failing with error : Cannot look up attributes in a type object

sample.pl

class MyInt is Int {
    method square(){
        self * self;
    }

    class Prime {
      has $.start is rw = 2;

      method is_prime(MyInt $val){
        my Bool $flag = True;
        loop {
            last if $!start >= $val;
            do {
                $flag = False;
                last;
            } if $val % $!start == 0;
            $!start += 1;
        }

        return $flag;
      }
    }

    method test(){
        Prime.is_prime(self);
    }
}

print (1..10).eager.map(->$x { MyInt.new($x) }).grep(*.test).map(*.square);