Swift's concurrency approach is emblematic of its complete lack of pragmatism. Runtime sees hey I think this code is running in the wrong thread (or context if you must) what do I do? Warn and ignore, because it's probably OK 99% of the time, nope. Warn and correct, because you have enough info to infer what is probably needed, nope. How about crash 100% of the time, yep, that's the one.