Prepare rest endpoint to be extended.

lib/compat/wordpress-6.1/rest-api.php

@@ -5,22 +5,6 @@
  * @package gutenberg
  */
 
-/**
- * Update `wp_template` and `wp_template-part` post types to use
- * Gutenberg's REST controller.
- *
- * @param array  $args Array of arguments for registering a post type.
- * @param string $post_type Post type key.
- */
-function gutenberg_update_templates_template_parts_rest_controller( $args, $post_type ) {
-	if ( in_array( $post_type, array( 'wp_template', 'wp_template-part' ), true ) ) {
-		$args['rest_controller_class'] = 'Gutenberg_REST_Templates_Controller';
-	}
-	return $args;
-}
-add_filter( 'register_post_type_args', 'gutenberg_update_templates_template_parts_rest_controller', 10, 2 );
-
-
 /**
  * Add the post type's `icon`(menu_icon) in the response.
  * When we backport this change we will need to add the

lib/compat/wordpress-6.2/class-gutenberg-rest-templates-controller-6-2.php

@@ -0,0 +1,63 @@
+<?php
+/**
+ * REST API: Gutenberg_REST_Templates_Controller_6_2 class
+ *
+ * @package    Gutenberg
+ * @subpackage REST_API
+ */
+
+/**
+ * Base Templates REST API Controller.
+ */
+class Gutenberg_REST_Templates_Controller_6_2 extends Gutenberg_REST_Templates_Controller {
+
+	/**
+	 * Registers the controllers routes.
+	 *
+	 * @return void
+	 */
+	public function register_routes() {
+		parent::register_routes();
+		// Get fallback template content.
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base . '/lookup',
+			array(
+				array(
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => array( $this, 'get_template_fallback' ),
+					'permission_callback' => array( $this, 'get_item_permissions_check' ),
+					'args'                => array(
+						'slug'            => array(
+							'description' => __( 'The slug of the template to get the fallback for', 'gutenberg' ),
+							'type'        => 'string',
+							'required'    => true,
+						),
+						'is_custom'       => array(
+							'description' => __( 'Indicates if a template is custom or part of the template hierarchy', 'gutenberg' ),
+							'type'        => 'boolean',
+						),
+						'template_prefix' => array(
+							'description' => __( 'The template prefix for the created template. This is used to extract the main template type ex. in `taxonomy-books` we extract the `taxonomy`', 'gutenberg' ),
+							'type'        => 'string',
+						),
+					),
+				),
+			)
+		);
+	}
+
+	/**
+	 * Returns the fallback template for a given slug.
+	 *
+	 * @param WP_REST_Request $request The request instance.
+	 *
+	 * @return WP_REST_Response|WP_Error
+	 */
+	public function get_template_fallback( $request ) {
+		$hierarchy         = get_template_hierarchy( $request['slug'], $request['is_custom'], $request['template_prefix'] );
+		$fallback_template = resolve_block_template( $request['slug'], $hierarchy, '' );
+		$response          = $this->prepare_item_for_response( $fallback_template, $request );
+		return rest_ensure_response( $response );
+	}
+}

lib/compat/wordpress-6.2/rest-api.php

@@ -5,6 +5,21 @@
  * @package gutenberg
  */
 
+/**
+ * Update `wp_template` and `wp_template-part` post types to use
+ * Gutenberg's REST controller.
+ *
+ * @param array  $args Array of arguments for registering a post type.
+ * @param string $post_type Post type key.
+ */
+function gutenberg_update_templates_template_parts_rest_controller( $args, $post_type ) {
+	if ( in_array( $post_type, array( 'wp_template', 'wp_template-part' ), true ) ) {
+		$args['rest_controller_class'] = 'Gutenberg_REST_Templates_Controller_6_2';
+	}
+	return $args;
+}
+add_filter( 'register_post_type_args', 'gutenberg_update_templates_template_parts_rest_controller', 10, 2 );
+
 /**
  * Registers the block pattern categories REST API routes.
  */

lib/load.php

@@ -44,6 +44,7 @@ function gutenberg_is_experiment_enabled( $name ) {
 	require_once __DIR__ . '/compat/wordpress-6.2/class-gutenberg-rest-block-patterns-controller-6-2.php';
 	require_once __DIR__ . '/compat/wordpress-6.2/class-gutenberg-rest-block-pattern-categories-controller.php';
 	require_once __DIR__ . '/compat/wordpress-6.2/class-gutenberg-rest-pattern-directory-controller-6-2.php';
+	require_once __DIR__ . '/compat/wordpress-6.2/class-gutenberg-rest-templates-controller-6-2.php';
 	require_once __DIR__ . '/compat/wordpress-6.2/rest-api.php';
 	require_once __DIR__ . '/compat/wordpress-6.2/block-patterns.php';
 	require_once __DIR__ . '/compat/wordpress-6.2/class-gutenberg-rest-global-styles-controller-6-2.php';