'Box' requires that 'CanFly' be a class type

Question

Background

Let's consider the following working code

protocol CanFly { }

protocol Container {
    associatedtype ContentType
    var value: ContentType? { get }
}

class Box<V>: Container {
    var value: V?
}

let box = Box<CanFly>()
box.value // <- has type `CanFly?`

Here Box accepts a protocol as generic type, beautiful isn't it?

Things get harder

Now I want to do a "small" change, I make the value property in Box weak

class Box<V>: Container {
    weak var value: V?
}

And of course I get this error

class Box<V>: Container {
    weak var value: V? // 'weak' must not be applied to non-class-bound 'V'; consider adding a protocol conformance that has a class bound
}

I try to fix it constraining V to be an AnyObject

class Box<V:AnyObject>: Container {
    weak var value: V?
}

And I get this error

let box = Box<CanFly>() // 'Box' requires that 'CanFly' be a class type

The problem

The problem here is that since I defined V:AnyObject I can no longer use the protocol CanFly as generic type of Box.

The question

I want

1. to be able to use a protocol as generic type of Box
2. AND the Box#value property to be weak

How can I do that?

Final considerations

I am looking for something like this

class Box<V:ProtocolForClassOnly>: Container {
    weak var value: V?
}

P.S. The closest question on StackOverflow I could find is this one but unfortunately hasn't helped me.

Answer 1

So constrain your CanFly protocol to type AnyObject:

protocol CanFly: AnyObject { }

protocol Container {
    associatedtype ContentType
    var value: ContentType? { get }
}

class Box<V>: Container {
    var value: V?
}

Then your code works:

func foo() {
    let box = Box<CanFly>()
    print(type(of:box.value))
}