Error with async generators

Ron Buckton (@rbuckton)

TypeScript Version: master

Search Terms:

Code

const g: <U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>) => Promise<U> = async <U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>): Promise<U> => {
  throw com;
};

Expected behavior:
pass
Actual behavior:

index.ts:1:7 - error TS2719: Type '<U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>) => Promise<U>' is not assignable to type '<U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>) => Promise<U>'. Two different types with this name exist, but they are unrelated.
  Type 'Promise<U | PromiseLike<U>>' is not assignable to type 'Promise<U>'.
    Type 'U | PromiseLike<U>' is not assignable to type 'U'.
      'U | PromiseLike<U>' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.
        Type 'PromiseLike<U>' is not assignable to type 'U'.
          'PromiseLike<U>' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.

1 const g: <U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>) => Promise<U> = async <U, R, S>(com: () => Iterator<S, U, R> | AsyncIterator<S, U, R>): Promise<U> => {
        ~

Playground Link:

Related Issues:

It doesn't seem to like the definitions of TNext | PromiseLike<TNext> and TReturn | PromiseLike<TReturn> for the next() and return() methods.

Yes, the actual type is not assignable to the declared type although they are the same type.

Ron Buckton (@rbuckton)

Ron Buckton (@rbuckton) Please don't forget this too.

Anders Hejlsberg (@ahejlsberg)

Anders Hejlsberg (@ahejlsberg) for reference

This appears to be unaffected by #32460. Best I can tell the issue is that Iterator and AsyncIterator are alike enough that inference succeeds in inferring from either to either--and therefore gets confused.

A simpler repro:

interface Foo<T> {
    test(value: T): void;
}

interface Bar<T> {
    test(value: T | PromiseLike<T>): void;
}

declare let f1: <T>(x: Foo<T> | Bar<T>) => Promise<T>;
declare let f2: <U>(x: Foo<U> | Bar<U>) => Promise<U>;

f1 = f2;  // Error
f2 = f1;  // Error

Navigation Menu

Repository navigation

Error with async generators #32247

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Error with async generators #32247

Description

Activity

rbuckton commented on Jul 4, 2019

falsandtru commented on Jul 4, 2019

falsandtru commented on Jul 17, 2019

rbuckton commented on Jul 17, 2019

ahejlsberg commented on Jul 23, 2019

ahejlsberg commented on Jul 25, 2019

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions