DispatchQueue crashing with main.sync in Swift

Question

Please explain to me why I am getting this crash?

Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

in this

Answer 1

As per Apple documentation on executing dispatch_sync on a queue you're currently on will crash your code:

Calling this function and targeting the current queue results in deadlock.