Defer generic awaited type #35064

jablko · 2019-11-12T20:31:18Z

Roll up ~~#33062~~, #33074, ~~#33103~~, ~~#33105~~, #33707 and #35284

Fixes ~~#27711~~
Fixes #22469
Fixes #28427
Fixes ~~#30390~~
Fixes #31722
Fixes #33559
Fixes #33562
Fixes #33752
Fixes ~~#34924~~
Fixes #34937
Fixes ~~#35136~~
Fixes ~~#35258~~

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orta · 2019-11-18T19:49:08Z

@typescript-bot test this
@typescript-bot user test this
@typescript-bot run dt

typescript-bot · 2019-11-18T19:49:13Z

Heya @orta, I've started to run the parallelized community code test suite on this PR at f8e58ec. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2019-11-18T19:49:13Z

Heya @orta, I've started to run the extended test suite on this PR at f8e58ec. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2019-11-18T19:49:13Z

Heya @orta, I've started to run the parallelized Definitely Typed test suite on this PR at f8e58ec. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2019-11-18T20:16:53Z

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

orta · 2020-01-22T18:08:44Z

@typescript-bot test this
@typescript-bot user test this
@typescript-bot run dt

typescript-bot · 2020-01-22T18:08:53Z

Heya @orta, I've started to run the extended test suite on this PR at 3c20282. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2020-01-22T18:08:53Z

Heya @orta, I've started to run the parallelized Definitely Typed test suite on this PR at 3c20282. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2020-01-22T18:08:53Z

Heya @orta, I've started to run the parallelized community code test suite on this PR at 3c20282. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2020-01-22T18:40:39Z

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

orta · 2020-01-23T21:54:16Z

Looks like most of that diff is just file number changing

There is a few errors though at this location - which I think could be correct. Does that feel right?

orta · 2020-01-23T22:07:11Z

Relates to #35998

ExE-Boss

The then method only needs to be assignable to Function.

ExE-Boss · 2020-01-25T17:26:50Z

src/lib/es5.d.ts

+// The undefined case is for strictNullChecks false, in which case
+// undefined extends PromiseLike<infer U> is true, which would otherwise
+// make Awaited<undefined> -> unknown.
+type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then(...args: any[]): any } ? unknown : T;


Suggested change

type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then(...args: any[]): any } ? unknown : T;

type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then: Function } ? unknown : T;

👍 Works, thanks for your help! Done. ✔️

typescript-bot · 2020-01-25T22:32:38Z

Heya @orta, I've started to run the extended test suite on this PR at b48a356. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2020-01-25T22:32:38Z

Heya @orta, I've started to run the parallelized community code test suite on this PR at b48a356. You can monitor the build here. It should now contribute to this PR's status checks.

typescript-bot · 2020-01-25T22:58:04Z

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

orta · 2020-01-27T18:11:43Z

These baselines look alright to me - jablko#7

rbuckton

I do not believe this is the correct solution to this problem.

rbuckton · 2020-01-28T06:24:27Z

src/compiler/checker.ts

+                return typeAsAwaitable.awaitedTypeOfType = type;
            }

-            const promisedType = getPromisedTypeOfPromise(type);


I don't agree with this change. This completely removes support for recursively unwrapping a non-native promise, which is a core capability of await and our type-system support for it which has existed since the feature was added.

Can a non-native promise resolve to another promise? If not, then await isn't recursive and I think this change is safe?

If it can, then I concede that implies a recursive awaited solution.

Well, non‑native promises can resolve to an object with a .then function property, which await will resolve recursively.

Wouldn't that violate Promises/A+?

@jablko: non-native promise-like implementations aren't all guaranteed to be Promise/A+. In the past, JQuery's defer created something promise-like (i.e. it has a callable then that accepted fulfill/reject callbacks), but it did not recursively unwrap. This is why native await and native Promise both recursively unwrap (to defend against non-Promise/A+-compliant promise-likes)

await is recursive for non-native promises. In https://tc39.es/ecma262/#await, the value is passed to PromiseResolve which adopts the foreign promise and performs recursive unwrapping.

Thanks! I concede that await is recursive for non-compliant promises. Does supporting non-compliant promises justify introducing a new kind of type?

There's only ever been incomplete support for non-compliant promises in TypeScript. TypeScript's native promise type hasn't recursively unwrapped non-compliant promises until now. For native and A+-compliant promises, await isn't recursive because they can't be nested.

Is a non-compliant promise more likely actually an error? Supporting non-compliant promises can be error prone, masking what are otherwise errors.

sandersn

It looks like there are outstanding comments on this PR, so I'm going to request changes to help me track our PR backlog.

You can include es2015.promise.d.ts without es2015.iterable.d.ts. It was moved to es2015.promise.d.ts in microsoft#31117

sandersn · 2020-02-27T18:43:16Z

This PR hasn't seen any activity for quite a while, so I'm going to close it to keep the number of open PRs manageable. Feel free to open a fresh PR or continue the discussion here.

jablko force-pushed the patch-22 branch 3 times, most recently from 4855cc2 to f8e58ec Compare November 18, 2019 14:18

typescript-bot mentioned this pull request Nov 18, 2019

🤖 User test baselines have changed for patch-22 jablko/TypeScript#5

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jablko force-pushed the patch-22 branch 5 times, most recently from 1c8c527 to 7f71fcd Compare November 26, 2019 15:29

jablko force-pushed the patch-22 branch 3 times, most recently from ad41b21 to 62a1d57 Compare November 29, 2019 22:25

jablko force-pushed the patch-22 branch from 62a1d57 to 1a5d3e9 Compare December 28, 2019 17:09

jablko force-pushed the patch-22 branch from 1a5d3e9 to 3c20282 Compare January 21, 2020 18:18

typescript-bot mentioned this pull request Jan 22, 2020

🤖 User test baselines have changed for patch-22 jablko/TypeScript#6

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ExE-Boss reviewed Jan 25, 2020

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typescript-bot mentioned this pull request Jan 25, 2020

🤖 User test baselines have changed for patch-22 jablko/TypeScript#7

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rbuckton reviewed Jan 28, 2020

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jablko force-pushed the patch-22 branch 4 times, most recently from 7da42e8 to 034a6fc Compare February 3, 2020 15:29

sandersn requested changes Feb 3, 2020

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jablko force-pushed the patch-22 branch 4 times, most recently from 340aaa9 to d802893 Compare February 10, 2020 20:01

jablko force-pushed the patch-22 branch 4 times, most recently from 5cb7c3f to 49ad9c9 Compare February 13, 2020 15:42

jablko force-pushed the patch-22 branch from 49ad9c9 to 4f4673e Compare February 21, 2020 00:06

jablko added 4 commits February 20, 2020 23:09

Move this Promise.race() overload back to es2015.iterable.d.ts

e9a1dae

You can include es2015.promise.d.ts without es2015.iterable.d.ts. It was moved to es2015.promise.d.ts in microsoft#31117

Add tests

8c5ed7f

Defer generic awaited type

1214b11

Add tests

1f3b7de

jablko force-pushed the patch-22 branch from 4f4673e to 1f3b7de Compare February 22, 2020 22:41

sandersn closed this Feb 27, 2020

jablko mentioned this pull request Mar 3, 2020

Design Meeting Notes, 2/28/2020 #37115

Closed

microsoft locked as resolved and limited conversation to collaborators Oct 21, 2025

typescript-bot assigned weswigham Oct 21, 2025

	type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then(...args: any[]): any } ? unknown : T;
	type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then: Function } ? unknown : T;

Defer generic awaited type #35064

Defer generic awaited type #35064

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