fix: Declare "EmptyObject" interface to wrap $CombinedState

[JacobLey]

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name: Declare EmptyObject interface to wrap $CombinedState
about: Adding interface for $CombinedState appeases latest version of Typescript, allows exporting result of combineReducers

PR Type

Does this PR add a new feature, or fix a bug?

Fix a bug with lastest version of Typescript

Why should this PR be included?

No changes to exports are included, but the internal change from type->interface allows Typescript to correctly reference/export usage of $CombinedState

Checklist

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Is there an existing issue for this PR?
  • closes Result of combineReducers is not exportable with latest typescript #4030
  • Previous PR, different attempt at solution fix: Export type of $CombinedState #4026
• Have the files been linted and formatted?
• Have the docs been updated to match the changes in the PR?
• Have the tests been updated to match the changes in the PR?
• Have you run the tests locally to confirm they pass?

New Features

What new capabilities does this PR add?

None, just maintains support of typescript with latest 4.2.x releases

What docs changes are needed to explain this?

None

Bug Fixes

What is the current behavior, and the steps to reproduce the issue?

Upgrade to typescript v2.4.2

Create a file example.ts with the following content:

import { combineReducers, $CombinedState } from 'redux';

export default combineReducers({})

make sure declarations: true is in your tsconfig.json

Typescript will emit an error on:
Default export of the module has or is using private name '$CombinedState'.

Also see #4029 for minimal reproducible issue

What is the expected behavior?

No error should be emitted

How does this PR fix the problem?

combineReducers<S>() returns a type of Reducer<CombinedState<S>>, and the CombinedState<S> is the type in question in this PR

https://www.typescriptlang.org/docs/handbook/advanced-types.html#interfaces-vs-type-aliases
Digging a bit into how Typescript works, but basically a type is an alias. So any usage of a type that internally uses $CombinedState means the resulting declaration must also use $CombinedState. Since that value is not exported, Typescript fails.

Using an interface however does not need direct access to $CombinedState, so the exporting of combineReducers will magically succeed.

[timdorr]

It looks like the issue is more that we're saying readonly [$CombinedState]?: undefined extends just [$CombinedState]: undefined. So, just a signature mismatch that's no longer allowed.

In any case, this both resolves it and keeps it from being a likely issue in the future. Thanks!