Analyzer did not infer the correct type after checking type of the class member

[RaymondChao]

In below example, the Dart analyzer did not show any errors:

class Bowl {
  dynamic animal;

  Bowl() {
    animal = Fish();
  }

  void shake() {
    if (animal is Fish) {
      animal.jump();
    }
  }
}

class Fish {
  swim() {
    print('swim');
  }
}

void main() {
  Bowl().shake();
}

[eernstg]

The method jump can be invoked on animal because the type of animal is dynamic.

You probably expected animal is Fish to promote animal such that its type would be Fish in the body of the if statement, but that doesn't happen because instance variables are never promoted (here are a couple of related issues: #34480, #25565).

We expect promotion to be revised in several ways (e.g., we may promote after if (animal is! Fish) return; which means that the control flow analysis is made stronger). But promotion of mutable instance variables is not so likely to happen, because any invocation of a method or function may allow such instance variables to be mutated and hence invalidate the type that we just tested for.

It would be useful to have the property that all promotions are associated with explicit syntactic constructs, which could mean that anything like if (animal is Fish) ... would be flagged (with a lint, hint, or something like that) because it does not promote animal. But that would be a breaking change and there could be many situations in existing code where such diagnostics would just be annoying noise. So there is no easy way to detect and report the situations where promotion was intended and expected, but didn't happen.

I'll close this issue with 'working as intended'.