Swift Expected argument type `_?`?

Question

Can anyone explain to me what this error means and what is wrong with this attempted implementation of a Weak wrapper in Swift.

Just copy this into a new Playground:

import Cocoa

public struct Weak<T> where T: AnyObject {
    weak var value: T?

    init (_ value: T?) {
        self.value = value
    }
}

public protocol MyProto: class {
}

public class Foo: MyProto {
}

var foos = [Weak<MyProto>]()
let foo = Foo()

for foo in foos {
    if let bar = foo.value {  // 'MyProto' is not convertible to 'AnyObject'
    }
}

foos.append(Weak(foo as MyProto))  // Cannot convert value of type 'MyProto' to expected argument type '_?'

I get the errors:

error: 'MyProto' is not convertible to 'AnyObject'

error: cannot convert value of type 'MyProto' to expected argument type '_?'