Lazy stored propertyとImplicitly Unwrapped Optionalを組み合わせた時のおもしろい挙動

a.swift

final class Person {
    var name: String
    lazy var greeting: ImplicitlyUnwrappedOptional<String> = {
        return "Hello, \(self.name)"
    }()
    init(_ name: String) {
        self.name = name
    }
}
var p = Person("Steve")
p.name // => steve
p.greeting // => Hello, steve
p.name = "Tim"
p.greeting = nil
p.greeting // => Hello, Tim