Result of `combineReducers` is not exportable with latest typescript

[JacobLey]

Prior Issues

Attempt to fix by directly exporting $CombinedState #4029

What is the current behavior?

When typescript is upgraded to the latest version 4.2.x the output of combineReducers throws an error.

Steps to Reproduce

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://codesandbox.io or similar.

What is the expected behavior?

See minimal reproducible error here: #4029
Checkout branch https://github.com/JacobLey/redux/tree/combinedStateRepro, run cd ./error && npm start

Typescript should run, and emit error Default export of the module has or is using private name '$CombinedState'

What should Redux be doing?

Exporting the result of combineReducers should not be a typescript error.

Environment Details

To my knowledge, all versions of Redux with Typescript declarations are impacted.
More importantly, this change happens on the latest 4.2.x versions of Typescript.

[JacobLey]

Opened this Issue at request of #4026 (comment)

[markerikson]

My first question is, why are you exporting it with an empty generic and an empty object? Does this happen with "normal" usage, ie, something like combineReducers({counter: counterReducer})?

Also, if something about TS 4.2 changed how it's interpreting the types... I'd like to know what about TS 4.2 changed so that we know why this is suddenly causing an error.

[JacobLey]

Updated error branch to give examples of failure with actual map.
For some reason createStore is required to trigger the issue

I too would like to know what the Typescript change is, but cannot easily find a related commit.
This type of error is not new to Typescript though: microsoft/TypeScript#6307