxxxx,
or http://sourceforge.net/projects/ghc/.


I was able to work around the problem by compiling without optimization.

--
Alastair Reid