RG.mmr.pl6

role Zub[::T] { 
    proto method CALL-ME(|c) { * }
    multi method CALL-ME(Mu:D:) {
	    note "Got nothing";
    }
    multi method CALL-ME(Mu:D: T $i) {
	    note "Got $i";
    }
}

class Foo {
    has $!bar = Mu.new;

    method bar(|c) is rw {
	    ($!bar but Zub[Int])(|c);
    }
}

Foo.new.bar(1);
Foo.new.bar();