es-get-iterator broken by IIFE now allowing contextual return type

[sandersn]

Broken by #45007:

interface Iterable<T, TReturn = unknown, TNext = undefined> {
    [Symbol.iterator](): Iterator<T, TReturn, TNext>;
}

type InferIterable<T> = T extends Iterable<infer TYield, infer TReturn, infer TNext>
    ? Iterator<TYield, TReturn, TNext>
    : unknown extends T
        ? Iterator<unknown> | undefined
        : undefined;

declare function getIterator<T>(iterable: T): InferIterable<T>;

var x = getIterator((function*() { yield "foo"; yield "bar"; })());

Expected: x: Iterator<"foo" | "bar", void, unknown>

Actual: x: Iterator<"foo" | "bar", void, any>

I think this is an acceptable break, but I want another round of thinking about it now that we have a concrete example.

[sandersn]

@DanielRosenwasser you probably want to know about this too.

[sandersn]

Smaller repro:

declare function getIterator<T>(iterable: T): void;
getIterator((function*() { yield "foo"; yield "bar"; })());

[sandersn]

This is an inconsistency in our iterator checking; with no contextual type, the hard-coded TNext default is unknown; with a defective, next-free contextual type, the hard-coded default is any instead.

We should maybe consider fixing this, but only if it doesn't break anybody, or if anybody thinks the current state is bad enough.

In any case, I think this change is fine to ship as-is.