iOS 17: Accessing @Dependency inside withTaskGroup crashes with swift_task_reportIllegalTaskLocalBindingWithinWithTaskGroup

[EmadBeyrami]

Hi 👋

I recently ran into an interesting issue while using swift-dependencies and wanted to check whether this is expected behavior or if it's something worth documenting.

The issue

On iOS 17, accessing a @Dependency from within the body of withTaskGroup (or withThrowingTaskGroup) causes a runtime trap:

swift_task_reportIllegalTaskLocalBindingWithinWithTaskGroup

For example:

@Dependency(\.connectionClient)
private var connectionClient

func checkConnections() async throws {
    try await withThrowingTaskGroup(of: Bool.self) { group in

        // Crashes on iOS 17
        group.addTask { [connectionClient] in
            try await connectionClient.isAvailable()
        }

        // Also crashes
        let state = try await connectionClient.currentState()
    }
}

The workaround is to resolve the dependency before entering the task group:

let connectionClient = self.connectionClient

try await withThrowingTaskGroup(of: Bool.self) { group in
    group.addTask {
        try await connectionClient.isAvailable()
    }
}

Observations

• Reproducible on iOS 17
• I cannot reproduce it on iOS 18
• The crash happens before the child task executes (capture lists don't avoid it)
• The stack trace ends in swift_task_reportIllegalTaskLocalBindingWithinWithTaskGroup

Question

Is this expected due to how swift-dependencies interacts with Swift's task-local values on the iOS 17 concurrency runtime?

If so, would it make sense to mention this in the documentation? It took me quite a while to track down because the code compiles cleanly and only fails on iOS 17.

I also wrote up the investigation here if it's useful:
https://dev.to/emadbeyrami/the-silent-ios-17-crash-hiding-in-dependency-and-withtaskgroup-g8c

Thanks!

[mbrandonw]

Hi @EmadBeyrami, this is just a bug in Swift that was fixed here and available in iOS 18. Not sure there's much we can say beyond that.

Also your AI summary should mention that this was a Swift bug that was fixed, not a swift-dependencies bug, and that it's perfectly fine to access dependencies in addTask without captures:

try await withThrowingTaskGroup(of: Bool.self) { group in
  group.addTask {
    try await connectionClient.isAvailable()  ✅
  }
}

And further (I would have to double check), I believe moving the capture to withTaskGroup should also fix it:

try await withThrowingTaskGroup(of: Bool.self) { [connectionClient] group in
  group.addTask {
    try await connectionClient.isAvailable()  ✅
  }
  let state = try await connectionClient.currentState()  ✅
}

But again, I would double check that before assuming that works.