Both the type_traits and function_types library discard parameter const-qualifiers when decomposing function types. Is this a bug or a feature? If a feature, then it's surprising enough behaviour that I think it warrants some documentation in both libraries. For the record, I'm using gcc 4.7.3 on Cygwin 1.7.22 in Windows 7, and here's a reproducible example of what I'm talking about: #include <boost/type_traits.hpp> #include <boost/function_types/parameter_types.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/assert.hpp> using namespace boost; typedef void (foo)(int const); int main() { typedef function_traits<foo>::arg1_type traits_deduced_type; BOOST_MPL_ASSERT(( is_same<traits_deduced_type, int> )); //Errors: //BOOST_MPL_ASSERT(( is_same<traits_deduced_type, int const> )); typedef boost::function_types::parameter_types<foo> ftypes_params; typedef boost::mpl::at<ftypes_params, boost::mpl::int_<0> >::type ftypes_deduced_type; BOOST_MPL_ASSERT(( is_same<ftypes_deduced_type, int> )); //Errors: //BOOST_MPL_ASSERT(( is_same<ftypes_deduced_type, int const> )); return 0; } Mostafa