John Maddock's fix to is_base_and_derived to make it complain about the use of incomplete types (see http://svn.boost.org/trac/boost/ changeset/39219) broke something within the parameter library. I've committed the following patch to the parameter library, which fixes the problem. Unless there was a particular reason not to include the "maybe" header from arg_list.hpp? - Doug Index: aux_/arg_list.hpp =================================================================== --- aux_/arg_list.hpp (revision 39245) +++ aux_/arg_list.hpp (working copy) @@ -11,6 +11,7 @@ #include <boost/parameter/aux_/default.hpp> #include <boost/parameter/aux_/parameter_requirements.hpp> #include <boost/parameter/aux_/yesno.hpp> +#include <boost/parameter/aux_/maybe.hpp> #include <boost/parameter/config.hpp> #include <boost/mpl/apply.hpp> @@ -158,10 +159,6 @@ no_tag operator*(empty_arg_list, KW*); template <class KW, class T> struct tagged_argument; -// Forward declaration for arg_list::operator[], with -// IS_XXX helper -struct maybe_base; - template <class T> struct is_maybe : is_base_and_derived<maybe_base, T>