Issue #2502021 by willzyx, Saphyel, dawehner, DuaelFr, b0unty, cilefen,...

Issue #2502021 by willzyx, Saphyel, dawehner, DuaelFr, b0unty, cilefen, larowlan, lauriii: Unhandled exception when trying to register a duplicate user

core/modules/user/src/Tests/UserRegistrationTest.php

@@ -8,6 +8,7 @@
 namespace Drupal\user\Tests;
 
 use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
@@ -264,6 +265,24 @@ function testRegistrationDefaultValues() {
     $this->assertEqual($new_user->init->value, $mail, 'Correct init field.');
   }
 
+  /**
+   * Tests username and email field constraints on user registration.
+   *
+   * @see \Drupal\user\Plugin\Validation\Constraint\UserNameUnique
+   * @see \Drupal\user\Plugin\Validation\Constraint\UserMailUnique
+   */
+  public function testUniqueFields() {
+    $account = $this->drupalCreateUser();
+
+    $edit = ['mail' => 'test@example.com', 'name' => $account->getUsername()];
+    $this->drupalPostForm('user/register', $edit, t('Create new account'));
+    $this->assertRaw(SafeMarkup::format('The username %value is already taken.', ['%value' => $account->getUsername()]));
+
+    $edit = ['mail' => $account->getEmail(), 'name' => $this->randomString()];
+    $this->drupalPostForm('user/register', $edit, t('Create new account'));
+    $this->assertRaw(SafeMarkup::format('The email address %value is already taken.', ['%value' => $account->getEmail()]));
+  }
+
   /**
    * Tests Field API fields on user registration forms.
    */

core/modules/user/src/UserAccessControlHandler.php

@@ -80,8 +80,11 @@ protected function checkFieldAccess($operation, FieldDefinitionInterface $field_
     $is_own_account = $items ? $items->getEntity()->id() == $account->id() : FALSE;
     switch ($field_definition->getName()) {
       case 'name':
-        // Allow view access to anyone with access to the entity.
-        if ($operation == 'view') {
+        // Allow view access to anyone with access to the entity. Anonymous
+        // users should be able to access the username field during the
+        // registration process, otherwise the username and email constraints
+        // are not checked.
+        if ($operation == 'view' || ($items && $account->isAnonymous() && $items->getEntity()->isAnonymous())) {
           return AccessResult::allowed()->cachePerPermissions();
         }
         // Allow edit access for the own user name if the permission is