Dart2js accepts "null" as argument to "!".

[lrhn]

Example:

main() {
  bool x = null;
  print(!x);  // Should throw, prints "true" in dart2js.
}

[rakudrama]

Will ! accept only a nonnullable operand with NNBD?

[lrhn]

Yes, the type expected by conditions and boolean operators will (overwhelmingly likely) be non-null bool.

All these operations currently throw if they get a null value (exept when implementations forget to check), so they are prime examples of operations whose operands will become non-nullable.

[eernstg]

Note that dart2js fails to raise a dynamic error because null occurs where a bool is required in other situations as well. #36193 (comment) reports that if (e) .. will run the else branch rather than raising a dynamic error. For more detail, check #36193 (comment).

I just tried another one (using dart2js from ac4eca7):

main() {
  print("Before loop");
  while (null) {
    print("In loop!");
    break;
  }
  print("After loop");
}

Surprisingly, this prints 'Before loop', then 'After loop', and then once more 'After loop'. With do-while, null counts as false (so it prints 'Before loop', 'In loop!', 'After loop' as semi-expected):

main() {
  print("Before loop");
  do {
    print("In loop!");
    break;
  } while (null);
  print("After loop");
}

[fishythefish]

https://dart-review.googlesource.com/c/sdk/+/98780 should fix this.