property observer bug

NumberString.swift

class NumberString {
    var number = 0
    var string: String = "" {
        didSet { number = string.toInt()! }
    }
    
    init(foo: String) {
        string = foo  // Will NOT cause didSet
    }

    init(bar: String) {
        setString(bar)
    }

    func setString(foo: String) {
        string = foo  // Will cause didSet
    }
}

var foo = NumberString(foo: "123")  // number = 0
foo.string = "456"                  // number = 456
var bar = NumberString(bar: "789")  // number = 789

This behavior is noted in The Swift Programming Language: Properties as following:

willSet and didSet observers are not called when a property is first initialized. They are only called when the property’s value is set outside of an initialization context.

Yes. But "string" property is initialized with "" already, therefore the line 8 isn't actually necessary to compile the code. (that's why the line 12 is possible without doing something like string = "").
If both initializers behave exactly the same, I can see it as consistent and not "a bug"

I understand. It's mismatch between "var is initialized" and "it's in initializer".
Currently, following condition takes priority.

They are only called when the property’s value is set outside of an initialization context.

I guess these comes from two different side "What is required before calling func in initializer?" and "When can property be observable?".
It's interesting.