"Parameter implicitly has an 'any' type." error when passing action-wrapped function with implicit argument types to optional property

[seanchas116]

Intended outcome:

action -wrapped function with implicit argument types can be assigned as an optional property of a object, and the types are correctly inferred.

import { action } from "mobx";

// Both should be OK

const tes1: {
  a: (x: number) => void;
} = {
  a: action((x) => console.log(x.toFixed()))
};

const test2: {
  a?: (x: number) => void;
} = {
  a: action((x) => console.log(x.toFixed()))
};

Actual outcome:

Implicit argument types in action -wrapped function assigned as an optional property of a object becomes any and causes an error in struct environments.

import { action } from "mobx";

// OK
const tes1: {
  a: (x: number) => void;
} = {
  a: action((x) => console.log(x.toFixed()))
};

// OK in MobX 5, Error in MobX 6 (x becomes any)
const test2: {
  a?: (x: number) => void;
} = {
  a: action((x) => console.log(x.toFixed()))
  // ↑ "Parameter 'x' implicitly has an 'any' type. ts(7006)" error in this line
};

Looks like MobX 5 works fine because undefined is added to T in this line. Adding undefined to T in Mobx6 action.d.ts fixed the problem in my environment.

How to reproduce the issue:

https://codesandbox.io/s/mobx-action-error-gnd8v

Versions

Occurs in MobX 6. MobX 5 works fine.

[kubk]

It's better to add both undefined and null to support promises: #2213
Mobx 5 also had unit tests for such cases: #2217
But they were removed in Mobx 6

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[kubk]

Can make a PR to fix it if anyone interested

[stale]

This issue has been automatically unmarked as stale. Please disregard previous warnings.

[mweststrate]

Should be fixed in 6.0.5