ValidReferenceConstraintValidator missing field entity

Removing the tags as per the guidelines: https://www.drupal.org/issue-tags. Generally not adding a tag unless aids discovery in a way that the other fields don't allow for. In this case we have the component, which I think covers it.

This fix makes total sense. Why not pass the entity through to the plugin manager if we have it. Small item of feedback:

+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php
@@ -91,8 +91,14 @@ public function validate($value, Constraint $constraint) {

@@ -91,8 +91,14 @@ public function validate($value, Constraint $constraint) {
+    if ($value_parent = $value->getParent()) {
+      $value_parent_entity = $value_parent->getValue();
+    } else {
+      $value_parent_entity = NULL;
+    }
...
+    $handler = $this->selectionManager->getSelectionHandler($value->getFieldDefinition(), $value_parent_entity);

The FieldItemListInterface specifies:

  /**
   * Gets the entity that field belongs to.
   *
   * @return \Drupal\Core\Entity\EntityInterface
   *   The entity object.
   */
  public function getEntity();

So I think it's safe to assume you'll always get an entity from ->getParent(). Thus I think we can simply pass $value->getParent() to $this->selectionManager->getSelectionHandler.

For me, this is right on the line for needing a test. It uses an API in a way which seems obviously more correct, but wondering if there is a test case which obviously exposes this as a bug we can use? We'd possibly need a test handler which uses the entity, but since it's an optional param, most handlers should work with NULL if possible anyway.

I just RTBC'd a very similar patch in #2879918: Send entity to selection handler on validate., so I'm going to close this one as a duplicate.

@grathbone, thanks for the patch, I asked the core committers to credit you in that issue :)