is! operator with ||

[tlserver]

class A {
}

class B extends A {
  int f;

  B(this.f);
}

main() {
  A b = B(1);
  print(b is B && b.f == 1); // true
  // apply De Morgan's laws
  print(b is! B || b.f != 1); // expecting false but actually Error: The getter 'f' isn't defined for the class 'A'.
}

Is it possible that making Dart compiler think that b is B instead of A in the second case? b is always B when b is! B === false.

Dart VM version: 2.3.0 (Fri May 3 10:32:31 2019 +0200) on "windows_x64"

[lrhn]

Dart's type promotion is still very simple.
It only prmoteos on the positive branch of x is T tests, not on anything else (including not on the negative branches of x is! T).

We pla ton improve this in the future, no later than when introducing non-nullable types (becuse if (x != null) .... definitely should promote x` to non-nullable). We are tracking this in the language repository; dart-lang/language#81 .

For now, you'll just have to De Morgan yourself back to positive is tests when you need promotion.

(So, behaviior is intended, and as a request for improvement, I'm deferring to the language repo issue).