Yes, I was playing around with it as well. It seems the support has been out there for a while, but officially since C++17 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n437... was accepted, s.t. std::vector, std::list and std::forward_list can work with incomplete types:

   struct example {
     std::vector<example> xs;
   };