Thread.isMainThread throwing compiler warnings

[ksluder (Kyle Sluder)]

@eskimo

import Foundation
 
let queue = DispatchQueue(label: "not-main")
 
func main() {
    queue.async {
        DispatchQueue.main.sync {
            // This doesn’t trap:
            dispatchPrecondition(condition: .onQueue(.main))
            // But this prints false:
            print(Thread.isMainThread)
        }
    }
    dispatchMain()
}
 
main()

What platform did you test this on? dispatch_sync is explicitly documented not to perform the same-thread optimization when targeting the main queue.