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NOT READY TO MERGE - Use conditional type inference for promise resolution #29671

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forivall
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This improves the quality of the type resolution for promise constructor
methods, as using conditional type inference is distributive, whereas
the inference provided by arguments is not.

Alternatively, the arguments based inference could be improved, but that
would be a completely different kind of change.

Fixes #

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This improves the quality of the type resolution for promise constructor
methods, as using conditional type inference is distributive, whereas
the inference provided by arguments is not.

Alternatively, the arguments based inference could be improved, but that
would be a completely different kind of change.
@forivall forivall force-pushed the forivall/improve-promise-typings branch from b8a9fd1 to e3522fd Compare January 31, 2019 23:22
@AlCalzone
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I think we get it @typescript-bot 😂

@j-oliveras
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@RyanCavanaugh @DanielRosenwasser I think the bot gone crazy 🤣

@forivall
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forivall commented Feb 7, 2019

I discovered how this will be a breaking change: if the user has manually defined their types in the <>, this change will cause a compile error.

I discovered this when i changed one of my own functions from

export const first = <T>(val: T | ReadonlyArray<T>): T =>
  Array.isArray(val) ? val[0] : val

to

export const first = <T>(val: T): T extends ReadonlyArray<infer U> ? U : T =>
  Array.isArray(val) ? val[0] : val

In some circumstance where I had defined first<string|undefined>(foo /* : string | string[] | undefined */), I had to remove the <string|undefined> for it to work. This was needed before changing the types to use conditionals, as typescript was unable to infer that that was the type needed, with the error

Argument of type 'string | string[] | undefined' is not assignable to parameter of type 'string | ReadonlyArray<string>'.
  Type 'undefined' is not assignable to type 'string | ReadonlyArray<string>

So, i would propose an improvement to the arguments-based inference to be distributive. Does anyone know if this issue has been reported?

@RyanCavanaugh RyanCavanaugh added the Experiment A fork with an experimental idea which might not make it into master label Apr 23, 2019
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@forivall Can you make an issue for this that explains the problem, the solution, and the drawback you discovered? It sounds like there's an associated inference change that's needed, and a PR doesn't seem like the right place for that discussion.

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Superceded by #45350, #33707

@forivall forivall closed this Nov 17, 2021
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