Does not compile with Xcode 9 and Swift 4 about async HOT 11 CLOSED

    private static func async<O>(after seconds: Double? = nil, block: @escaping () -> O, queue: GCD) -> AsyncBlock<Void, O> {
        let reference = Reference<O>()
        let block = DispatchWorkItem(block: {
            reference.value = block()
        })

        if let seconds = seconds {
            let time = DispatchTime.now() + seconds
            queue.queue.asyncAfter(deadline: time, execute: block)
        } else {
            queue.queue.async(execute: block)
        }

        // Wrap block in a struct since @convention(block) () -> Swift.Void can't be extended
        return AsyncBlock<Void, O>(block, output: reference)
    }