Swift协议继承和通用函数

import Foundation

protocol StringInitable {
    init( String:string )
}

class A : StringInitable {
    var stored:string

    required init ( String:string ) {
        stored = String
    }
}

class B : A /*,StringInitable */ {
    var another_stored:string

    required init ( String:string ) {
        another_stored = "B-store"

        super.init(String: String)
    }
}

func maker<T:stringInitable>(String:string) -> T {
    return T(String: String)
}

let instanceA = A(String: "test-maker-A")
let instanceB = B(String: "test-maker-B")

let makerA:A = maker("test-maker-A")
let makerB:B = maker("test-maker-B")

let typeInstanceA = _stdlib_getTypename(instanceA)
let typeMakerA = _stdlib_getTypename(makerA)

let typeInstanceB = _stdlib_getTypename(instanceB)
let typeMakerB = _stdlib_getTypename(makerB)

从结果来看,编译器似乎推断出了正确的类型,但未能调用正确的初始化器.为什么我必须在B类中显式实现StringInitable(通过删除B类定义中的注释来测试)以使通用函数“maker”调用正确的初始化器？