Support generic state

[parloti]

Since this change pull/2967 Typescript can't compile my reducer based on generic state.

The following error is being generated:
Argument of type '(state: TState, action: object & { type: string; }) => TState' is not assignable to parameter of type 'OnReducer<TState, [ActionCreator<string, Creator<any[], object>>]>'.
Type 'TState' is not assignable to type 'TState extends object ? { [P in keyof TState]: TState[P]; } : TState'.
Type 'object' is not assignable to type 'TState extends object ? { [P in keyof TState]: TState[P]; } : TState'.(2345)

Minimal reproduction of the bug/regression with instructions:

Reproduction in the Typescript playground.

Describe any alternatives/workarounds you're currently using

I am currently casting the state returned by the reducer to 'any'.

Other information:

If accepted, I would be willing to submit a PR for this feature

[ ] Yes (Assistance is provided if you need help submitting a pull request)
[X ] No

[HaanJeroen]

same here! would be nice to get a solution

[sommcz]

same here.. the way i only found is to cast reducer return type manually. solved by downgrade to ^11.0 for now.