[Reg 6.5.1->6.5.3] Regression in formatting for implicit type conversion from QDateTime to QML string

Regression from Qt 6.5.1 to 6.5.3: The string format for QDateTime when assigned to a string property in QML changed. (See attached isolated example.) Intentional or not, hard to tell, but it did break our application when upgrading.

(Context: QML doesn't allow passing C++ QDateTime through QML property var to a C++ Q_INVOKABLE function with a QDateTime argument: "TypeError: Passing incompatible arguments to C++ functions from JavaScript is not allowed." So, if a model has a QDateTime type role, and this is used in QML as in the example, and the datetime is passed to C++ again, it must either be a string all the way through, or the Q_INVOKABLE must take a QString argument and parse the string representation of the QDateTime. This is what happened in our application, and that parsing broke (as in the attached example).)

Brief: If a QVariant storing a QDateTime is passed in from C++, and accessed from QML, its conversion to string behavior is different in Qt 6.5.1 and 6.5.3. The attached example, inlined QML below, exposes the QDateTime type as a named role, "dateTime", through a custom model "Model". This model is used in a ListView whose delegate uses a Text to demonstrate the differences in conversion.

The difference lies in what happens when the type is converted on the fly, perhaps through implicit left-to-right type deduction, vs when it's converted when stored in a string property. See screenshots; output when run with the two versions:

1. Qt 6.5.1:
   qml: dateTime:Wed Feb 28 14:35:12 2024 GMT+0100
   dateTimeStr:2024-02-28T14:35:12.456
   valid? true

1. Qt 6.5.3:
   qml: dateTime:Wed Feb 28 14:36:10 2024 GMT+0100
   dateTimeStr:Wed Feb 28 14:36:10 2024 GMT+0100
   valid? false

ListView {
    width: 400
    height: 100

    model: Model {
        id: testModel
    }

    delegate: Text {
        property string dateTimeStr: dateTime
        text: "dateTime:" + dateTime + "\ndateTimeStr:" + dateTimeStr + "\nvalid? " + testModel.verifyDate(dateTimeStr)
    }
}