Check return type inside generators

[agentcooper]

Fixes #18726.

I saw another pull request for this only after I did my fix and mine seems somewhat lighter (I maybe missed something).

Test case

async function* makeGenerator(): AsyncIterableIterator<number> {
  yield 1;
  yield 2;
  yield 3;
  yield 4;
  return "abc";
}

async function pull(gen: AsyncIterableIterator<number>) {
  const { done, value } = await gen.next();
  console.log(value);
  if (!done) {
    await pull(gen);
  }
}

pull(makeGenerator());

Command

node built/local/tsc.js --lib 'esnext,esnext.asynciterable' hello.ts

Before

No errors.

After

hello.ts:6:3 - error TS2322: Type '"abc"' is not assignable to type 'number'.

6   return "abc";
    ~~~~~~~~~~~~~

[msftclas]

All CLA requirements met.

[mhegazy]

@rbuckton can you take a look.

[agentcooper]

@mhegazy @rbuckton gentle ping

[agentcooper]

@rbuckton can you please take a look at this one?

[mhegazy]

Please see my response in #18726 (comment)