diff --git a/core/core.services.yml b/core/core.services.yml
index 212a84a..efa4b34 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -475,10 +475,10 @@ services:
       - { name: service_collector, tag: 'module_install.uninstall_validator', call: addUninstallValidator }
     arguments: ['@app.root', '@module_handler', '@kernel', '@router.builder']
     lazy: true
-  module_listing:
+  extension.list.module:
     class: Drupal\Core\Extension\ModuleExtensionList
-    arguments: ['@app.root', 'module', '@cache.default', '@info_parser', '@module_handler', '@config.factory', '@profile_listing']
-  profile_listing:
+    arguments: ['@app.root', 'module', '@cache.default', '@info_parser', '@module_handler', '@config.factory', '@extension.list.profile']
+  extension.list.profile:
     class: Drupal\Core\Extension\ProfileExtensionList
     arguments: ['@app.root', 'profile', '@cache.default', '@info_parser', '@module_handler']
   content_uninstall_validator:
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 79c1e10..5f5a19a 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -201,14 +201,17 @@ function drupal_get_filename($type, $name, $filename = NULL) {
   }
 
   if ($type === 'module' || $type === 'profile') {
-    $service = $type . '_listing';
+    $service_id = 'extension.list.' . $type;
+    /** @var \Drupal\Core\Extension\ExtensionList $extension_list */
+    $extension_list = \Drupal::service($service_id);
     if (isset($filename)) {
       // Manually add the info file path of an extension.
-      return \Drupal::service($service)->setFilename($name, $filename);
+      $extension_list->setFilename($name, $filename);
+      return;
     }
     else {
       try {
-        return \Drupal::service($service)->getFilename($name);
+        return $extension_list->getFilename($name);
       }
       catch (\InvalidArgumentException $e) {
         // Catch the exception and trigger error to maintain existing behavior.
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index a232757..f83bc7c 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -780,9 +780,10 @@ protected function initializeContainer() {
     if (isset($this->container)) {
       // Save the id of the currently logged in user.
       if ($this->container->initialized('current_user')) {
+        /** @var \Drupal\Core\Session\AccountProxyInterface $current_user */
         $current_user = $this->container->get('current_user');
         // Ensure to not accidentally initialize the user.
-        $current_user_id = $current_user->accountIsInitialized() ? $current_user->id() : 0;
+        $current_user_id = $current_user->hasAccount() ? $current_user->id() : 0;
       }
 
       // If there is a session, close and save it.
diff --git a/core/lib/Drupal/Core/Extension/ExtensionList.php b/core/lib/Drupal/Core/Extension/ExtensionList.php
index 3d3ed04..925d319 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionList.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionList.php
@@ -12,7 +12,7 @@
 abstract class ExtensionList {
 
   /**
-   * The type of the extension, either 'module', 'theme' or 'profile'.
+   * The type of the extension, such as "module" or "theme".
    *
    * @var string
    */
@@ -33,9 +33,9 @@
   protected $cache;
 
   /**
-   * Default information added to every extension.
+   * Default values to be merged into *.info.yml file arrays.
    *
-   * @var array
+   * @var mixed[]
    */
   protected $defaults = [];
 
@@ -54,7 +54,7 @@
   protected $moduleHandler;
 
   /**
-   * The static cached extensions.
+   * The statically cached extensions.
    *
    * @var \Drupal\Core\Extension\Extension[]
    */
@@ -63,7 +63,8 @@
   /**
    * Static caching for extension info.
    *
-   * @var array
+   * @var array[]
+   *   Keys are extension names, and values their info arrays (mixed[]).
    */
   protected $extensionInfo;
 
