diff --git a/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
index 1ecdc19d66..063d065f8d 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/TemplateDiscovery.php
@@ -13,26 +13,27 @@
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 
 /**
- * Discovers plugins from Twig files.
+ * Discovers plugins from Twig template files.
  */
 abstract class TemplateDiscovery implements DiscoveryInterface {
 
   use DiscoveryTrait;
 
   /**
-   * Defines the key in the discovered data where the file path is stored.
+   * The key in the output discovered data for the file path.
    */
   const FILE_KEY = '_discovered_file_path';
 
   /**
-   * An array of directories to scan, keyed by the provider.
+   * Directories to scan.
    *
-   * The value can either be a string or an array of strings. The string values
-   * should be the path of a directory to scan.
+   * Array whose keys are provider (extension) short names, and whose values can
+   * be either a single string path to scan for plugins, or an array of paths
+   * for this provider.
    *
    * @var array
    */
-  protected $directories = [];
+  protected $directories;
 
   /**
    * The suffix for the file cache key.
@@ -42,30 +43,52 @@ abstract class TemplateDiscovery implements DiscoveryInterface {
   protected $fileCacheKeySuffix;
 
   /**
-   * The key contained in the discovered data that identifies it.
+   * The key in the output discovered data that contains the ID.
    *
    * @var string
    */
   protected $idKey;
 
+  /**
+   * The regular expression filter used to find relevant files.
+   *
+   * @var string
+   */
+  protected $fileFilter;
+
+  /**
+   * An extension that is allowed to provide plugins on behalf of others.
+   *
+   * @var string
+   */
+  protected $masterProvider;
+
   /**
    * Constructs a TemplateDiscovery object.
    *
    * @param array $directories
-   *   An array of directories to scan, keyed by the provider. The value can
-   *   either be a string or an array of strings. The string values should be
-   *   the path of a directory to scan.
+   *   Array whose keys are provider (extension) short names, and whose values
+   *   can be either a single string path to scan for plugins, or an array of
+   *   paths for this provider.
    * @param string $file_cache_key_suffix
-   *   The file cache key suffix. This should be unique for each type of
-   *   discovery.
-   * @param string $key
-   *   (optional) The key contained in the discovered data that identifies it.
-   *   Defaults to 'id'.
+   *   The file cache key suffix. This should be unique for each class that
+   *   extends this abstract class.
+   * @param string $master_provider
+   *   (optional) The name of a provider that is allowed to provide plugins on
+   *   behalf of other extensions.
+   * @param string $id_key
+   *   (optional) The key to use in the output discovered data for the plugin
+   *   ID. Defaults to 'id'.
+   * @param string $file_filter
+   *   (optional) Regular expression pattern to filter file names. Defaults to
+   *   a pattern that finds files with extension .html.twig.
    */
-  public function __construct(array $directories, $file_cache_key_suffix, $key = 'id') {
+  public function __construct(array $directories, string $file_cache_key_suffix, string $master_provider = '', string $id_key = 'id', string $file_filter = '/\.html\.twig$/i') {
     $this->directories = $directories;
     $this->fileCacheKeySuffix = $file_cache_key_suffix;
-    $this->idKey = $key;
+    $this->masterProvider = $master_provider;
+    $this->idKey = $id_key;
+    $this->fileFilter = $file_filter;
   }
 
   /**
@@ -74,7 +97,7 @@ public function __construct(array $directories, $file_cache_key_suffix, $key = '
   public function getDefinitions() {
     $plugins = $this->findAll();
 
-    // Flatten definitions into what's expected from plugins.
+    // Flatten definitions array.
     $definitions = [];
     foreach ($plugins as $list) {
       foreach ($list as $id => $definition) {
@@ -86,13 +109,26 @@ public function getDefinitions() {
   }
 
