Raku constructor question

question.raku

# Close, in the sense that class B has no idea of the implementation
# of some-A-method, but B.new returns the wrong type of instance.

    class A {
        has $.a;

        method new ($x) {
            return self.bless: :a($x / 2);
        }

        method some-A-method {
            return $!a;
        }
    }

    class B is A {
        has $.b = 66;

        method new ($x) {
            A.new($x);
        }

    }

    my $b = B.new(42);

        # Expecting 21.
    say $b.some-A-method;

        #  Expecting 66.
    say $b.b;

Thanks for your response. Turns out I had not explained precisely enough what I was after (it even became clear to me only as we went back and forth discussing it on IRC (raku channel on libera). The following solution (fixed up a bit by me), the thing I was after (class B knowing nothing of class A's implementation), was proposed by gfldex:

class A {
    has $!a;

    submethod TWEAK {
        $!a = $*x;
    }

    method some-A-method {
        return $!a / 2;
    }
}

class B is A {

    method new ($*x) { 
        self.bless;
    }
}

say B.new(42).some-A-method;