interface I { I clone(); }
interface J { J clone(); }
class C : I
{
    override C clone()   // covariant overriding of I.clone()
    {
        return new C;
    }
}

// C.clone() can override I.clone(), indeed.
static assert(isCovariantWith!(typeof(C.clone), typeof(I.clone)));

// C.clone() can't override J.clone(); the return type C is not implicitly
// convertible to J.
static assert(!isCovariantWith!(typeof(C.clone), typeof(J.clone)));