Compiler: Loses type information in lambda with generic return type

[btrautmann]

Dart SDK version: 2.13.3 (stable) (Wed Jun 9 12:44:44 2021 +0200) on "macos_x64"

We noticed this issue when migrating a part of our codebase to null safety. To reproduce, simply add this code to a working Flutter program:

Future<void> _testFunction(String testString) async {
  final route = MaterialPageRoute<String>(builder: (_) => Container());
  final result = await () async {
    return Navigator.of(context).push(route);
  }();
  _testFunction2(result);
  }

void _testFunction2(String? testString) {
  print('This worked!');
}

When flutter test is run or just running tests through the IDE, you will get this compilation error, even though the analyzer (as seen through VSCode in this case) does not complain:

lib/main.dart:60:20: Error: The argument type 'Object?' can't be assigned to the parameter type 'String?'.
 - 'Object' is from 'dart:core'.
    _testFunction2(result);

This can be fixed by changing

return Navigator.of(context).push(route);

to

return Navigator.of(context).push<String?>(route);

(by explicitly adding the type information to push()).

[mraleph]

seems like an inference difference between CFE and analyzer. /cc @johnniwinther @stefantsov

[chloestefantsova]

I'm working on the issue at https://dart-review.googlesource.com/c/sdk/+/211023.