Expose completingModel as ProxyModel

When I attach a QCompleter, I can use its completionModel as input to, e.g., a QListView. This is great for using a QLineEdit + QCompleter combination as a filter input to allow the user to search/filter items in the list.

The problem arises when I need to interact with items in the list. Let's have a look at the signature:

QAbstractItemModel *QCompleter::completionModel() const

This signature hides the fact that QCompleter uses a (subclass of) QAbstractProxyModel. This renders signals that provide me with a ModelIndex pretty much useless, as I cannot call QAbstractProxyModel::mapToSource().

I suggest to change the signature such as:

QAbstractProxyModel *QCompleter::completionModel() const

My current workaround is ugly:
auto proxy = qobject_cast<const QAbstractProxyModel*>(completer->completionModel());