Can't create optional thunk payload

The Problem

I am building out a store using TypeScript. In it, I have a thunk which should accept an optional payload. However, I do not know how to properly define this in TypeScript.

This is the type of my thunk.

fetchStuff: Thunk<
  IStuffModel,
  { language: Language; } | undefined,
  void,
  IStoreModel
>;

The Goal

The goal is to be able to call this thunk in two ways.

fetchStuff();
fetchStuff({ language: 'en' });

However, whenever I call it like fetchStuff() I receive the following type error.

Expected 1 arguments, but got 0.ts(2554)
index.d.ts(56, 4): An argument for 'payload' was not provided.

If I define the thunk payload type as {language: Language } | void I receive an error inside the thunk itself where I use payload?.language. The error says that language does not exist on type void. The same goes if I use null.

Not sure if this is a bug or if I am just missing something silly.

[ctrlplusb]

Ok, I refined this a bit more and have updated the docs. The payload will only be optional if you union with undefined.

If you union with null then we will expect you to be passing null or the alternative type into the payload. I think there are genuine cases where you may want to use null as a signifying value.

We can’t use void as it will cause some regressions with our capability to support generic models.

The correct declaration for what you are trying to achieve is now:

Thunk<
  StoreModel,
  string | undefined
>;

This patch will be released shortly.

[DanielPe05]

This still seems to be an issue when the type is not primitive. So for example: Given

... Model
interface Payload {
  a?: string
  b?: number
}
myThunk: Thunk<MayModel, Payload | undefined, unknown, StoreModel>

... Implementation
myThunk: thunk((actions, payload = {}, { getStoreActions }) => {
    if (payload?.a) {
       // do something
    }

    actions.doSomethingElse(payload?.b || 1)
  }),

We get a Expected 1 arguments, but got 0. ts(2554) warning in the IDE (code still runs) when calling this await getActions.myThunk()

Any ideas why this might be?