Improper Type Inference in Async Generator Callback

[DScheglov]

🔎 Search Terms

async generator inference, async generator

🕗 Version & Regression Information

• This is a crash
• This changed between versions 4.0.5 and 5.4.3
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?target=99&jsx=0&ts=5.4.3#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-oDgaDwZDochWmAEDmJ0bw6n0zH3eTKdTac78BPSAAiMy2QgAfS1tgAqmYEbepcvZWuFRuMLbruC5iAeDadhmp4UlSNJ0ryBCgVAd4Ps+r4fl+P4GJ4xhiDCiqbsBe6BtUg65khACMhRMpIPBBoWR6xjaTIMHCrxIpIbHnFuVQIImkIAFTII8dAMGJAFKsgdHVPIE5TKwwAQDgPhCXA9RwIIW4QDuxFBrRJGFtILHicgfC5sokh6qKkiPJayD3o+L7vp+T5+rYtgANK3rC-gEWA0hGcZDB+bONY4tWY4uApSkqbC6maURSH6UGfBwBFUCGUFYlmTmFlWdkNnug5aHOQiT5HG+hAABK2Kg3lwhJ0IBQYJnIGCYBYFAIAznwvXSeE6Vlj5yATmRw7pWk4m6NI-w4XhjVAdpu6jUOSEAEzUQwyXVAxHbHnIZIwRe8EVNeJFjUhKGOehLluZ53mcsVTkYeVlU1XVUpPMZXEIBxP08ec-EgEJIlZSF0mySR8mKcpqnxYtOkTttmWtQwOV5aq1m2VQT03WVd1ecNfnNWDgGhWBxyDVFMOxWpGkIyB6Xbal6Uo6j6MgJZmMFdj9moc9LkVdVtX1b5gEk617Wdd1oa9Xw-Xk7WujDSt5ETcZ01pEAA

💻 Code

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 fetchBook: (id: string) => Promise<Result<Book, "NOT_FOUND">>;

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

export const fetchBookWithAuthor1 =
  (bookId: string) =>
    asyncDo(async function* () {
      const book: Book = yield* await fetchBook(bookId)
        .then(mapErr(() => "NOT_FOUND_BOOK" as const))

      const author: Author = yield* await fetchAuthor(book.authorId)
        .then(mapErr(() => "NOT_FOUND_AUTHOR" as const))

      return { ...book, author } as BookWithAuthor;
    });

export const fetchBookWithAuthor2 =
  (bookId: string): Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">> =>
    asyncDo(async function* () {
      const book: Book = yield* await fetchBook(bookId)
        .then(mapErr(() => "NOT_FOUND_BOOK" as const))

      const author: Author = yield* await fetchAuthor(book.authorId)
        .then(mapErr(() => "NOT_FOUND_AUTHOR" as const))

      return { ...book, author } as BookWithAuthor;
    });

🙁 Actual behavior

Compilation Error in the function fetchBookWithAuthor2

Type 'Author | BookWithAuthor' is not assignable to type 'Author'.
Property 'name' is missing in type 'BookWithAuthor' but required in type 'Author'.(2322)
input.ts(17, 29): 'name' is declared here.

🙂 Expected behavior

No errors.
compilation result must be the same as for fetchBookWithAuthor1.

The only difference between two versions of the function is a return type specified.

The defenition file is emitted correctly:

interface Result<T, E> {
    [Symbol.iterator](): Generator<E, T>;
}
type Book = {
    id: string;
    title: string;
    authorId: string;
};
type Author = {
    id: string;
    name: string;
};
type BookWithAuthor = Book & {
    author: Author;
};
export declare const fetchBookWithAuthor1: (bookId: string) => Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">>;
export declare const fetchBookWithAuthor2: (bookId: string) => Promise<Result<BookWithAuthor, "NOT_FOUND_BOOK" | "NOT_FOUND_AUTHOR">>;
export {};

Additional information about the issue

No response

[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;
});