Why is there a warning on this Java generic method definition?

I noticed that if I use generics on a method signature to accomplish something similar to co-variant return types, it works like I think it would, except it generates a warning:

interface Car {
    <T extends Car> T getCar();
}

class MazdaRX8 implements Car {
    public MazdaRX8 getCar() { // "Unchecked overriding" warning
        return this;
    }
}

With the code above, my IDE gives the warning: "Unchecked overriding: return type requires unchecked conversion. Found: 'MazdaRX8', required 'T'"

What does this warning mean?

It makes little sense to me, and Google didn't bring up anything useful. Why doesn't this serve as a warning-free replacement for the following interface (which is also warning free, as co-variant return types are allowed by Java)?

interface Car {
    Car getCar();
}