Sitting in the backyard. Beautiful day, got up at 3am to get something figured out (for proposed boost profiler \ introspection library). Good news is I figured out the MPL code needed to accomplish my goal. After typing in an incremental version of the code, I got the dreaded internal compiler error (ICE). I am chewing on this- and thought I would post it so to provide an opportunity for someone to show how incredibly stupid I am because I did something that was obviously wrong. (you can even rub my nose in it if you like :) ) here is the code: I am tearing it apart\deconstructing to try to figure out what is kicking in the ICE. (I am also going to try it with an eval_if (which I should be using anyway) just to see what happens) I am using boost 1.32 and vc7.1 struct do_copy_col { typedef do_copy_col type; do_copy_col() { std::cout << "do_copy_col" << std::endl; } // todo- implement operator()(dest,src) }; struct do_not_copy_col { typedef do_not_copy_col type; do_not_copy_col() { std::cout << "do_not_copy_col" << std::endl; } // todo- implement operator()(dest,src) }; template <class type_t, class sequence_t> struct copy_col_if_in_dest : public if_< typename is_same< typename deref<typename boost::mpl::find<sequence_t, type_t>::type >::type, type_t>, do_copy_col, do_not_copy_col>::type { }; template <class src_t, class dest_t> void copy_union(src_t & src, dest_t & dest) { copy_col_if_in_dest<boost::mpl::at<src::typelist, boost::mpl::int_<0> >::type,src::typelist>(); }