text: //
  // 2.4.default.template.param.cpp
  // chapter 2 language usability
  // modern cpp tutorial
  //
  // created by changkun at changkun.de
  // https://github.com/changkun/modern-cpp-tutorial
  //
  
  #include <iostream>
  
  template<typename T = int, typename U = int>
  auto add(T x, U y) -> decltype(x+y) {
      return x+y;
  }
  
  int main() {
      std::cout << add(1, 2) << std::endl;
  }