This shouldn't work, but it works. Thus undefined behaviour.

UndefinedBehaviour.swift

func update(_ arg: inout String) {
    arg = "UPDATED \(arg)"
}


public class SystemWork {
    let mainQueue = DispatchQueue.init(label: "mainQueue", qos: .userInitiated, attributes: [], autoreleaseFrequency: .inherit, target: nil)
    let workerQueue =  DispatchQueue.init(label: "workerQueue", qos: .userInitiated, attributes: [.concurrent], autoreleaseFrequency: .inherit, target: nil)
    
    let systemQueue =  DispatchQueue.init(label: "systemQueue", qos: .userInitiated, attributes: [.concurrent], autoreleaseFrequency: .inherit, target: nil)
    
    
    var results: Array<Int> = []
    let waiter = DispatchGroup()
    var dict = Dictionary<Int, String>()
    let range = (0..<100)
    
    public init() {
        self.range.forEach { self.dict[$0] = "HI \($0)" }
    }
    
    public func work() {
        print("WORK START")
//                        range.forEach { id in
//                            withUnsafeMutablePointer(to: &self.dict[id, default: ""]) { data in
//                            self.workerQueue.async(group: self.waiter) {
//                                    update(&data.pointee)
//                                }
//                            }
//                        }
        
        
        self.mainQueue.async(group: self.waiter) {

            self.systemQueue.async(group: self.waiter) {

                self.range.reduce(into: Array<Array<Int>>()) { result, int in
                    if (result.last?.count ?? 0) <= 64 {
                        var x = result.popLast() ?? Array<Int>()
                        x.append(int)
                        result.append(x)
                    }
                      else { result.append([int]) }
                    }
                    .forEach { ids in


								ids.forEach { id in
                                  withUnsafeMutablePointer(to: &self.dict[id, default: ""]) { data in
                        
                                  self.workerQueue.async(group: self.waiter) {
                                      update(&data.pointee)

                                }

                            }
                        }
                }
            }
        }

        self.waiter.wait()
        print("FINSIHED")

        let finished = self.dict.values.filter { $0.hasPrefix("UPDATED ") }
        print(finished.count)
    }
    
}