@@ -128,25 +129,43 @@ protected function getExtensionDiscovery() {
    */
   public function reset() {
     $this->extensions = NULL;
-    $this->cache->delete($this->getCacheId());
+    $this->cache->delete($this->getListCacheId());
     $this->extensionInfo = NULL;
-    $this->cache->delete("system.{$this->type}.info");
+    $this->cache->delete($this->getInfoCacheId());
     $this->fileNames = NULL;
-    $this->cache->delete("system.{$this->type}.files");
+    $this->cache->delete($this->getFilenameCacheId());
     $this->addedFileNames = NULL;
     return $this;
   }
 
   /**
-   * Returns the used cache ID.
+   * Returns the extension list cache ID.
    *
    * @return string
    */
-  protected function getCacheId() {
+  protected function getListCacheId() {
     return 'core.extension_listing.' . $this->type;
   }
 
   /**
+   * Returns the extension info cache ID.
+   *
+   * @return string
+   */
+  protected function getInfoCacheId() {
+    return "system.{$this->type}.info";
+  }
+
+  /**
+   * Returns the extension filenames cache ID.
+   *
+   * @return string
+   */
+  protected function getFilenameCacheId() {
+    return "system.{$this->type}.files";
+  }
+
+  /**
    * Determines if an extension exists in the filesystem.
    *
    * @param string $name
@@ -205,12 +224,12 @@ public function listExtensions() {
     if (isset($this->extensions)) {
       return $this->extensions;
     }
-    if ($cache = $this->cache->get($this->getCacheId())) {
+    if ($cache = $this->cache->get($this->getListCacheId())) {
       $this->extensions = $cache->data;
       return $this->extensions;
     }
     $extensions = $this->doListExtensions();
-    $this->cache->set($this->getCacheId(), $extensions);
+    $this->cache->set($this->getListCacheId(), $extensions);
     $this->extensions = $extensions;
     return $this->extensions;
   }
@@ -266,7 +285,7 @@ protected function doListExtensions() {
    *   The name of an extension whose information shall be returned. If
    *   $name does not exist or is not enabled, an empty array will be returned.
    *
-   * @return array
+   * @return mixed[]
    *   An associative array of extension information.
    *
    * @throws \InvalidArgumentException
@@ -287,18 +306,19 @@ public function getInfo($name) {
    * This function returns the contents of the .info.yml file for each installed
    * extension.
    *
-   * @return array
+   * @return array[]
    *   An associative array of extension information keyed by name. If no
    *   records are available, an empty array is returned.
    */
   public function getAllInfo() {
     if (!isset($this->extensionInfo)) {
-      if ($cache = $this->cache->get("system.{$this->type}.info")) {
+      $cache_key = "system.{$this->type}.info";
+      if ($cache = $this->cache->get($cache_key)) {
         $info = $cache->data;
       }
       else {
         $info = $this->recalculateInfo();
-        $this->cache->set("system.{$this->type}.info", $info);
+        $this->cache->set($cache_key, $info);
       }
       $this->extensionInfo = $info;
     }
@@ -311,7 +331,7 @@ public function getAllInfo() {
    * The information is placed in cache with the "system.{extension_type}.info"
    * key.
    *
-   * @return array
+   * @return array[]
    *   An array of arrays of .info.yml entries keyed by the extension name.
    */
   protected function recalculateInfo() {
@@ -329,12 +349,13 @@ protected function recalculateInfo() {
    */
   public function getFilenames() {
     if (!isset($this->fileNames)) {
-      if ($cache = $this->cache->get("system.{$this->type}.files")) {
+      $cache_id = $this->getFilenameCacheId();
+      if ($cache = $this->cache->get($cache_id)) {
         $file_names = $cache->data;
       }
       else {
         $file_names = $this->recalculateFilenames();
-        $this->cache->set("system.{$this->type}.files", $file_names);
+        $this->cache->set($cache_id, $file_names);
       }
       $this->fileNames = $file_names;
     }
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandler.php b/core/lib/Drupal/Core/Extension/ModuleHandler.php
index a428389..c41a650 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandler.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandler.php
@@ -717,7 +717,7 @@ public function getModuleDirectories() {
    * {@inheritdoc}
    */
   public function getName($module) {
-    return \Drupal::service('module_listing')->getName($module);
+    return \Drupal::service('extension.list.module')->getName($module);
   }
 
