Unable to use done property of IteratorResult to narrow types in typescript 3.6.2

[brainkim]

TypeScript Version: 3.6.2

Search Terms:
async iterator IteratorResult iterable next return throw type narrowing

Code
file: iterator-result.ts

declare const result: IteratorResult<string, number>;
if (!result.done) {
  let value: string = result.value;
  console.log(value);
}

Expected behavior:
The code compiles. Checking the negation of result.done narrows the type of result.value to string.

Actual behavior:

iterator-result.ts(3,7): error TS2322: Type 'string | number' is not assignable to type 'string'.
  Type 'number' is not assignable to type 'string'.

I thought the whole point of strictly typed iterators was that we could use the done to narrow the type of value, but it seems like a late-game change (making IteratorYieldResult.done optional) prevents us from doing this.

I’m fine with having result.done be optional for IteratorYieldResult but this raises the question why we didn’t also allow result.value to be optional in IteratorReturnResult, when it is idiomatic javascript to return { done: true } without a value in custom iterators.

Playground Link:

Typescript 3.6.2 is not available in the playground yet.

Related Issues:
#30790 <- pr which introduced this change.
#2983
#10564
#11375
#27059

[MattiasBuelens]

According to this comment on #27059, this bug only happens when strictNullChecks is off. If you can, you should enable strictNullChecks (or strict).

[brainkim]

Can confirm, was testing with a plain tsc call which did not enable strict.

[rbuckton]

This is currently a limitation of control-flow when strictNullChecks is not enabled. The problem is that null and undefined are assignable to a type of true, therefore we cannot remove the done: true discriminant from the union in the negative case of !result.done. Please note that the following conditions do work:

if (result.done) {
  let value: number = result.value;
}
if (result.done === false) {
  let value: string = result.value;
}