Spin-off from/ .
That issue adds actual validation on entity fields.
This means that implicit or explicit constraints contained in FieldItem::getPropertyDefinitions() (e.g 'type' => 'uri' in LinkItem, 'type' => 'integer' for 'tid' in TaxonomyTermReferenceItem...) that were just "dormant" so far in HEAD, are now fired.
When those constraints fail, the generated violation objects do not point to any specific delta: the propertyPath is just the name of the property, e.g. 'url', not '0[url]'. It is thus impossible to assign them back to the right form element.
TypedDataManager::getConstraints() has a "@todo: Figure out how to handle nested constraint structures as collections.", which seems to be about exactly that ? But fixing this is above my head right now...