Assignment isn't working using generics in a simple way

Assignment isn't working using generics in a simple way

I would very apprciate if someone could explain to me why u = bar(u,r) in following code isn't working. I just can't find the right explanation.
class R {
}

class U {
}

public class Foo {

    public static <T> T bar(T x, T y) {
        return x;
    }

    public static void main(String[] args) {

        R r = new R();
        U u = new U();

        u = bar(u,r); // why is this not working?
    }
}