无法在swift中加载类数组中的数组数据

extension Array where Element: ArrayProtocol {
    func flatten() -> [Element.Element] {
        var result: [Element.Element] = []
        for array in self {
            result.append(contentsOf: array)
        }
        return result
    }
}

let array1 = [1, 2, 3]
let array2 = [4, 5, 6]
let array3 = [7, 8, 9]

let arrayOfArrays = [array1, array2, array3]
let flattenedArray = arrayOfArrays.flatten()

print(flattenedArray) // 输出：[1, 2, 3, 4, 5, 6, 7, 8, 9]