Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony-cmf/routing/Enhancer/RouteEnhancerInterface.php

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+<?php
+
+/*
+ * This file is part of the Symfony CMF package.
+ *
+ * (c) 2011-2014 Symfony CMF
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Cmf\Component\Routing\Enhancer;
+
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * A route enhancer can change the values in the route data arrays
+ *
+ * This is useful to provide information to the rest of the routing system
+ * that can be inferred from other parameters rather than hardcode that
+ * information in every route.
+ *
+ * @author David Buchmann
+ */
+interface RouteEnhancerInterface
+{
+    /**
+     * Update the defaults based on its own data and the request.
+     *
+     * @param array   $defaults the getRouteDefaults array.
+     * @param Request $request  the Request instance.
+     *
+     * @return array the modified defaults. Each enhancer MUST return the
+     *               $defaults but may add or remove values.
+     */
+    public function enhance(array $defaults, Request $request);
+}