@propertyWrapper
struct Delayed<Value> {

    var wrappedValue: Value {
        get {
            guard let value = value else {
                preconditionFailure("Property \(String(describing: self)) has not been set yet")
            }
            return value
        }
        set {
            guard value == nil else {
                preconditionFailure("Property \(String(describing: self)) has already been set")
            }
            value = newValue
        }
    }

    private var value: Value?

}

class Foo {

    @Delayed var bar: Bar {
        didSet { print("bar has been set to \(bar)") }
    }

}