为什么可以在没有任何错误的情况下为非可选项分配非可选项？

Question

我正在阅读Apple的Optional Chaining，我对它的含义感到有些困惑

class Person {
    var residence: Residence?
}

class Residence {
    var numberOfRooms = "fifty"
}

let john = Person()
john.residence = Residence()

let roomCountSafe = john.residence?.numberOfRooms // here numberOfRooms is no longer a String but instead is a String? right?

john.residence?.numberOfRooms = "thirty" // Line A: how can we assign a nonOptional to an Optional?

print(john.residence?.numberOfRooms)

var x = "aString"

但不能将可选项分配给nonOptional

x = john.residence?.numberOfRooms // Line B: error: value of optional type
 'String?' not unwrapped; did you mean to use '!' or '?'?

或者这是否解释了选项是'枚举'并且你要么进入'case'集合，要么就是我在A行做的事情。为什么A行返回一个可选的（“30”是因为它可能因为可选链接的性质而失败？

（我确实理解了lineB，但是当涉及到B行与A行时，我有点困惑）