Swift 4：使用Codable`通用参数’T’无法推断

无法推断出通用参数“T”

在线：让data = try encoder.encode(obj)

这是代码

import Foundation

struct User: Codable {
    var firstName: String
    var lastName: String
}

let u1 = User(firstName: "Ann",lastName: "A")
let u2 = User(firstName: "Ben",lastName: "B")
let u3 = User(firstName: "Charlie",lastName: "C")
let u4 = User(firstName: "David",lastName: "D")

let a = [u1,u2,u3,u4]

var ret = [[String: Any]]()
for i in 0..<a.count {
        let param = [
            "a" : a[i],"b" : 45
            ] as [String : Any]
    ret.append(param)
}


let obj = ["obj": ret]

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try encoder.encode(obj) // This line produces an error
print(String(data: data,encoding: .utf8)!)

我究竟做错了什么？