The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#include <iostream>
#include "cororef.hpp"

int main()
  auto coroTask = coro(3);             // OOPS: creates reference to temporary/literal
  std::cout << "coro(3) started\n";
  coro(375);                           // another temporary coroutine
  std::cout << "coro(375) started\n";

  // loop to resume the coroutine until it is done:
  while (coroTask.resume()) {          // ERROR: undefined behavior
    std::cout << "coro() suspended\n";

  std::cout << "coro() done\n";