Better assignability check for IteratorResult<T,U>

[skishore]

I'm opening this bug to discuss a blocker for #2983 that seems significant enough to merit discussion on its own. In the course of trying to fix that issue, I found that Typescript cannot assign the type:

{done: boolean, value: number}

to the type:

{done: false, value: number} | {done: true, value: any}

What's going on here is that assignment from a non-union type T to a union type A | B is currently only allowed if T can be assigned to A or to B. However, in this particular case, casework on the type T shows that is indeed a subtype of A | B.

I don't propose to fix this problem in full generality. In fact, that seems pretty much infeasible to me, as I think we can express arbitrary SAT instances as instances of this problem. However, would people be okay with an improvement to the assignment relation specific to this IteratorResult case?

[mhegazy]

Duplicate of #2983 and #11375

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.