<Thread: 0x00007fd110000c60> begin main <Thread: 0x00007fd114002dd0> 0 end main begin f <Thread: 0x00007fd114002dd0> 1 end f

stderr:

<stdin>:13:9: warning: 'async(priority:operation:)' is deprecated: `async` was replaced by `Task.init` and will be removed shortly. async { ^