Function f2

example/when_all.cpp:19–24

}

task<void> f2() {
    printf("f2 start.\n");
    co_await timeout(std::chrono::seconds{2});
    printf("f2 done.\n");
    co_return;
}

task<> run() {
    // co_spawn f1 & f2 & f3, and wait for them