diff --git a/core/lib/Drupal/Core/DependencyInjection/Compiler/TwigExtensionPass.php b/core/lib/Drupal/Core/DependencyInjection/Compiler/TwigExtensionPass.php
index 35dd690..caade2d 100644
--- a/core/lib/Drupal/Core/DependencyInjection/Compiler/TwigExtensionPass.php
+++ b/core/lib/Drupal/Core/DependencyInjection/Compiler/TwigExtensionPass.php
@@ -11,9 +11,10 @@
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 
 /**
- * Adds twig_extension_hash parameter to the container.
+ * Adds the twig_extension_hash parameter to the container.
  *
- * A hash of all extensions for Twig template invalidation.
+ * twig_extension_hash is a hash of all extension mtimes for Twig template
+ * invalidation.
  */
 class TwigExtensionPass implements CompilerPassInterface {
 
@@ -23,7 +24,10 @@ class TwigExtensionPass implements CompilerPassInterface {
   public function process(ContainerBuilder $container) {
     $twig_extension_hash = '';
     foreach (array_keys($container->findTaggedServiceIds('twig.extension')) as $service_id) {
-      $twig_extension_hash .= $container->getDefinition($service_id)->getClass();
+      $reflecton = new \ReflectionClass($container->getDefinition($service_id)->getClass());
+      // We use mtime to invalidate both on new extensions and changes of
+      // existing ones.
+      $twig_extension_hash .= filemtime($reflecton->getFileName());
     }
 
     $container->setParameter('twig_extension_hash', $twig_extension_hash);