Issue #2977882 by Wim Leers: Term entity type's "parent" field does not specify "target_bundles" setting: a term from *any* vocabulary can be chosen

core/modules/taxonomy/src/Entity/Term.php

@@ -176,6 +176,16 @@ class Term extends ContentEntityBase implements TermInterface {
     return $fields;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public static function bundleFieldDefinitions(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
+    // Only terms in the same bundle can be a parent.
+    $fields['parent'] = clone $base_field_definitions['parent'];
+    $fields['parent']->setSetting('handler_settings', ['target_bundles' => [$bundle => $bundle]]);
+    return $fields;
+  }
+
   /**
    * {@inheritdoc}
    */

core/modules/taxonomy/src/Tests/TermTest.php

@@ -72,6 +72,15 @@ class TermTest extends TaxonomyTestBase {
       ->save();
   }
 
+  /**
+   * The "parent" field must restrict references to the same vocabulary.
+   */
+  public function testParentHandlerSettings() {
+    $vocabulary_fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('taxonomy_term', $this->vocabulary->id());
+    $parent_target_bundles = $vocabulary_fields['parent']->getSetting('handler_settings')['target_bundles'];
+    $this->assertIdentical([$this->vocabulary->id() => $this->vocabulary->id()], $parent_target_bundles);
+  }
+
   /**
    * Test terms in a single and multiple hierarchy.
    */