The BOOST_PARAMETER_KEYWORD(tag, name) macro expands, in part, to something like: keyword<tag::name> & name = ...; Shouldn't this be: keyword<tag::name> const & name = ...; ^^^^^ What is the reason for using a non-const ref here? These things are not mutable. In fact, it is causing me problems. Consider a placeholder type with an implicit conversion operator: struct default_constructed_t { template<typename T> operator T() const { return T(); } }; default_constructed_t const default_constructed = {}; BOOST_PARAMETER_KEYWORD(tag, value) Now, when I do this: "value = default_constructed;" I get this error: main.cpp:306: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: /cygdrive/c/boost/consulting/svn/main/boost/1.33.1/backports/boost_1_33_1/boost/parameter/ keyword.hpp:59: note: candidate 1: const typename boost::parameter::aux::tag<Tag, const T>::type boost::parameter::keyword<T>::operator=(const T&) const [with T = default_constructed_t, Tag = tag::value] /cygdrive/c/boost/consulting/svn/main/boost/1.33.1/backports/boost_1_33_1/boost/parameter/ keyword.hpp:32: note: candidate 2: boost::parameter::keyword<tag::value>& boost::parameter::keyword<tag::value>::operator=(const boost::parameter::keyword<tag::value>&) Had the keyword object been const, the call would not have been ambiguous. -- Eric Niebler Boost Consulting www.boost-consulting.com