---------------------------------------------------------------------------------- struct extract_rule {

template <typename Sig> struct result;

template <typename Holder> struct result< extract_rule(const Holder) > { typedef const typename Holder::type& type; };

template <typename Holder> typename result< extract_rule(const Holder) >::type operator()( const Holder& h ) const { return h.rule; } }; ----------------------------------------------------------------------------------

1> with 1> [ 1> Sig=extract_rule (const onevariantrule ) 1> ]

The template onevariantrule<> is defined _before_ extract_rule.

------------------------------------------------------------------------------------------- template struct onevariantrule { typedef .... type; }; -------------------------------------------------------------------------------------------

Holder matches onevariantrule. That template instantiation is indeed a class.

Both specializations template <typename Holder> struct result< extract_rule(const Holder) > { }; and template <typename Holder> struct result< extract_rule(const Holder& ) > { ///// the reference was missing here, doh! typedef const typename Holder::type& type; }; were needed. Sorry for the spam,