Issue #3441434 by kunal.sachdev, narendrar, omkar.podey, bbrala, wim leers,...

Issue #3441434 by kunal.sachdev, narendrar, omkar.podey, bbrala, wim leers, catch, smustgrave, yash.rode, alexpott: Add validation constraints to core.menu.schema.yml

core/config/schema/core.menu.schema.yml

 core.menu.static_menu_link_overrides:
   type: config_object
   label: 'Static menu link overrides'
+  constraints:
+    FullyValidatable: ~
   mapping:
     definitions:
       type: sequence
@@ -12,9 +14,24 @@ core.menu.static_menu_link_overrides:
           menu_name:
             type: string
             label: 'Menu name'
+            # This is the id of system.menu.* config.
+            # @see core/modules/system/config/schema/system.schema.yml
+            ConfigExists:
+              prefix: 'system.menu.'
           parent:
             type: string
             label: 'Parent'
+            # Menu link plugins specify the empty string if there is no parent.
+            # So this can be an empty string('') or valid menu link plugin ID.
+            # @see \Drupal\Core\Menu\MenuLinkInterface::getParent()
+            constraints:
+              AtLeastOneOf:
+                constraints:
+                  - PluginExists:
+                      manager: plugin.manager.menu.link
+                      interface: 'Drupal\Core\Menu\MenuLinkInterface'
+                  - IdenticalTo:
+                      value: ''
           weight:
             type: weight
             label: 'Weight'

core/lib/Drupal/Core/Validation/ConstraintManager.php

@@ -12,6 +12,7 @@
 use Symfony\Component\Validator\Constraints\Blank;
 use Symfony\Component\Validator\Constraints\Callback;
 use Symfony\Component\Validator\Constraints\Choice;
+use Symfony\Component\Validator\Constraints\IdenticalTo;
 use Symfony\Component\Validator\Constraints\Image;
 use Symfony\Component\Validator\Constraints\NotBlank;
 
@@ -122,6 +123,11 @@ public function registerDefinitions() {
       'class' => Image::class,
       'type' => ['string'],
     ]);
+    $this->getDiscovery()->setDefinition('IdenticalTo', [
+      'label' => new TranslatableMarkup('IdenticalTo'),
+      'class' => IdenticalTo::class,
+      'type' => FALSE,
+    ]);
   }
 
   /**