diff --git a/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
index 4f9f737..3f466a6 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
@@ -115,7 +115,7 @@
    *
    * @var string
    */
-  protected $masterProvider;
+  protected $rootProvider;
 
   /**
    * Constructs a TemplateDiscovery object.
@@ -127,7 +127,7 @@
    * @param string $file_cache_key_suffix
    *   The file cache key suffix. This should be unique for each class that
    *   extends this abstract class.
-   * @param string $master_provider
+   * @param string $root_provider
    *   (optional) The name of a provider that is allowed to provide plugins on
    *   behalf of other extensions.
    * @param string $id_key
@@ -137,10 +137,10 @@
    *   (optional) Regular expression pattern to filter file names. Defaults to
    *   a pattern that finds files with extension .html.twig.
    */
-  public function __construct(array $directories, string $file_cache_key_suffix, string $master_provider = '', string $id_key = 'id', string $file_filter = '/\.html\.twig$/i') {
+  public function __construct(array $directories, string $file_cache_key_suffix, string $root_provider = '', string $id_key = 'id', string $file_filter = '/\.html\.twig$/i') {
     $this->directories = $directories;
     $this->fileCacheKeySuffix = $file_cache_key_suffix;
-    $this->masterProvider = $master_provider;
+    $this->rootProvider = $root_provider;
     $this->idKey = $id_key;
     $this->fileFilter = $file_filter;
   }
@@ -184,7 +184,7 @@ public function getDefinitions() {
    *
    * @see \Drupal\Component\FrontMatter\FrontMatter
    */
-  public function findAll() {
+  public function findAll(): array {
     $all = [];
 
     $files = $this->findFiles();
@@ -204,7 +204,7 @@ public function findAll() {
         $plugin_id = substr(basename($file), 0, -10);
         list($file_name_provider,) = explode('.', $plugin_id, 2);
         if ($provider !== $file_name_provider &&
-          (empty($this->masterProvider) || $provider != $this->masterProvider)) {
+          (empty($this->rootProvider) || $provider != $this->rootProvider)) {
           throw new DiscoveryException("$file file name should begin with '$provider'");
         }
         $data = [
@@ -260,14 +260,14 @@ public function findAll() {
    *   An array whose keys are files containing plugins, and whose values are
    *   the provider (extension defining the plugin).
    */
-  protected function findFiles() {
+  protected function findFiles(): array {
     $file_list = [];
     foreach ($this->directories as $provider => $directories) {
       $directories = (array) $directories;
       foreach ($directories as $directory) {
         if (is_dir($directory)) {
-          /** @var \SplFileInfo $fileInfo */
           $iterator = new RegexDirectoryIterator($directory, $this->fileFilter);
+          /** @var \SplFileInfo $fileInfo */
           foreach ($iterator as $fileInfo) {
             $file_list[$fileInfo->getPathname()] = $provider;
           }