Class Drawable

example/concepts.cpp:15–21

namespace te = boost::te;

struct Drawable {
  void draw(std::ostream &out) const {
    te::call([](auto const &self,
                auto &out) -> decltype(self.draw(out)) { self.draw(out); },
             *this, out);
  }
};

struct Square {
  void draw(std::ostream &out) const { out << "Square"; }