Constrained genericity

subset Eatable of Any where { .^can('eat') };

class Cake { method eat() {...} }

role FoodBox[Eatable] {
    has %.foodbox;
}

class Yummy does FoodBox[Cake] { }      # composes correctly
# class Yucky does FoodBox[Int] { }     # fails to compose

my Yummy $foodbox .= new;
say $foodbox.perl;

Yummy.new(foodbox => {})