Argument type infer not work correctly without specify types of function params

[Jokcy]

Bug Report

🔎 Search Terms

• Argument type infer not work correctly

🕗 Version & Regression Information

seems to be every version of typescript shows this behavior

⏯ Playground Link

Playground Link

In this example, if you try the use commits in actions.a, you will see the type of commits is:

Record<string, (state: {
    name: string;
    arr: never[];
}, ...args: any) => void>

If I update the commit function to use state: State,

...
{
  commits: {
    a: (state: State, newName: string) => 'a'
  }
}
...

then the commits type in actions.a will be exactly what commits is, eg:

{
    a: (state: State, newName: string) => string;
}

🙁 Actual behavior

Without specify state: State, the commits type infer in actions.a not correct. I suppose TS will infer what state is, and there should not be different with/without specify state: State.

🙂 Expected behavior

there should not be different with/without specify state: State.

[jcalz]

Feels like #38872

[Jokcy]

@jcalz So in this situation is there any better soluation to make it work?

[jcalz]

Split the single object into multiple parameters like this? (Note that the specific type named State will not magically be inferred anywhere since you never refer to it anywhere inside the call to defineStore().)

[Jokcy]

@jcalz THX very much, help a lot. Is there any article about context sensitive of typescript? I still not quite understand why the demo you provide solve this issue. I looked at the linked issue (also some issue related by it), I assume (state, newName: string) => 'a' is a context sensitive function and it will not be inferred at phase1 type inference. Or maybe because different args will be inferred in order instead together?

[jcalz]

A context sensitive function is not fully annotated and so the compiler must use contextual typing to infer something about its type.

According to this comment the compiler performs a phase of inference for every function parameter which is context sensitive, but it does not perform multiple phases of inference for a single function parameter.

[RyanCavanaugh]

I opened #47599 to track further work in the area

[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.

[AtLastDopamine]

像这样将单个对象拆分为多个参数？（请注意，命名的特定类型State不会在任何地方神奇地推断出来，因为您永远不会在调用中的任何地方引用它defineStore()。）

@jcalz Your answer is very helpful, but I still don't understand how splitting into multiple parameters works.

In your example, if I switch the position of 'commits' and 'actions', the type derivation still does not work properly

[jcalz]

Some inference only works from left-to-right in order of parameters, see this comment for example.