Saving the current default revision as a non-default revision is disallowed

It is possible to save the current default revision of an entity as the non-default revision. This results in inconsistent data as it only updates the revision data table but not the data table.

For example the following code snippet:

$entity = Node::load(1);
$entity->set('title', 'new title');
$entity->isDefaultRevision(FALSE);
$entity->save();

Loading the entity or the revision will then produce a different result for that title.

This is now no longer allowed and attempting to do so will throw an exception. It is also not allowed to save a new entity as a non-default revision. This will be ignored half-way through the process, but insert/update hooks were in the past invoked with the default revision flag set to FALSE, possibly causing inconsistent behavior.

Essentially, saving an existing default revision as a non-default revision must be combined with saving it as a new revision or it must be kept as the default revision as having an entity without a default revision is not a valid state.

Either:

$entity = Node::load(1);
$entity->set('title', 'new title');
$entity->isDefaultRevision(FALSE);
$entity->setNewRevision(TRUE);
$entity->save();

Or:

$entity = Node::load(1);
$entity->set('title', 'new title');
// This should be the default in this scenario and this call should not be necessary.
$entity->isDefaultRevision(TRUE);
$entity->save();