Error: The getter 'prop' isn't defined for the type 'Object'.

[vsevolod19860507]

Is it a bug or a feature?

abstract class A {
  int prop;
}

class B {
  B(this.obj);

  Object obj;

  void meth1(Object obj) {
    if (obj is A) {
      print(obj.prop); // Ok.
    }
  }

  void meth2() {
    if (obj is A) {
      print(obj.prop); // Error: The getter 'prop' isn't defined for the type 'Object'.
    }
  }
}

[eernstg]

Instance variables cannot soundly be promoted (so obj doesn't get the type A in the if), because they can be overridden by a getter that could return a different object each time it is invoked. It is being debated whether we should allow an unsound mechanism (so each access would be checked dynamically), or maybe support easy introduction of a fresh variable whose initial value is the value of the instance variable.

A good place to start is dart-lang/language#1187.

[vsevolod19860507]

Thank you.