一. 在swift函数的申明语法中，可以给参数设置默认值，代码如下:

func test(name: String = "") {
    print(#function)
}
test() /// 调用一
test(name: "tom") /// 调用二

• 有默认值的函数，可以省略参数的传递 调用一
• 按正常函数调用 调用二

二. 这种函数是怎么实现的呢？

猜测：编译器会对应生成多个函数：

func test(name: String) {
    /// -> 相当于1种写法
    /// test(name: String)
    print(#function)
}
func test(name: String, address: String = "") {
    /// -> 相当于2种写法
    /// test(name: String)
    /// test(name: String, address: String)
    print(#function)
}

从上面的函数来看 test(name: String) 就被重复申明了，那么应该报错啊！不不不，我们只能说它可能报错，请看下面的代码，代码比较长，请耐心阅读

• 正确情况:

/// 同名不同参
func test(name: String) {
    /// -> 相当于1种写法
    /// test(name: String) -> 和自身行式一样的，假设为特殊，可以覆盖其他（不能被其它覆盖）specialFunc
    print(#function)
}
func test(name: String, address: String = "") {
    /// -> 相当于2种写法
    /// test(name: String)
    /// test(name: String, address: String) specialFunc
    print(#function)
}
func test(name: String, age: Int = 18) {
    /// -> 相当于2种写法
    /// test(name: String)
    /// test(name: String, age: Int) specialFunc
    print(#function)
}
func test(name: String, age: Int = 18, address: String = "") {
    /// -> 相当于4种写法
    /// test(name: String)
    /// test(name: String, age: Int)
    /// test(name: String, address: String)
    /// test(name: String, age: Int, address: String) specialFunc
    print(#function)
}

/// 由于specialFunc的覆盖和唯一性
/// 上面的函数最终得到如下4个(仅有)唯一函数
/// test(name: String) specialFunc
/// test(name: String, address: String) specialFunc
/// test(name: String, age: Int) specialFunc
/// test(name: String, age: Int, address: String) specialFunc

/// 这里的调用一一对应specialFunc函数, 没有歧义，运行正常
test(name: "tom") /// test(name:)
test(name: "tom", address: "") /// test(name:address:)
test(name: "tom", age: 10) /// test(name:age:)
test(name: "tom", age: 10, address: "") /// test(name:age:address:)

• 会产生错误的情况如下：

func parameter(name: String, address: String = "", age: Int = 18) {
    /// -> 相当于4种写法
    /// parameter(name: String)
    /// parameter(name: String, address: String)
    /// parameter(name: String, age: Int)
    /// parameter(name: String, address: String, age: Int) specialFunc
    print(#function)
}
func parameter(name: String, age: Int = 18, address: String = "") {
    /// -> 相当于4种写法
    /// parameter(name: String)
    /// parameter(name: String, age: Int)
    /// parameter(name: String, address: String)
    /// parameter(name: String, age: Int, address: String) specialFunc
    print(#function)
}

/// 上面的到6个有歧义的函数:
/// parameter(name: String)
/// parameter(name: String, address: String)
/// parameter(name: String, age: Int)
/// 和
/// parameter(name: String)
/// parameter(name: String, age: Int)
/// parameter(name: String, address: String)

/// 无法确认函数的唯一性，无法正确的调用函数，报错!
/// parameter(name: "")
/// parameter(name: "", age: 10)
/// parameter(name: "", address: "")

/// 上面的函数得到两个唯一函数:
/// parameter(name: String, address: String, age: Int) specialFunc
/// parameter(name: String, age: Int, address: String) specialFunc
/// 调用唯一函数，正确
parameter(name: "", address: "", age: 18)
parameter(name: "", age: 18, address: "")

• 如上的代码可以直接运行来校验猜测
• 就specialFunc而言，确实是出于对结果的构想（假定就是这样来推断后面的结果）

三. 以上是我个人的猜测。如有更好的解释，请各位多多指教。