Why will using __construct instead of widget_class_name when creating widget trigger out of memory error

I find that if I use

class widget_name extends WP_Widget {
    function __construct() { ... }
}

instead of

class widget_name extends WP_Widget {
    function widget_name() { ... }
}

I get an error like

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 523800 bytes) in /var/www/vhosts/klifmedia.net/httpdocs/jm/km/wp-content/themes/km/functions.php on line 77