   /**
-   * Returns an array of discoverable items.
+   * Discovers plugins and returns them.
    *
    * @return array
-   *   An array of discovered data keyed by provider.
+   *   An array of discovered plugin data. The outer array is keyed by
+   *   provider. For each provider, the inner array is an associated array of
+   *   plugin data, keyed by plugin ID. Each array of plugin data contains the
+   *   following elements:
+   *   - The plugin ID (the file name without the extension). The key name
+   *     for this can be overridden in the constructor of this class, but
+   *     defaults to 'id'.
+   *   - provider: Short name of the extension that defined the plugin.
+   *   - _discovered_file_path: Path to the file (see static::FILE_KEY).
+   *   - Additional information from front matter in the file, as returned
+   *     by the extending class's implementation of
+   *     $this->validateAndSaveFrontMatter().
    *
    * @throws \Drupal\Component\Discovery\DiscoveryException
    *   Exception thrown if there is a problem during discovery.
+   *
+   * @see \Drupal\Component\FrontMatter\FrontMatter
    */
   public function findAll() {
     $all = [];
@@ -103,7 +139,7 @@ public function findAll() {
 
     // Try to load from the file cache first.
     foreach ($file_cache->getMultiple(array_keys($files)) as $file => $data) {
-      $all[$files[$file]][$data['id']] = $data;
+      $all[$files[$file]][$data[$this->idKey]] = $data;
       unset($files[$file]);
     }
 
@@ -113,13 +149,14 @@ public function findAll() {
       foreach ($files as $file => $provider) {
         $plugin_id = substr(basename($file), 0, -10);
         list($file_name_provider,) = explode('.', $plugin_id, 2);
-        if ($provider !== $file_name_provider) {
+        if ($provider !== $file_name_provider &&
+          (empty($this->masterProvider) || $provider != $this->masterProvider)) {
           throw new DiscoveryException("$file file name should begin with '$provider'");
         }
         $data = [
           // The plugin ID is derived from the filename. The extension
           // '.html.twig' is removed.
-          'id' => $plugin_id,
+          $this->idKey => $plugin_id,
           'provider' => $file_name_provider,
           static::FILE_KEY => $file,
         ];
@@ -133,8 +170,8 @@ public function findAll() {
           throw new DiscoveryException(sprintf('Malformed YAML in template "%s": %s.', $file, $e->getMessage()));
         }
 
-        $data = $this->validateAndSave($front_matter, $data);
-        $all[$provider][$data['id']] = $data;
+        $data = $this->validateAndSaveFrontMatter($front_matter, $data, $file);
+        $all[$provider][$data[$this->idKey]] = $data;
         $file_cache->set($file, $data);
       }
     }
@@ -143,27 +180,31 @@ public function findAll() {
   }
 
   /**
-   * Saves and validates front matter for a discovered template plugin.
+   * Validates and saves front matter for a discovered template plugin.
+   *
+   * Your method should throw an exception of there is an error in the front
+   * matter.
    *
    * @param array $front_matter
    *   Array of parsed front matter from the template.
    * @param array $data
-   *   Array of plugin information, with keys:
-   *   - id: Plugin ID from the file name.
-   *   - provider: Short name of the provider of the plugin.
-   *   - _discovered_file_path: Path to the template.
+   *   Array of plugin definition information to add the front matter data to.
+   * @param string $file
+   *   File name for this plugin.
    *
    * @return array
-   *   $data with the necessary front matter added to the plugin definition.
-   *   Throw an exception of there is an error in the front matter.
+   *   $data with the front matter data added to the array.
+   *
+   * @see \Drupal\Component\FrontMatter\FrontMatter
    */
-  abstract function validateAndSave(array $front_matter, array $data);
+  abstract protected function validateAndSaveFrontMatter(array $front_matter, array $data, string $file);
 
   /**
-   * Returns an array of providers keyed by file path.
+   * Finds files containing plugins.
    *
    * @return array
-   *   An array of providers keyed by file path.
+   *   An array whose keys are files containing plugins, and whose values are
+   *   the provider (extension defining the plugin).
    */
   protected function findFiles() {
     $file_list = [];
@@ -172,7 +213,7 @@ protected function findFiles() {
       foreach ($directories as $directory) {
         if (is_dir($directory)) {
           /** @var \SplFileInfo $fileInfo */
-          $iterator = new RegexDirectoryIterator($directory, '/\.html\.twig$/i');
+          $iterator = new RegexDirectoryIterator($directory, $this->fileFilter);
           foreach ($iterator as $fileInfo) {
             $file_list[$fileInfo->getPathname()] = $provider;
           }