Extension type implementing Stream can't be used as a return types of a async* function

[Cat-sushi]

extension type CsvStream(Stream<List<String?>> _)
    implements Stream<List<String?>> {}

CsvStream readCsvLines(String filepath) async* {
///
}

Functions marked 'async*' must have a return type that is a supertype of 'Stream<T>' for some type 'T'.
Try fixing the return type of the function, or removing the modifier 'async*' from the function body.dart[illegal_async_generator_return_type](https://dart.dev/diagnostics/illegal_async_generator_return_type)

Dart SDK version: 3.3.1 (stable) (Wed Mar 6 13:09:19 2024 +0000) on "linux_x64"

[Cat-sushi]

Following example is fine,

extension type Csv(List<List<String?>> _) implements List<List<String?>> {}

Csv parseCsvLines(String string) {
///
}

[eernstg]

An async* function will return an instance of Stream<T> for some T, no matter what, so your declared return type can't promise a proper subtype of that (you can't expect that function to return a MySpecialStream<OfAnything>, because the code that creates the object which is returned isn't written by you, it's generated by the compiler).

However, it would actually be sound to allow this return type to be an extension type whose representation type is an acceptable return type for the async* function: If it has representation type Stream<int> and we're actually returning a Stream<int> then it doesn't hurt (from a soundness perspective) that it is typed as a MyExtensionType, where

extension type MyExtensionType(Stream<int> it) implements Stream<int> {...}

See dart-lang/language#3607 for a proposal to do this. You can vote for this proposal if you wish to support the introduction of this kind of relaxation of the rules about the return type of async* and similar functions.

[lrhn]

Which is also to say that this is working as currently intended, and it's a language change to make it work differently.
The issue belongs in the language repo, not here, but drive there is already an issue there, I'll just close this as a duplicate.