Incorrect type inference of the action callback arguments

[kubk]

I have an:
[x] Issue:

• Provide a minimal sample reproduction: https://codesandbox.io/s/react-mobx-starter-t8xju
  Go to components/App.tsx
• Did you check this issue wasn't filed before? It looks like there was a fix in 2016, which solved the same issue: Typescript: Add granular overloads for action, which fixes "Parameter XXX implicitly has an 'any' type" #500
• Elaborate on your issue. What behavior did you expect?
  I am expecting correct type inference for the arguments of action callback. All arguments have type 'unknown':
  
  Expected type 'number'.
• State the versions of MobX and relevant libraries. Which browser / node / ... version? Mobx 5.9.4, TypeScript 3.4.5
• What problem would it solve for you? It would allow to use type inference instead of manually specifying each argument type of the action callback.
• Do you think others will benefit from this change as well and it should in core package (see also mobx-utils)? Yes, because it looks like type inference worked with previous Mobx/TypeScript versions.
• Are you willing to (attempt) a PR yourself? I already tried to fix the issue but with no success.

[danielkcz]

Um, since you obviously have CodeSandbox in that picture, can you please share the link as well? The one you linked above does not have such code in App.tsx.

[kubk]

@FredyC Sorry, I updated the link and removed unrelated code.

[danielkcz]

Thanks. Well, I am not so sure action can infer that argument properly. And if it would be possible, it would be a lot of hassle for a little gain. Have a look at the current typings for actions.

mobx/src/api/action.ts

Line 20 in db4b3e7

<A1, A2, A3, A4, A5, A6, R, T extends (a1: A1, a2: A2, a3: A3, a4: A4, a6: A6) => R>(fn: T): T &

It's already crazy overload hell. Expanding that to get the correct argument type might not be worth it. I think this is one of the cases where explicit type casting will be necessary ... action((arg: number) => ....

I will leave this open if someone else wants to chime.

[lock]

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