Dear All, I'm trying to write a metafunction rebind which, when fed a specialization T<P1,...,Pn> of an n-ary template T, returns T<mpl::_, ...., mpl::_>. This is straightforward if default template arguments are not considered when matching template template parameters. (See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#150.) E.g., template<typename T> struct rebind { typedef T type; } template<template<typename> class T, typename P> struct rebind< T<P> > { typedef T<mpl::_> type; }; template<template<typename, typename> class T, typename P1, typename P2> struct rebind< T<P1, P2> > { typedef T<mpl::_, mpl::_> type; }; etc. On GCC 3.4.1 (and I assume earlier versions too) this leads to ambiguity errors. To workaround this problem, I've resorted to using enable_if and mpl::aux::template_arity, e.g., template<typename T, typename Enabler = void> struct rebind { typedef T type; } template<template<typename> class T, typename P> struct rebind< T<P>, typename enable_if< mpl::equal_to< typename mpl::aux::template_arity< T<P> >::type, mpl::int_<1> > >::type > { typedef T<mpl::_> type; }; I have two questions: 1. is my analysis correct? 2. is there a better solution? Best Regards, Jonathan