Can't using an attached property to target in Connections

import QtQuick.Window 2.15
import QtQuick 2.15

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Item {
        id: item
    }

    Connections {
        target: item.Window
        function onWindowChanged() {
            console.log("window changed of item")
        }

        Component.onCompleted: {
            console.log(target, item.Window)
            console.assert(target !== item, "Target is the item self, and is not its attached object.")
        }
    }
}