My environment is OS 10.9.4 and Xcode 6 beta 5. I have tried to use Async in my iOS app, but it failed in the simulator (iOS 8).
I didn't modify anything, just run the two projects "AsyncExample iOS" and "AsyncExample iOSTests".

"AsyncExample iOS" printed these:
A: This is run on the Main (expected Background)
B: This is run on the Main (expected Main), after the previous block

"AsyncExample iOSTests" just succeed for the functions "testAsyncMain", "testAfterMain" and "testCancel". Other functions failed because qos_class_self() returned 0, which wasn't equal to any qos class.

Is MAC OS version the reason?