I've been having a very difficult time with a problem illustrated by the following program: It looks like the eval_if in line 51 isn't working correctly. That is, the metafuction get_promotion_policy is being invoked when both types are not safe types. I've been working on this for two days and can't see what's going on. Any help from a TMP genious would be much appreciated. #include <type_traits> #include <boost/mpl/and.hpp> #include <boost/mpl/or.hpp> #include <boost/mpl/if.hpp> #include <boost/mpl/eval_if.hpp> #include <boost/mpl/identity.hpp> struct safe_tag {}; template<class T, class P = void> struct safe : public safe_tag { T m_t; typedef P PromotionPolicy; }; template<class T> struct is_safe : public //std::is_arithmetic<T> std::is_base_of<safe_tag, T> {}; template<class T, class U> struct common_policies { static_assert( boost::mpl::or_< is_safe<T>, is_safe<U> >::value, "at least one type must be a safe type" ); template<typename Z> struct get_promotion_policy { static_assert( is_safe<Z>::value, "only safe types have promotion policies" ); typedef typename Z::PromotionPolicy type; }; // if both types are safe, the policies have to be the same! static_assert( boost::mpl::eval_if< boost::mpl::and_< is_safe<T>, is_safe<U> >, typename std::is_same< typename get_promotion_policy<T>::type, typename get_promotion_policy<U>::type >::type, boost::mpl::identity<boost::mpl::true_> >::type::value, "if both types are safe, the policies have to be the same!" ); // now we've verified that there is no conflict between policies // return the one from the first safe type typedef typename boost::mpl::if_< is_safe<T>, T, typename boost::mpl::if_< is_safe<U>, U, // boost::mpl::void_ >::type >::type safe_type; typedef typename get_promotion_policy<safe_type>::type promotion_policy; }; common_policies<int, safe<int> > t; int main(){ return 0; }