M.pm6

my package EXPORTHOW {
    class SUPERSEDE::class is Metamodel::ClassHOW {
        method is_trusted(Mu $obj, Mu $claimant) {
            $claimant.^compute_mro(); # force it to be computed for typecheck below
            callsame() || $claimant.^type_check($obj.WHAT)
        }
    }
}

x.p6

use M;
class A { method !p() { say 'yay' } }
class B is A { method x() { self!A::p } }
B.x;