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Promise.all return type incorrect when some promise types are unions of Promise types, rather than Promise of union of types. #37216
Description
TypeScript Version: 3.8.3 (regression - does not occur in 3.7)
Search Terms: Promise.all return type
Code
declare const conditional: boolean;
async function foo(): Promise<void> {
// Note: promiseA cannot possibly resolve to undefined
const promiseA = Promise.resolve("hello");
const promiseB = conditional ? Promise.resolve(10) : Promise.resolve(undefined);
const [resultA, resultB] = await Promise.all([promiseA, promiseB]);
// Unexpected error here: resultA is possibly undefined
alert(resultA.toUpperCase());
}```
**Expected behavior:**
Type of `resultA` should be `string`;
**Actual behavior:**
Type of `resultA` is `string | undefined`;
**Workaround:**
Tack on a `as const` to the tuple of promises being passed to `Promise.all()`:
```ts
const [resultA, resultB] = await Promise.all([promiseA, promiseB] as const);
Activity
UselessPickles commentedon Mar 4, 2020
I think I just realized that
undefinedhas nothing to do with this, but instead that one of the promises' type is a union of different Promise types?[-]Promise.all return type incorrect when some promises may resolve to undefined[/-][+]Promise.all return type incorrect when some promise types are unions of Promise types, rather than Promise of union of types.[/+]nmain commentedon Mar 4, 2020
Probable duplicate of #35626
Probable duplicate of #34937
Probable duplicate of #33562
I think these all trace back to #33057, which is my fault, so I am sorry.
UselessPickles commentedon Mar 4, 2020
Nicholai Main (@nmain) Thanks. I don't know what's wrong with my eyes. Looking at the search results again it seems obvious that those were likely duplicates I should have looked at.
typescript-bot commentedon Mar 7, 2020
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.