WP_REST_Posts_Controller::check_template

	public function check_template( $template, $request ) {

		if ( ! $template ) {
			return true;
		}

		if ( $request['id'] ) {
			$current_template = get_page_template_slug( $request['id'] );
		} else {
			$current_template = '';
		}

		// Always allow for updating a post to the same template, even if that template is no longer supported.
		if ( $template === $current_template ) {
			return true;
		}

		// If this is a create request, get_post() will return null and wp theme will fallback to the passed post type.
		$allowed_templates = wp_get_theme()->get_page_templates( get_post( $request['id'] ), $this->post_type );

		if ( isset( $allowed_templates[ $template ] ) ) {
			return true;
		}

		return new WP_Error(
			'rest_invalid_param',
			/* translators: 1: Parameter, 2: List of valid values. */
			sprintf( __( '%1$s is not one of %2$s.' ), 'template', implode( ', ', array_keys( $allowed_templates ) ) )
		);
	}