yield*: Delegating to other generators is not an array

[JPeer264]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)
This issue might be a duplicate of #17276 (but it addresses another issue)

Code

The following example is straight out of the MDN docs and copied it right into TS Playground:

function* g1() {
  yield 2;
  yield 3;
  yield 4;
}

function* g2() {
  yield 1;
  yield* g1(); // <- gives an error: "Type 'IterableIterator<>' is not an array type"
  yield 5;
}

var iterator = g2();

Expected behavior:
Expect that the error won't appear.

Actual behavior:
Typescript throws an error that g1() is not an array type.

[DanielRosenwasser]

I'll have to fix the playground up, but make sure that

1. you have downlevelIteration on.
2. you're using es2015 for your lib option.

[JPeer264]

downlevelIteration was not set. That solved it. Thanks.

@DanielRosenwasser The documentation about generators are not there yet, should I update those?

[TomasHubelbauer]

I wish this was better documented, or ideally, the error message was much better. I keep running into this here and there and it would be really cool if the compiler could just let me know to enable this to make it work.

[rdsedmundo]

Definitely agree with you @TomasHubelbauer. I had the same problem, and fortunately this was posted here before, otherwise, I'd spent so much time to figure out what was the real problem.