WordPress theme debug template name

functions.php

<?php

if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {

	/**
	 * Adds the current template info for debugging.
	 *
	 * @param string $t The template.
	 */
	function var_template_include( $t ) {
		$GLOBALS['current_theme_template'] = basename( $t );
		return $t;
	}
	add_filter( 'template_include', 'var_template_include', 1000 );

	/**
	 * Prints an HTML comment with the current template's name.
	 */
	function print_current_template_html_comment() {
		if ( ! isset( $GLOBALS['current_theme_template'] ) ) {
			return false;
		}
		echo '<!-- Current template: ' . $GLOBALS['current_theme_template'] . '-->';
	}
	add_action( 'wp_head', 'print_current_template_html_comment' );

}