Consider : #include <boost/fusion/include/vector.hpp> #include <boost/fusion/include/push_back.hpp> #include <boost/fusion/include/size.hpp> #include <boost/mpl/assert.hpp> #include <boost/type_traits/is_same.hpp> int main() { using namespace boost; using namespace fusion; typedef vector<int, int> seq1; typedef result_of::push_back<vector<int>, int>::type seq2; BOOST_MPL_ASSERT((is_same<result_of::size<seq1>::type, result_of::size<seq2>::type>)); } One would not expect this to fail, however it does : 'boost::mpl::assertion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boost::is_same<T,U>::* ***********' to 'boost::mpl::assert<false>::type' 1> with 1> [ 1> T=boost::mpl::int_<2>, 1> U=boost::mpl::integral_c<int,2> 1> ] J-L