diff -u b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscovery.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscovery.php
@@ -12,22 +12,22 @@
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 
 /**
- * Provides discovery for files containing FronMatter within a given set of directories.
+ * Provides discovery for files containing FrontMatter.
  */
 class FrontMatterDiscovery implements DiscoveryInterface {
 
   use DiscoveryTrait;
 
   /**
-   * Plugin may appear not on top level of frontmatter.
+   * Plugin may appear not on top level of FrontMatter.
+   *
+   * @var array
    *
    * @code
    * plugins:
    *   layouts:
    *     layout_onecolumn:
    * @endcode
-   *
-   * @var array
    */
   protected $arrayPosition = [];
 
@@ -55,28 +55,27 @@
   /**
    * Contains an array of translatable properties passed along to t().
    *
-   * @see \Drupal\Core\Plugin\Discovery\FrontMatterDiscovery::addTranslatableProperty()
-   *
    * @var array
+   *
+   * @see \Drupal\Core\Plugin\Discovery\FrontMatterDiscovery::addTranslatableProperty()
    */
   protected $translatableProperties = [];
 
-
   /**
    * Constructs a FrontMatterDiscovery object.
    *
    * @param array $directories
    *   An array of directories to scan, keyed by the provider.
-   * @param string $file_filter
-   *   (optional) Regular expression pattern to filter file names. Defaults to
-   *   a pattern that finds files with extension .frontmatter.html.twig.
    * @param string $file_cache_key_suffix
    *   The file cache key suffix. This should be unique for each class that
    *   extends this abstract class.
    * @param array $array_position
-   *   Deepth of plugin in FrontMatter array.
+   *   Depth of plugin in FrontMatter array.
+   * @param string $file_filter
+   *   (optional) Regular expression pattern to filter file names. Defaults to
+   *   a pattern that finds files with extension .frontmatter.html.twig.
    */
-  public function __construct(array $directories, string $file_filter = '/\.frontmatter\.html\.twig$/i', string $file_cache_key_suffix, array $array_position = []) {
+  public function __construct(array $directories, string $file_cache_key_suffix, array $array_position = [], string $file_filter = '/\.frontmatter\.html\.twig$/i') {
     $this->arrayPosition = $array_position;
     $this->directories = $directories;
     $this->fileFilter = $file_filter;
@@ -181,10 +180,11 @@
     return $all;
   }
 
-    /**
+  /**
    * Returns an array of file paths, keyed by provider.
    *
    * @return array
+   *   An array of file paths keyed by provider.
    */
   protected function findFiles() {
     $file_list = [];
@@ -203,7 +203,7 @@
   }
 
   /**
-   * Gets an iterator to recursive loop over the files in the provided directory.
+   * Gets an iterator to recursive loop over files in the provided directory.
    *
    * @param string $directory
    *   The directory to scan.
@@ -221,2 +221,3 @@
   }
+
 }
diff -u b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscoveryDecorator.php b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscoveryDecorator.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscoveryDecorator.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/FrontMatterDiscoveryDecorator.php
@@ -5,10 +5,10 @@
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
 
 /**
- * Enables Frontmatter discovery for plugin definitions.
+ * Enables FrontMatter discovery for plugin definitions.
  *
  * You should normally extend this class to add validation for the values in the
- * Frontmatter data or to restrict use of the class or derivatives keys.
+ * FrontMatter data or to restrict use of the class or derivatives keys.
  */
 class FrontMatterDiscoveryDecorator extends FrontMatterDiscovery {
 
@@ -24,14 +24,19 @@
    *
    * @param \Drupal\Component\Plugin\Discovery\DiscoveryInterface $decorated
    *   The discovery object that is being decorated.
-   * @param string $name
-   *   The file name suffix to use for discovery; for instance, 'test' will
-   *   become 'MODULE.test.yml'.
    * @param array $directories
    *   An array of directories to scan.
+   * @param string $file_cache_key_suffix
+   *   The file cache key suffix. This should be unique for each class that
+   *   extends this abstract class.
+   * @param array $array_position
+   *   Depth of plugin in FrontMatter array.
+   * @param string $file_filter
+   *   The file name suffix to use for discovery; for instance, 'test' will
+   *   become 'MODULE.test.yml'.
    */
-  public function __construct(DiscoveryInterface $decorated, $directories, $file_filter, $file_cache_key_suffix, $array_position) {
-    parent::__construct($directories, $file_filter, $file_cache_key_suffix, $array_position);
+  public function __construct(DiscoveryInterface $decorated, array $directories, string $file_cache_key_suffix, array $array_position, string $file_filter) {
+    parent::__construct($directories, $file_cache_key_suffix, $array_position, $file_filter);
 
     $this->decorated = $decorated;
   }
@@ -45,8 +50,16 @@
 
   /**
    * Passes through all unknown calls onto the decorated object.
+   *
+   * @param string $method
+   *   The method to call on the decorated plugin discovery.
+   * @param array $args
+   *   The arguments to send to the method.
+   *
+   * @return mixed
+   *   The method result.
    */
-  public function __call($method, $args) {
+  public function __call($method, array $args) {
     return call_user_func_array([$this->decorated, $method], $args);
   }