ghc-6.4.20050220: panic! eval_data2tag...

Hi,

I get the following error when I compile a particular program with "ghc 
--make -O ...", but not if I omit -O. ghc-6.2.2 works fine in either 
case. I can supply more details on demand (I am a lazy functional 
programmer, after all :-)

--
Thomas H
------------------------------------------------------------------------

ghc-6.4.20050220: panic! (the `impossible' happened, GHC version 
6.4.20050220):
       eval_data2tag
   GHCziPrim.dataToTagzh{(w) v 95f}
     @ (HsExpStruct.EI{tc r7yx} i{tv aD9V}
                                (<recnt>SyntaxRec.HsExpI{tc rD7P} i{tv 
aD9V})
                                (<recnt>SyntaxRec.HsPatI{tc rD7L} i{tv 
aD9V})
                                [<recnt>SyntaxRec.HsDeclI{tc rD7T} i{tv 
aD9V}]
                                (<recnt>SyntaxRec.HsTypeI{tc rD7H} i{tv 
aD9V})
                                [<recnt>SyntaxRec.HsTypeI{tc rD7H} i{tv 
aD9V}])
     (__coerce (SyntaxRec.BaseExpI{tc rD81} i{tv aD9V}) ds1{v sFN5})