Bounded generics do not work with inheritance

public class TestGenerics { 
    static interface Z {} 
    static class X implements Z {} 
    static class Y extends X {} 

    static class A <T extends Z> { void a(T t) { System.out.println(t); } } 
    static class B extends A<Y> {} 
    static class C extends A<X> {} 

    public static void main(String[] args) { 
        new C().a(new Y()); 
    } 
}