qmllint (current dev) suggests to replace "var" property with function,
for code bellow.

Property "nextCheckState" is a variant property. It may or may not be a method. Use a regular function instead.

But this suggestion have no sense for me.

QML doesn't support function "override" in the same manner as C++, hence to allow user change part of logic of your component this logic should be inside property,
and only "var" type is allowed if you want to save "function() {}" inside property.
So how it possible to "use regular function instead" in such case ?

Rectangle {
    id: control

    property var nextCheckState: function(prevState) {
        return !prevState;
    }

    BoolValue {
        id: flag
    }
   MouseArea {
        anchors.fill: parent
        onClicked: {
            flag.setOn(control.nextCheckState(flag.isOn));
        }
    }
}