I want to try concurrent processing of an array for the first time. The compiler gives this warning: “Mutation of captured var ‘scaledImage’ in concurrently-executing code; this is an error in Swift 6”.

I then revised the code by copying the source array a local let variable; thus all the data are now local but the warning persisted. Since all the data are now local and the array “scaledImage” is a ‘sink’ array I don’t understand why the warning persists nor what to do about it.

    // Concurrent processing 1st attempt
    func processArrayConcurrently() async -> [UInt8] {
        var scaledImage = [UInt8]()
        scaledImage.reserveCapacity(self.backingImage!.count)
        // constants for the data
        let dMin = Double(self.backingMinMax[0])
        let dRange = Double(self.backingRange)
        let backingImage = self.backingImage!

        let concurrentQueue = DispatchQueue(label: "com.example.concurrentQueue", attributes: .concurrent)

        DispatchQueue.concurrentPerform(iterations: backingImage.count) { index in
            let processedValue = UInt8(backingImage[index] * 2) // Example processing
            concurrentQueue.async(flags: .barrier) {  // WARNING next line
                scaledImage[index] = processedValue
            }
        }

        // Wait for all tasks to complete
        concurrentQueue.sync(flags: .barrier) {}

        return scaledImage
    }