[boost] [type_erasure] Force any to have typedef

Hi, Assume I have the following function template: template <typename Container> typename Container::value_type get_front(const Container& c) { return c.front(); } Is there a way I can force type_erasure::any<> to have a `value_type` typedef? I can create a concept which requires the underlying type to have it, but I'd like to make `any` to have such a typedef. Thanks, Benedek