Improper Type Inference in Async Generator Callback about typescript

[RyanCavanaugh]

Slightly reduced (could go farther probably) + annotated

interface Result<T, E> {
  [Symbol.iterator](): Generator<E, T>
}

type NotResultOf<T> = T extends Result<any, any> ? never : T;
type ErrTypeOf<T> = T extends Result<unknown, infer E> ? E : never;
type OkTypeOf<T> = T extends Result<infer R, unknown> ? R : never;
type AsyncJob<T, E> = () => AsyncGenerator<E, T>;

declare const asyncDo: <T, E>(job: AsyncJob<T, E>) => Promise<Result<
  OkTypeOf<T> | NotResultOf<T>,
  E | ErrTypeOf<T>
>>;
declare const mapErr: <E, F>(mapper: (error: E) => F) => <T>(result: Result<T, E>) => Result<T, F>;

type Book = { id: string; title: string; authorId: string };
type Author = { id: string; name: string };
type BookWithAuthor = Book & { author: Author };

declare const fetchAuthor: (id: string) => Promise<Result<Author, "NOT_FOUND">>;

let authorId: string = "";

const f1 = asyncDo(async function* () {
  const author: Author = yield* await fetchAuthor(authorId)

  return null! as BookWithAuthor;
});

const f2: Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">> = asyncDo(async function* () {
  
  // Without yield*, the type of test1 is
  //   Result<Author, "NOT_FOUND_AUTHOR>
  const test1 = await fetchAuthor(authorId).then(mapErr(() => "NOT_FOUND_AUTHOR" as const))
  //     ^?

  // With yield*, the type of test2 is
  //    Author | BookWithAuthor
  // But this codepath has no way to produce BookWithAuthor
  const test2 = yield* await fetchAuthor(authorId).then(mapErr(() => "NOT_FOUND_AUTHOR" as const))
  //    ^?

  return null! as BookWithAuthor;
});