Swift闭包语法

我有以下代码:

let obsScan = source.scan(0, accumulator: +)
let obsReduce = source.reduce(0, accumulator: +)

let obs = Observable.zip(obsScan, obsReduce) { scan, reduce in
        return "scan - \(scan), reduce - \(reduce)"
}

我想用自动完成闭包语法重写它,结果是:

let obs = Observable.zip(obsScan, obsReduce, resultSelector: { (scan, reduce) -> _ in
         return "scan - \(scan), reduce - \(reduce)"
})

闭包参数列表的上下文类型需要2个参数,其中 不能隐式忽略行上的连续语句必须为 由“;”分隔预期表达式

我不明白为什么我用tab来自动完成带有闭包的函数,当我填充参数时,我得到了一个错误?

整个函数声明如下:

 public static func zip<O1, O2>(_ source1: O1, _ source2: O2, resultSelector: @escaping (O1.E, O2.E) throws -> RxSwift.Observable.E) -> RxSwift.Observable<RxSwift.Observable.E> where O1 : ObservableType, O2 : ObservableType