Quote from Qt Core 5.9 documentation:

[virtual] Qt::DropActions QAbstractItemModel::supportedDragActions() const

Returns the actions supported by the data in this model.

The default implementation returns supportedDropActions(). Reimplement this function if you wish to support additional actions.

supportedDragActions initialize as -1 in QAbstractItemModelPrivate::QAbstractItemModelPrivate

Qt 5.9.0 default implementation:

Qt::DropActions QAbstractItemModel::supportedDragActions() const
 {
 Q_D(const QAbstractItemModel);
 if (d->supportedDragActions != *Qt::IgnoreAction*)
 return d->supportedDragActions;
 return supportedDropActions();
 }

Qt 5.8.0 default implementation:

Qt::DropActions QAbstractItemModel::supportedDragActions() const
 {
 Q_D(const QAbstractItemModel);
 if (d->supportedDragActions != *-1*)
 return d->supportedDragActions;
 return supportedDropActions();
 }

supportedDragActions() doesn't work by default.