Issue #2646100 by gapple: Exception on php7 + APCu without backwards compatibilty enabled

core/lib/Drupal/Core/DrupalKernel.php

@@ -963,11 +963,13 @@ protected function initializeSettings(Request $request) {
       }
     }
 
-    // If the class loader is still the same, possibly upgrade to the APCu class
+    // If the class loader is still the same, possibly upgrade to the APC class
     // loader.
+    // ApcClassLoader does not support APCu without backwards compatibility
+    // enabled.
     if ($class_loader_class == get_class($this->classLoader)
         && Settings::get('class_loader_auto_detect', TRUE)
-        && function_exists('apcu_fetch')) {
+        && extension_loaded('apc')) {
       $prefix = Settings::getApcuPrefix('class_loader', $this->root);
       $apc_loader = new \Symfony\Component\ClassLoader\ApcClassLoader($prefix, $this->classLoader);
       $this->classLoader->unregister();