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Explain use of never[] as most generic function argument #2714

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Explain use of never[] as most generic function argument #2714

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2 changes: 2 additions & 0 deletions packages/documentation/copy/en/handbook-v2/Type Manipulation/Conditional Types.md
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Expand Up @@ -204,6 +204,8 @@ type Str = GetReturnType<(x: string) => string>;
type Bools = GetReturnType<(a: boolean, b: boolean) => boolean[]>;
// ^?
```
If you are curious about the use of `never[]` as the most generic type for function arguments, there is an excellent explanation in this [reddit](https://www.reddit.com/r/typescript/comments/muyl55/comment/gv9ndij/?utm_source=reddit&utm_medium=web2x&context=3).
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I think that we probably need an explainer about this, but I don't think that this wording or a link to reddit is appropriate. Note that TS 5.0 greatly refined the type relationship between the different function "top types" such that this answer may not longer be accurate. Though the end result of "you probably want never[]" may still apply.

Related PR: microsoft/TypeScript#52282

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Honestly, I think the example should replace never[] with any[]. We try to avoid having to include these “by the way did you know about this very confusing thing???” asides in the handbook specifically. An explainer might be better suited in the reference pages of the docs, but I don’t know if there’s any existing content where it would make sense to slot in.

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I did some sleuthing to figure out the origin of this; it seems like this has been there since the beginning of the new handbook in microsoft/TypeScript-New-Handbook@1a21a7f; @DanielRosenwasser do you have an opinion on this one?

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I think that any[] should be sufficient for the purposes of this example, especially since we use any[] in the definition of Flatten<T> further up on the same page.



When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the _last_ signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

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