How to set non-selectable default text on QComboBox?

It doesn't appear that case was anticipated in the Combo Box API. But with the underlying model flexibility it seems you should be able to add your --Select Country-- as a first "legitimate" item, and then keep it from being user selectable:

QStandardItemModel* model =
        qobject_cast<QStandardItemModel*>(comboBox->model());
QModelIndex firstIndex = model->index(0, comboBox->modelColumn(),
        comboBox->rootModelIndex());
QStandardItem* firstItem = model->itemFromIndex(firstIndex);
firstItem->setSelectable(false);

Depending on what precise behavior you want, you might want to use setEnabled instead. Or I'd personally prefer it if it was just a different color item that I could set it back to:

Qt, How do I change the text color of one item of a QComboBox? (C++)

(I don't like it when I click on something and then get trapped to where I can't get back where I was, even if it's a nothing-selected-yet-state!)

One way you can do something similar is to set a placeholder:

comboBox->setPlaceholderText(QStringLiteral("--Select Country--"));
comboBox->setCurrentIndex(-1);

This way you have a default that cannot be selected.