Totally reasonable as long as it's a read-only property. I'm not quite going to tag this as a starter bug because I'm not sure what all the moving pieces would be, but I don't think it would be too hard.

Maybe this is basic stuff, but...

if the subclass B had properties (e.g. var foo: Int = 0) and B().copy() instantiates parent A, which doesn't have foo property and only casts it to B, how come code doesn't crash on runtime when you do e.g. B().copy().foo = 5?

Comment by LinQingmo (JIRA)

@Moximillian It will not crash.

func copy() -> Self {
    let copy = Self.init() // When call B().copy(), this is let copy = B.init(), so it can cast to B.
    print(type(of: copy)) // When call B().copy(), the type of copy is B not A.
    return copy
}

Right, the required in the declaration of the init means it can be invoked on the dynamic type. If you just said A.init(), you'd get an error message about not returning the right type.

Oh, inherited methods operate on instance of subclass, so is more similar to protocol extension, in a way... All makes sense now, thanks!

PR filed to make declaring properties of type `Self` an error: #23485

#23485 #23613