TypedefTest.hx

typedef A = { a:Int };
typedef B = { > A, b:Int };

class TypedefTest {

   public static function f(x:A):A
        return x.a == 0 ? x : null;

    static function main() {
        var b:B = { a:1, b:2 };
        var fb:B = f(b);

        // compiler complains:
        // A should be B
        // { a : Int } should be B
        // { a : Int } should be { b : Int, a : Int }
        // { a : Int } has no field b
        
        // casting returns null.
    }
}