Perl6 hangs on Proxy referring to self

proxy-doc-ex1-refer-self.p6

class c {
    sub double() is rw {
        my $storage = 0;
        Proxy.new(
            FETCH => method ()     { say self.WHAT; $storage * 2    },
            STORE => method ($new) { $storage = $new },
        )
    }

    method m {
        my $doubled := double();
        $doubled = 4;
        say $doubled;       # OUTPUT: «8␤»
    }
}
c.m