When must you pass io_context to boost::asio::spawn? (C++)

I was surprised to find that the code below works without passing io_context as the first argument to spawn. Could somebody please explain why I don't need to pass it in this case, and in what situations you must explicitly pass it.

#include <boost/asio/spawn.hpp>
#include <boost/asio/deadline_timer.hpp>
#include <iostream>

int main() {

  boost::asio::io_context io_context;
  boost::asio::deadline_timer timer(io_context);

  boost::asio::spawn([&](boost::asio::yield_context yield){ // don't need to pass io_context?!
    std::cout << "started spawn" << std::endl;
    timer.expires_from_now(boost::posix_time::seconds(5));
    timer.async_wait(yield);
    std::cout << "finished spawn" << std::endl;
  });

  std::cout << "running io_context" << std::endl;
  io_context.run();
  std::cout << "finished running io_context" << std::endl;

}