When the same concurrency-related code fails to compile in Xcode 16 but builds cleanly in Xcode 26, what’s your first instinct? Mine was that the compiler had gotten smarter — but reality turned out to be more nuanced. This post documents a recent debugging journey: starting from a test failure, tracing all the way down to the Core Data SDK interface, and ultimately discovering that the key change had nothing to do with the Swift compiler itself — it was how NSManagedObjectContext is imported into Swift that had changed.