QML and nullable "primitive" types

Let's say I want return `std::optional<int>` or `std::optional<QColor>` or `std::optional<QString>` from C++ to QML via `Q_INVOKABLE` function.

I see only one option to do it now: usage of ​`QVariant()` and `QVariant(T)`.
In other words it would be dynamic typing: ​type `T` or invalid type.

I can define new types like `OptionalT`:

struct OptionalT {
   Q_GADGET
   bool isValid;
   T data;
   Q_PROPERTY(...)
};

because of `Q_GADGET` not supported as return type for `Q_INVOKABLE` (https://bugreports.qt.io/browse/QTBUG-79330).

The propblem that on QML type become `var`, and qmllint can not reports any warning for code like this:

        const depTime = obj.getDepartureTime()
        if (depTime) {
            return depTime.toLocalString(Qt.locale(), "hh:mm\nt") // <-- here error, no "e" on Locale
        } else
            return qsTr("Departure time")

It would be nice if there is option to annotate nullable types on C++/QML border.