Extends<T>

Test.hx

class Test {
  static function main() {
    var a:Extends<A> = extended({a: "hello", b: "world"});
    $type(a); // Extends<A>
    trace(haxe.Json.stringify(a));
    
    a.a = "bye";
    // a.a = 42; // Int should be String
    // a.b = "nope"; // Extends<A> has no field b
    trace(haxe.Json.stringify(a));
    
    // --------
    
    var props:WrapperProps = {
      whatever: true,
      AProps: extended({a: "hello", b: "world"})
    };
    trace(haxe.Json.stringify(props));
  }
  
  static function extended<Base, T:Base>(obj:T):Extends<Base>
    return cast (obj :Extended<Base, T>);
}

typedef A = {
  var a:String;
}

typedef WrapperProps = {
  var whatever:Bool;
  var AProps:Extends<A>;
}
  
typedef Extends<T> = T;
typedef Extended<T, TExtended:T> = TExtended;