Is -warn-concurrency working?

I just added Other Swift Flags = -warn-concurrency to a large existing project in Xcode 13.3 (RC), and got zero extra warnings. That feels almost certainly incorrect. Is there a larger problem with -warn-concurrency here? Is it intended to be working in Swift 5.6? Or is there a different way I should be adding it?

$ /Applications/Xcode\ -version
swift-driver version: 1.45.2 Apple Swift version 5.6 (swiftlang- clang-1316.0.20.8)

You have to add this to Other Swift Flags:

-Xfrontend -warn-concurrency

Curious. Neither SE-0337 nor the Xcode release notes mention requiring -Xfrontend. That does indeed produce new warnings and errors, though. Seems like we could document that better.

It's also not clear to me why some things are warnings and others are errors with -warn-concurrency enabled. Is there a guiding principle?

I agree.

I don't know what exactly -warn-concurrency does. This recent post from @Douglas_Gregor seems somewhat related:

-warn-concurrency might not be the right name; it's applying stricter logic for concurrency-related checks across the entire module, not just in code that has "adopted concurrency."

If it's wrong, or if the errors are indeed not fixable, please file bugs so we can improve it!