Swift 6 concurrency check not complaining about Objc block property

[brustolin]

Description

I work on an ObjC framework which contains an Option class with a beforeSend property block callback like so (names changed for simplicity):

typedef Event *_Nullable (^ BeforeSendCallback)(Event *_Nonnull event);

@interface Option 
@property (nonatomic, copy) BeforeSendCallback beforeSend;
//... More properties
@end

The beforeSend callback can be called from any thread, and when we do call it from a thread other than the one the user used to create the option object, the app crashes with EXC_BREAKPOINT, if the user didn't mark the closure with @Sendable.

I would think that the compiler should complain about this in Swift 6 mode. Am I missing something?

Reproduction

This crashes in runtime when beforeSend is used in a background thread:

MySDK.start { option in
 option.beforeSend = { event in
     //process event
     return event
  }
}

This does not crashes in runtime:

MySDK.start { option in
 option.beforeSend = { @Sendable event in
     //process event
     return event
  }
}

Expected behavior

I would expected the compiler to complain about the missing @Sendable in the closure or any other error.

Environment

swift-driver version: 1.115 Apple Swift version 6.0.2 (swiftlang-6.0.2.1.2 clang-1600.0.26.4)
Target: arm64-apple-macosx15.0

Additional information

No response

[jamieQ]

i'm not entirely sure what the expected behavior is in this case, but you may find this section of the Swift 6 migration guide of interest. in particular, there are some annotations that can be applied to objc code to communicate the intended swift concurrency behaviors of an API.