[REG 6.5.3 -> 6.8.3] ListModel set() on existing index fails if setting data retrieved from C++ QVariantMap containing a QVariantList role

Hello,

While porting my app from 6.5.3 to 6.8.3, I found out that set() function of QML ListModel no longer works when we pass a QVariantList of QVariantMaps to it, if previous item already exists, and produces the following warning:

<Unknown File>: Can't assign to existing role 'b' of different type [VariantMap -> List]

This used to work previously, since when a property of the object passed to set() is an array, it's supposed to create an internal ListModel for it and apply the roles of the inner object to it.

Pure QML/JS works properly, this seems to affect only C++ code:

Working:

Item {
    id: testItem

    ListModel {
        id: testModel
    }

    function getSomeData(count) {
        var list = [];
        for (var i = 0; i < count; ++i)
            list.push({ r: i, s: String(i) });
        return { a: 2, b: list };
    }

    Component.onCompleted: function() {
        testModel.set(0, getSomeData(10));
        testModel.set(0, getSomeData(10));

        console.log(testModel.count);
        console.log(testModel.get(0)["b"].get(1)["r"]);
    }
}

Then if we instead get the data from a C++ class exposed to QML, that produces the same but returning a QVariantList of QVariantMaps, this no longer works:

QVariantMap getSomeData(int count)
{
    QVariantList list;

    for (int i = 0; i < count; ++i)
    {
        QVariantMap item;
        item[QLatin1String("r")] = i;
        item[QLatin1String("s")] = QString::number(i);
        list.append(item);
    }

    QVariantMap data;
    data[QLatin1String("a")] = 2;
    data[QLatin1String("b")] = list;
    return data;
}

There seem to have been related changes around setArrayLike(), maybe with new "if" statements, it now goes to setArrayLike() while it would earlier go through others ifs, I am not sure.

Passing a QVariantMap without list inside still works properly.

As a workaround, we can do JSON.parse(JSON.stringify(data)) but this is not great for performance.

Feel free to ask for more details if needed,

Thank you,
Louis