Given a type T, I am trying to transform a type list into another containing the common type between T and the individual types in the type list. When I compile the example below, I get the following error: /usr/local/include/boost/type_traits/common_type.hpp:111:42: error: incompatible operand types ('mpl_::arg<1>' and 'int') typedef decltype(declval<bool>() ? declval<T>() : declval<U>()) type; ^ ~~~~~~~~~~~~ ~~~~~~~~~~~~ It appears that the placeholder is not expanded to its actual type. If I use, say, decay<> instead of common_type<>, the example works just fine. Does anybody know how I can make it work? Example code: #include <boost/mpl/vector.hpp> #include <boost/mpl/transform.hpp> #include <boost/type_traits/common_type.hpp> int main() { typedef boost::mpl::vector<int, double> sequence_types; typedef boost::mpl::transform< sequence_types, // This is causing the problem boost::common_type<boost::mpl::_1, int>::type >::type transformed_types; return 0; }