Issue #3344744 by alexpott: A route with a default title of 0 does not work

core/lib/Drupal/Core/Controller/TitleResolver.php

@@ -57,10 +57,11 @@ public function getTitle(Request $request, Route $route) {
       $arguments = $this->argumentResolver->getArguments($request, $callable);
       $route_title = call_user_func_array($callable, $arguments);
     }
-    elseif ($title = $route->getDefault('_title')) {
+    elseif ($route->hasDefault('_title') && strlen($route->getDefault('_title')) > 0) {
+      $title = $route->getDefault('_title');
       $options = [];
-      if ($context = $route->getDefault('_title_context')) {
-        $options['context'] = $context;
+      if ($route->hasDefault('_title_context')) {
+        $options['context'] = $route->getDefault('_title_context');
       }
       $args = [];
       if (($raw_parameters = $request->attributes->get('_raw_variables'))) {

core/tests/Drupal/Tests/Core/Controller/TitleResolverTest.php

@@ -70,6 +70,17 @@ public function testStaticTitle() {
     $this->assertEquals(new TranslatableMarkup('static title', [], [], $this->translationManager), $this->titleResolver->getTitle($request, $route));
   }
 
+  /**
+   * Tests a static title of '0'.
+   *
+   * @see \Drupal\Core\Controller\TitleResolver::getTitle()
+   */
+  public function testStaticTitleZero() {
+    $request = new Request();
+    $route = new Route('/test-route', ['_title' => '0', '_title_context' => '0']);
+    $this->assertEquals(new TranslatableMarkup('0', [], ['context' => '0'], $this->translationManager), $this->titleResolver->getTitle($request, $route));
+  }
+
   /**
    * Tests a static title with a context.
    *