isset() 在5.6報錯，在7.1不報錯，什么原因？

const A  = 'f';
class DEMO{
    const X = [A=>1];
    static function bb($b){
        if( isset(self::X[$b]) ){
            echo 111;
        }else{
            echo 222;
        }
    }
}
DEMO::bb('f');

PHP Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in /usercode/file.php on line 7