`Stream.asyncMap` bug in weak mode + nnbd sdk

[jakemac53]

If you return null from the closure passed Stream.asyncMap in weak mode (with the nnbd sdk) today you will hit an unreachable error.

Repro program:

void main() {
  var stream = Stream.fromIterable([1, 2, 3, 4]);
  var eventsStream =
      stream.asyncMap((i) => i % 2 == 0 ? i : null).where((i) => i != null);
  eventsStream.listen(print);
}

[kevmoo]

CC @lrhn

[franklinyow]

Can I get an assessment on the bug? Is it a blocker for unfork?

[natebosch]

Is it a blocker for unfork?

Yes. This will break existing code. It should be considered a blocker.

[vsmenon]

@lrhn @leafpetersen

What is the right resolution here? Do we need (and can we) undo the change that triggers the error?

[vsmenon]

Or is this just a plain bug in the new SDK?

[leafpetersen]

The issue is the following code:

   if (newValue is Future<E>) {
          subscription.pause();
          newValue.then(add, onError: addError).whenComplete(resume);
        } else {
          if (newValue is! E) {
            throw "unreachable"; // TODO(lrn): Remove when type promotion works.
          }
          controller.add(newValue);
        }

which assumes that given a value x of type FutureOr<E>, either x is Future<E> or x is E will be true. This doesn't hold in pre-nnbd/weak mode Dart because null is Future<E> and null is E are both false unless E is a nullable type (or a top type).

The code above should be changed to something like:

   if (newValue is Future<E>) {
          subscription.pause();
          newValue.then(add, onError: addError).whenComplete(resume);
        } else {
          if (newValue is! E && newValue != null) {
            throw "unreachable"; // TODO(lrn): Remove when type promotion works.
          }
          controller.add(newValue);
        }

We should probably audit the ported core libraries for similar patterns. Shouldn't be hard to fix. I can take a look, probably tomorrow.

[leafpetersen]

https://dart-review.googlesource.com/c/sdk/+/143333

[leafpetersen]

Fix landed.