Moreover, your result are wrong result_of<T()> will always return void. You need somethign like: struct arg_tag {}; namespace proto = boost::proto; namespace mpl = boost::mpl; using namespace std; template<int N> struct arg : proto::or_<proto::nullary_expr<arg_tag, mpl::int_<N> > > {}; template<typename T> struct readonly : proto::callable { template<typename Sig> struct result; template<typename This,class X> struct result<This(X)> { typedef const T& type; }; }; template<typename T> struct readwrite : proto::callable { template<typename Sig> struct result; template<typename This,class X> struct result<This(X)> { typedef T& type; }; }; template<int N, typename T> struct argtype : proto::or_< proto::when<proto::assign<arg<N>, proto::_>, readwrite<T>(proto::_)> , proto::otherwise<readonly<T>(proto::_)>
{}; proto::nullary_expr<arg_tag, mpl::int_<0> >::type const _1 = {{}}; proto::nullary_expr<arg_tag, mpl::int_<1> >::type const _2 = {{}}; namespace boost { namespace proto { template<class T> struct is_callable<readwrite<T> > : true_ {}; template<class T> struct is_callable<readonly<T> > : true_ {}; }}