Dispatch precondition false positive?

[DevAndArtist (Adrian Meister)]

Queues are not necessarily unique.

What is the default target queue for a serial queue?

I just recently noticed that dispatch queues always target another queue by default even if not explicitly specified. For concurrent queues it will target the global queue with QoS .default. However the precondition in the following code will fail if I try to use it on serial queue. Does a serial queue has a default target queue at all? What behavior should I expect? let serial = DispatchQueue(label: "serial") let concurrent = DispatchQueue(label: "concurrent", attributes: .concurrent) …

Check if anything related applies in your setup.