Type unsafety of ??

[duzenko]

Consider the following code

    num n = 1;
    String s = n ?? List();

It gives an obvious runtime error.
I'm not so much interested in why it's considered a valid syntax as in getting the analyzer report this at compile time. I have bumped into this a few times too many.

[srawlins]

I suspect this is an implicit cast. The same thing could be done with:

num n = 1;
String s = n == null ? List() : n;

The type of the right-hand-side is the Least Upper Bound of List<dynamic> and num, which is Object. So an Object is being assigned to a String, via an implicit downcast. See dart-lang/sdk#31410 for some history. This is going away with the Non-nullable type system. You can also ban it from your codebase with implicit-casts: false in your analysis_options.yaml.

[duzenko]

I suspect this is an implicit cast. The same thing could be done with:

num n = 1;
String s = n == null ? List() : n;

The type of the right-hand-side is the Least Upper Bound of List<dynamic> and num, which is Object. So an Object is being assigned to a String, via an implicit downcast. See dart-lang/sdk#31410 for some history. This is going away with the Non-nullable type system. You can also ban it from your codebase with implicit-casts: false in your analysis_options.yaml.

Sorry, I think implicit-casts: false is a bit of an overreaction to such a localized issue.
I don't have a problem with implicit downcast per se. It's about implicit upcast/downcast combination that the compiler is doing to implement this single operator.
And FWIW I would like the same analyzer hint coming up with ?: as well.

[munificent]

Closing this because it will get addressed by NNBD which removes implicit downcasts.

[natebosch]

Closing this because it will get addressed by NNBD which removes implicit downcasts.

This case won't be fixed assuming we continue to allow implicit downcast from dynamic.

#192 (comment)

[duzenko]

reopen?

[natebosch]

Hmm, trying to repro I'm seeing that the type of n ?? List() is Object and not dynamic like I thought, so this case will indeed be solved.

[leafpetersen]

This is mostly solved. The case that isn't solved is if one of elements of the lub is dynamic.

void main() {
  dynamic y;
  int x = y ?? "hello";
}

[duzenko]

No, dynamic is not a problem