Rectangle {
    id: rect
    width: 360
    height: 360

    property variant src: ShaderEffectSource { sourceItem: srcRect }

    Rectangle {
        id: srcRect
        color: "red"
        width: 100
        height: 100
    }

    ShaderEffect {
        anchors.fill: parent

        // Changing this to variant works
        property alias source: rect.src
    }
}