X is not a type when defining function parameters in Qml

#include <QQmlEngine>

class GadgetTest
{
    Q_GADGET
    QML_ANONYMOUS
    QML_VALUE_TYPE(vtmGadgetTest)
public:
    GadgetTest();
};
Q_DECLARE_METATYPE(GadgetTest)

function onGadgetSignal(s : vtmGadgetTest) { // Error: vtmGadgetTest is not a type
    console.log("Gadget: " + s);
}

function onGadgetSignal(s : GadgetTest) { // Error: GadgetTest is not a type
    console.log("Gadget: " + s);
}

function onGadgetSignal(s ) { // works
    console.log("Gadget: " + s);
}