No private field promotion on an extension type

[sgrekhov]

The following tests fails in both CFE and Analyzer

// SharedOptions=--enable-experiment=inline-class

extension type ET(int? _id) {
  void test() {
    if (_id is int) {
      _id.isOdd; // Analyzer COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
                 // CFE Error: Property 'isOdd' cannot be accessed on 'int?' because it is potentially null.
    }
  }
}

main() {
  ET et = ET(1);
  et.test();
  if (et._id != null) {
    et._id.isEven;  // Analyzer COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
                    // CFE Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
  }
}

cc @stereotype441 @eernstg

Tested on the exdge SDK (Sept 6, 2023) on Linux x64

[eernstg]

Right, the representation variable of an extension type should be promotable (private or not).

[Edit: The rules were changed after I wrote this: An extension type representation variable is only promotable if it is private. However, it is then unconditionally promotable, and it doesn't matter that other declarations in the same library have the same name.]

[stereotype441]

Thanks for the report! Since this concerns an unreleased feature (extension types) I'll plan to look at it after all other field pormotion work is complete.