Type promotion doesn't work when a closure is used

[johnpryan]

Result checkLottery(bool isLucky) {
  String? nullString;
  if (isLucky) {
    nullString = 'You win!';
  }

  if (nullString != null) {
    // Use this to fix:
    // var nonNullString = nullString;
    return () {
      return Result(nullString);
      //            ^ The argument type 'String?' can't be assigned to the parameter type 'String'.
    }();
  }

  return Result('You lose.');
}

@leafpetersen is there already an issue for this? I think this is called out in #44900

DartPad example

[eernstg]

Cf. dart-lang/language#1247.

[ghost]

This issue was moved by keertip to dart-lang/language#1582.