Class DrawableConcept

test/te.cpp:868–880

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

  auto empty() {
    return te::call<int>(
        [](auto &self) -> decltype(self.empty()) { return self.empty(); },
        *this);
  }
};

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