basic_auth module was assumed

src/AltloginServiceProvider.php

@@ -6,7 +6,7 @@ use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\DependencyInjection\ServiceProviderBase;
 
 /**
- * Modifies Drupal's services
+ * Modifies basic auth service to accept other usernames.
  */
 class AltLoginServiceProvider extends ServiceProviderBase {
 
@@ -14,8 +14,9 @@ class AltLoginServiceProvider extends ServiceProviderBase {
    * {@inheritdoc}
    */
   public function alter(ContainerBuilder $container) {
-    $container->getDefinition('basic_auth.authentication.basic_auth')
-      ->setClass('Drupal\alt_login\Authentication\Provider\BasicAuth');
+    if ($def = $container->getDefinition('basic_auth.authentication.basic_auth')) {
+      $def->setClass('Drupal\alt_login\Authentication\Provider\BasicAuth');
+    }
   }
 
 }