 }
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
index e41b55b..1cc2d35 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -308,7 +308,7 @@ public function getModuleDirectories();
    *   in if no matching module is found.
    *
    * @deprecated in Drupal 8.2.0, will be removed before Drupal 9.0.0.
-   *   Use \Drupal::service('module_listing')->getName() instead.
+   *   Use \Drupal::service('extension.list.module')->getName() instead.
    */
   public function getName($module);
 
diff --git a/core/lib/Drupal/Core/Extension/ModuleInstaller.php b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
index ef41a0f..73064ef 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstaller.php
@@ -166,7 +166,7 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
             $module_filenames[$name] = $current_module_filenames[$name];
           }
           else {
-            $module_path = \Drupal::service('module_listing')->getPath($name);
+            $module_path = \Drupal::service('extension.list.module')->getPath($name);
             $pathname = "$module_path/$name.info.yml";
             $filename = file_exists($module_path . "/$name.module") ? "$name.module" : NULL;
             $module_filenames[$name] = new Extension($this->root, 'module', $pathname, $filename);
@@ -182,10 +182,10 @@ public function install(array $module_list, $enable_dependencies = TRUE) {
         $this->moduleHandler->load($module);
         module_load_install($module);
 
-        // Clear the static cache of module_listing to pick up the
-        // new module, since it merges the installation status of modules into
-        // its statically cached list.
-        \Drupal::service('module_listing')->reset();
+        // Clear the static cache of the "extension.list.module" service to pick
+        // up the new module, since it merges the installation status of modules
+        // into its statically cached list.
+        \Drupal::service('extension.list.module')->reset();
 
         // Update the kernel to include it.
         $this->updateKernel($module_filenames);
@@ -437,10 +437,10 @@ public function uninstall(array $module_list, $uninstall_dependents = TRUE) {
       // Remove any potential cache bins provided by the module.
       $this->removeCacheBins($module);
 
-      // Clear the static cache of module_listing to pick up the
-      // new module, since it merges the installation status of modules into
-      // its statically cached list.
-      \Drupal::service('module_listing')->reset();
+      // Clear the static cache of the "extension.list.module" service to pick
+      // up the new module, since it merges the installation status of modules
+      // into its statically cached list.
+      \Drupal::service('extension.list.module')->reset();
 
       // Clear plugin manager caches.
       \Drupal::getContainer()->get('plugin.cache_clearer')->clearCachedDefinitions();
diff --git a/core/lib/Drupal/Core/Session/AccountProxy.php b/core/lib/Drupal/Core/Session/AccountProxy.php
index 63cf9a1..7c48619 100644
--- a/core/lib/Drupal/Core/Session/AccountProxy.php
+++ b/core/lib/Drupal/Core/Session/AccountProxy.php
@@ -45,7 +45,7 @@ public function setAccount(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function accountIsInitialized() {
+  public function hasAccount() {
     return isset($this->account);
   }
 
diff --git a/core/lib/Drupal/Core/Session/AccountProxyInterface.php b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
index 8969bb2..b7525aa 100644
--- a/core/lib/Drupal/Core/Session/AccountProxyInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountProxyInterface.php
@@ -6,6 +6,7 @@
  * Defines an interface for a service which has the current account stored.
  *
  * @ingroup user_api
+ * @internal
  */
 interface AccountProxyInterface extends AccountInterface {
 
@@ -44,11 +45,11 @@ public function getAccount();
   public function setInitialAccountId($account_id);
 
   /**
-   * Returns whether the account is already initialized.
+   * Checks whether an account is currently wrapped.
    *
    * @return bool
-   *   TRUE, when the account was initialized already.
+   *   TRUE, if an account is currently wrapped. FALSE otherwise.
    */
-  public function accountIsInitialized();
+  public function hasAccount();
 
 }