X.java

import java.util.function.Function;

import org.derive4j.Data;

class Y{}
class Z{}

@Data
public abstract class X<YZ> {
    X(){}
    interface Cases<YZ, R> {
        R y(Y y, Function<Y, YZ> id);
        R z(Z z, Function<Z, YZ> id);
    }
    public abstract <R> R match(Cases<YZ, R> cases);

    public static void main(String[] args) {
        // type variable of X<YZ> is constrained to be either of: 
        X<Y> xy = Xs.y(new Y());
        X<Z> xz = Xs.z(new Z());
    }
}