False positive unnecessary cast in firstWhere with orElse

[TadashiCZ]

Dart version: Dart SDK version: 2.17.6 (stable) (Tue Jul 12 12:54:37 2022 +0200) on "macos_arm64"

The problem is that this code creates unnecessary cast warning:

final RxList<XFile> selectedFiles = <XFile>[].obs;
final XFile? xFile = (selectedFiles as RxList<XFile?>).firstWhere((element) => file.name == element?.name, orElse: () => null);

but when using the suggestion it changes code below which is wrong because it cannot return null from the orElse clause.

final XFile xFile = (selectedFiles).firstWhere((element) => file.name == element?.name, orElse: () => null);

[lrhn]

The warning seems unnecessary, but the cast isn't actually working, so some warning is warranted.

You have an RxList<XFile>, which I assume is not an RxList<XFile?> (because then you could have written that as the type in the first line.)

The (selectedFiles as RxList<XFile?>) cast up-casts to the supertype, relying on unsafe covariance to be valid. However, the firstWhere method is precisely one which is not safe to use covariantly.

When you do:

final XFile? xFile = (selectedFiles as RxList<XFile?>).firstWhere((element) => file.name == element?.name, orElse: () => null);

you pass a () => null method as the orElse argument to an RxList<XFile>, which is going to cause a run-time error. The function passed as orElse must return an XFile, because that's what RxList<XFile> requires (because it's a List<XFile>).

The warning you get is not correct (the cast is not unnecessary), but it's correct to give a warning here (the use of the cast value is not safe).