Regression: warning when Binding points to the property of a value type

import QtQuick 2.3
import QtQuick.Window 2.2

Window {
    width: 800 * root.bar.x
    height: 400
    visible: true

    Binding {
        target: root
        property: "bar.x"
        value: 0.5
    }

    Item {
        id: root

        property rect bar: Qt.rect(0, 0, 0, 0)
    }
}

qrc:/main.qml:9:5: QML Binding: Property 'bar.x' does not exist on Item.