when has optional elements in tuple types, Promise.all has error

TypeScript Version: 3.1.3

Search Terms:
Promise.all, optional elements in tuple types

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

const arr: [Promise<number>, Promise<string>?] = [
  Promise.resolve(1)
];

arr[1] = Promise.resolve('1');

Promise.all(arr);    // Error

// or

const arr2: Array<Promise<number> | Promise<string>> = [Promise.resolve(1)];
arr2.push(Promise.resolve('1'));

Promise.all(arr2);   // Error

Expected behavior:

Promise.all(arr) should be correct

Actual behavior:

The same (or similar) error happens when working with an array that can contain undefined items, which is OK according to the spec:

declare const foo: (Promise<number> | undefined)[]
Promise.all(foo) // error

Since TS now has conditional types, I could imagine that the definition should be something like:

type UnwrapPromise<T> = T extends PromiseLike<infer U> ? U : T

declare function promiseAll<T>(values: T[]) : UnwrapPromise<T>;
promiseAll(foo) // (number | undefined)[]

Better typings for Promise.all(), like microsoft#31117

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when has optional elements in tuple types, Promise.all has error #28427

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when has optional elements in tuple types, Promise.all has error #28427

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grassator commented on Feb 15, 2019

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