TypeScript unexpectedly inferring 'any' type.

[CyrusNajmabadi]

Given the following code:

interface Widget { x: number }

declare type Unwrap<T> = T extends Promise<infer U1> ? UnwrapSimple<U1> : UnwrapSimple<T>;
declare type primitive = Function | string | number | boolean | undefined | null;

declare type UnwrapSimple<T> = T extends primitive ? T : T extends Array<infer U> ? UnwrappedArray<U> : T extends object ? UnwrappedObject<T> : never;

export interface UnwrappedArray<T> extends Array<Unwrap<T>> {
}

export declare type UnwrappedObject<T> = {
    [P in keyof T]: Unwrap<T[P]>;
};

declare var w: Widget;
declare type TupleType = [Widget, string];
declare var o: UnwrappedObject<TupleType>;

TypeScript oddly infers a type of [any, string] for o. This is surprising to us given how we would think these type operators would work. First, given the definition UnwrappedObject i would expect UnwrappedObject<TupleType> to expand like so:

UnwrappedObject<TupleType>  ->
UnwrappedObject<[Widget, string]> ->
[Unwrap<Widget>, Unwrap<string>] ->
[UnwrappedObject<Widget>, string] ->
[{ x: Unwrap<number> }, string] ->
[{ x: number }, string]

HOwever, instead of getting { x: number } (which is just the Widget type), we end up with 'any'. Cany anyone shed any light on this? Thanks!

[CyrusNajmabadi]

Tagging @DanielRosenwasser

[CyrusNajmabadi]

@RyanCavanaugh do you have any ideas here? Thanks!

[RyanCavanaugh]

This problem was fixed by #29740

[CyrusNajmabadi]

@RyanCavanaugh Thanks! Is there a way to tell what release of TS that may have made it out into?

[RyanCavanaugh]

The PR was 3 days ago, so it'd be in any nightly starting approximately yesterday

[CyrusNajmabadi]

@RyanCavanaugh wasn't it almost a month ago?

This is part of github that i'm not really familiar with!

[RyanCavanaugh]

I thought today was February 7th 🤦‍♂️

This fix wasn't in 3.3, so "nightly" is still the right answer.

The incantation (if you have a local TS repo) to check is

D:\github\TypeScript>git branch -r --contains d9ee867
  origin/HEAD -> origin/master
  origin/master
  origin/no-mkdir-race
  upstream/add-globalThis
  upstream/applyChangesToOpenFiles
  upstream/circularConstraintErrors
  upstream/contextualGenericRestParameter
  upstream/convert-to-named-parameters
  upstream/incrementalBuildInfo
  upstream/master
  upstream/perfTimeStamp
  upstream/revertExecFileSync
  upstream/usePrependToSkipBuild
  upstream/weswigham-patch-1

[CyrusNajmabadi]

I thought today was February 7th 🤦‍♂️

That's ok. I still think it's 2017 occasionally. :D

[CyrusNajmabadi]

Thanks. Have confirmed this is fixed in the latest nightly. I'm going to keep this issue open though until that is released.

@RyanCavanaugh @DanielRosenwasser Any general idea of when 3.4 might be released? Trying to make plans depending on if it's more like in a week or so, or more like a month. Thanks much!

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[RyanCavanaugh]

See #30281