<Thread: 0x00007f3114000c60> begin f <Thread: 0x00007f3118003360> 1 end f begin main <Thread: 0x00007f3114000c60> 1 end main

stderr:

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