Covariant return vs. As3

[Simn]

class Base {
    public function new() { }
    public function covariant():Base {
        return this;
    }
}

class Child extends Base {
    public override function covariant():Child {
        return this;
    }
}

class Main {
    static function main() {
        var c = new Child();
        var v = c.covariant();
    }
}

Generated As3:

package  {
    public class Main {
        static public function main() : void {
            var c : Child = new Child();
            var v : Child = c.covariant();
        }

    }
}

Fails in mxmlc with:

Main.as(6): col: 22 Error: Implicit coercion of a value with static type Base to a possibly unrelated type Child.

            var v : Child = c.covariant();