In the code below, the compiler complains with q.cc:39: error: no matching function for call to `inc2(boost::fusion::map, boost::fusion::void_, boost::fusion::void_, boo st::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::vo id_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>&)' id_t<K> is the SAME type as the expanded BF::map type in inc1; it even works as the return type of inc1, yet I can't use it as an argument to inc2. What am I missing? This happens both on gcc 3.4 and SunCC 12. #include #include #include #include #include #include #include #include namespace detail { namespace BF = boost::fusion; struct KEY; template<typename K> struct id_t { typedef BF::map > type; }; template<typename K> inline typename id_t<K>::type& inc1(BF::map > &id) { ++BF::at_key<K>(id); return id; } template<typename K> inline typename id_t<K>::type& inc2(typename id_t<K>::type &id) { ++BF::at_key<K>(id); return id; } void f() { BF::map > m(12); inc1(m); inc2(m); } }