Issue #1948702 by chx: module_hook() is broken for disabled modules and included hooks.

core/lib/Drupal/Core/Extension/ModuleHandler.php

@@ -215,10 +215,12 @@ class ModuleHandler implements ModuleHandlerInterface {
     }
 
     $name = $name ?: $module;
-    $file = DRUPAL_ROOT . '/' . dirname($this->moduleList[$module]) . "/$name.$type";
-    if (is_file($file)) {
-      require_once $file;
-      return $file;
+    if (isset($this->moduleList[$module])) {
+      $file = DRUPAL_ROOT . '/' . dirname($this->moduleList[$module]) . "/$name.$type";
+      if (is_file($file)) {
+        require_once $file;
+        return $file;
+      }
     }
 
     return FALSE;

core/tests/Drupal/Tests/Core/Extension/ModuleHandlerUnitTest.php

@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Extension\ModuleHanderUnitTest.
+ */
+
+namespace Drupal\Tests\Core\Extension;
+
+if (!defined('DRUPAL_ROOT')) {
+  define('DRUPAL_ROOT', dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__)))));
+}
+
+use Drupal\Core\Extension\ModuleHandler;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the ModuleHandler class.
+ *
+ * @group System
+ */
+class ModuleHandlerUnitTest extends UnitTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'ModuleHandler functionality',
+      'description' => 'Tests the ModuleHandler class.',
+      'group' => 'System',
+    );
+  }
+
+  function setUp() {
+    parent::setUp();
+    $this->moduleHandler = new ModuleHandler;
+  }
+
+  function testloadInclude() {
+    // Make sure that load include does not throw notices on nonexisiting
+    // modules.
+    $this->moduleHandler->loadInclude('foo', 'inc');
+  }
+}