fix PreloadedState type for TS 4.3 (#949)

etc/redux-toolkit.api.md

@@ -7,10 +7,10 @@
 import { Action } from 'redux';
 import { ActionCreator } from 'redux';
 import { AnyAction } from 'redux';
+import { CombinedState } from 'redux';
 import { default as createNextState } from 'immer';
 import { createSelector } from 'reselect';
 import { current } from 'immer';
-import { DeepPartial } from 'redux';
 import { Dispatch } from 'redux';
 import { Draft } from 'immer';
 import { freeze } from 'immer';
@@ -20,6 +20,7 @@ import { original } from 'immer';
 import { OutputParametricSelector } from 'reselect';
 import { OutputSelector } from 'reselect';
 import { ParametricSelector } from 'reselect';
+import { PreloadedState } from 'redux';
 import { Reducer } from 'redux';
 import { ReducersMapObject } from 'redux';
 import { Selector } from 'reselect';
@@ -127,7 +128,7 @@ export interface ConfigureStoreOptions<S = any, A extends Action = AnyAction, M
     devTools?: boolean | EnhancerOptions;
     enhancers?: StoreEnhancer[] | ConfigureEnhancersCallback;
     middleware?: ((getDefaultMiddleware: CurriedGetDefaultMiddleware<S>) => M) | M;
-    preloadedState?: DeepPartial<S extends any ? S : S>;
+    preloadedState?: PreloadedState<CombinedState<NoInfer<S>>>;
     reducer: Reducer<S, A> | ReducersMapObject<S, A>;
 }
 

src/configureStore.ts

@@ -10,8 +10,9 @@ import {
   AnyAction,
   StoreEnhancer,
   Store,
-  DeepPartial,
-  Dispatch
+  Dispatch,
+  PreloadedState,
+  CombinedState
 } from 'redux'
 import {
   composeWithDevTools,
@@ -24,7 +25,7 @@ import {
   curryGetDefaultMiddleware,
   CurriedGetDefaultMiddleware
 } from './getDefaultMiddleware'
-import { DispatchForMiddlewares } from './tsHelpers'
+import { DispatchForMiddlewares, NoInfer } from './tsHelpers'
 
 const IS_PRODUCTION = process.env.NODE_ENV === 'production'
 
@@ -74,11 +75,16 @@ export interface ConfigureStoreOptions<
    * function (either directly or indirectly by passing an object as `reducer`),
    * this must be an object with the same shape as the reducer map keys.
    */
-  // NOTE: The needlessly complicated `S extends any ? S : S` instead of just
-  // `S` ensures that the TypeScript compiler doesn't attempt to infer `S`
-  // based on the value passed as `preloadedState`, which might be a partial
-  // state rather than the full thing.
-  preloadedState?: DeepPartial<S extends any ? S : S>
+  /* 
+  Not 100% correct but the best approximation we can get:
+  - if S is a `CombinedState` applying a second `CombinedState` on it does not change anything.
+  - if it is not, there could be two cases:
+    - `ReducersMapObject<S, A>` is being passed in. In this case, we will call `combineReducers` on it and `CombinedState<S>` is correct
+    - `Reducer<S, A>` is being passed in. In this case, actually `CombinedState<S>` is wrong and `S` would be correct.
+    As we cannot distinguish between those two cases without adding another generic paramter, 
+    we just make the pragmatic assumption that the latter almost never happens.
+  */
+  preloadedState?: PreloadedState<CombinedState<NoInfer<S>>>
 
   /**
    * The store enhancers to apply. See Redux's `createStore()`.
@@ -173,5 +179,5 @@ export function configureStore<
 
   const composedEnhancer = finalCompose(...storeEnhancers) as any
 
-  return createStore(rootReducer, preloadedState as any, composedEnhancer)
+  return createStore(rootReducer, preloadedState, composedEnhancer)
 }