Strong mode allows Object to be inserted into List<String>

[harryterkelsen]

a.dart:

Object dummy() => new Object();
main() {
  var l = new List<String>();
  l.add(dummy());
}

$ dartanalyzer --strong a.dart
Analyzing [a.dart]...
No issues found

@leafpetersen @vsmenon @jmesserly

[vsmenon]

Yeah, that's probably because we allow implicit casting. Can you try running in DDC? You should get a runtime error.

[jmesserly]

another way to see the implicit downcast is to use a composite type:

Object dummy() => new Object();
main() {
  var l = new List<List<String>>();
  l.add(dummy());
}

[warning] Unsound implicit cast from Object to List<String> (test.dart, line 4, col 9)

Right now this behavior is By Design. Whether it will be that way in the future, not sure :)

[harryterkelsen]

I think there should be a warning for any code that would crash in checked mode

[jmesserly]

I think there should be a warning for any code that would crash in checked mode

Ironically, the current warning heuristics are the exact opposite of that.

We assume that no one has existing Dart1 code like above, because it would crash in VM/dart2js checked mode. That's why we allow it in Strong Mode.

We warn for composite types, because that could be passing Dart1 checked mode but will fail at runtime in Strong Mode.

It was a useful distinction to get code ported over to Strong Mode with minimal changes, but I agree in the long term I would love to see this changed and require explicit casts.

[leafpetersen]

I'm going to close this as a duplicate and make #26583 the canonical issue for this.