TypeSafeStream<int> from Stream<Object> failing in Dart 1.25.0-dev.10.0

[whesse]

With Dart version 1.25.0-dev.10.0, some conversions from
Stream to TypeSafeStream are failing.

If this is not the intent of the test, and the conversion should
succeed, it needs to be fixed in the library.

Filed a CL that fixes the test by changing some Stream to Stream.

[srawlins]

I'm pretty sure this is because of dart-lang/sdk#30339, so the tests should not have been passing earlier :)

These tests rely on re-ified types, if I'm not mistaken, so should only run in DDC or Dart 2.0 tools.

[whesse]

So the tests should be annotated so they only run in strong mode or on DDC. There are no status files for package:test tests, so I think the annotations have to go on the tests themselves. Until this is done, the tests will be failing on the package waterfall.

[nex3]

For reference, the failures we're seeing are of the form:

01:05 +833 -9: [VM] test/typed_wrapper/stream_test.dart: with valid types,
forwards where() [E]
  type 'List<Object>' is not a subtype of type 'List<int>' of 'function result'
where
    List is from dart:core
    Object is from dart:core
    Object is from dart:core
    Object is from dart:core
    List is from dart:core
    int is from dart:core
  
  package:async/src/typed/stream.dart 104:7   TypeSafeStream.toList
  test/typed_wrapper/stream_test.dart 297:44  main.<fn>.<fn>
  ===== asynchronous gap ===========================
  dart:async                                  _Completer.completeError
  package:async/src/typed/stream.dart 104:54  TypeSafeStream.toList
  ===== asynchronous gap ===========================
  dart:async                                  _asyncThenWrapperHelper
  package:async/src/typed/stream.dart 103:34  TypeSafeStream.toList
  test/typed_wrapper/stream_test.dart 297:44  main.<fn>.<fn>

The code throwing the error is:

  Future<List<T>> toList() async =>
      DelegatingList.typed<T>(await _stream.toList());

In Dart 1.0 semantics, I would expect this to ignore the type parameter on DelegatingList.typed() and thus return a Future<List<dynamic>>, which is a subtype of Future<List<T>>. It seems that it's instead returning a Future<List<Object>>, which indicates that this might be broken for real-world users. @leafpetersen, can you shed light on why this is a List<Object>?

[leafpetersen]

Maybe because of this:

 static List<E> typed<E>(List base) =>
      base is List<E> ? base : new TypeSafeList<E>(base);

On the VM, this will always return base, no? Presumably here, base has type List<Object>.

[nex3]

If DelegatingList.typed() is returning base as-is, that means base is List<int> is true. But if that's the case, shouldn't base be a valid return type for toList()?

[leafpetersen]

We're talking about Dart 1.0, right?

In Dart 1.0 semantics, I would expect this to ignore the type parameter on DelegatingList.typed()

it does, so the question that is being asked at runtime is base is List<dynamic>, which is always true. This test program when run on the VM prints "true":

bool foo<T>(base) {
  return base is List<T>;
}

void main() {
  print(foo<int>(<String>[]));
}

[nex3]

Oh, right. In that case, I guess the test is legitimately 2.0-only, since we really do want to test what happens when the underlying type is Object. We don't currently have a way to mark that in test, since test doesn't actually support any Dart 2.0 platforms currently, so maybe the thing to do is just to skip the test for now...