Inference of type bounds in as checks

[dcharkes]

Would it be possible to infer type bounds in as checks?

Consider the following example:

class A<T extends Object> {
  Object? get value => null;
}

class B<T extends Object> extends A<T> {
  @override
  final T value;

  B(this.value);
}

void example() {
  final A<int> a = B<int>(123);
  if (a is B) { // inferred as `a is B<Object>`. It would be nice if this would be inferred as B<int>
    final x = a.value; // inferred as `final Object? x` due to `a` not being promoted to `B<int>` and staying `A<int>`. 
  }
  if (a is B<int>) { // manually specifying `B<int>` promotes `a` to `B<int>` from `A<int>`
    final x = a.value; // which means `x` is inferred as `int`
  }
}

[FMorschel]

This is similar to #3837