Port TS type fixes from the 4.x branch

Author: markerikson

src/types/store.ts

@@ -33,25 +33,26 @@ declare const $CombinedState: unique symbol
  * `$CombinedState` symbol property. Without the property, this type would
  * match any object. The symbol doesn't really exist because it's an internal
  * (i.e. not exported), and internally we never check its value. Since it's a
- * symbol property, it's not expected to be enumerable, and the value is
+ * symbol property, it's not expected to be unumerable, and the value is
  * typed as always undefined, so its never expected to have a meaningful
- * value anyway. It just makes this type distinguishable from plain `{}`.
+ * value anyway. It just makes this type distinquishable from plain `{}`.
  */
-export type CombinedState<S> = { readonly [$CombinedState]?: undefined } & S
+interface EmptyObject {
+  readonly [$CombinedState]?: undefined
+}
+export type CombinedState<S> = EmptyObject & S
 
 /**
  * Recursively makes combined state objects partial. Only combined state _root
  * objects_ (i.e. the generated higher level object with keys mapping to
  * individual reducers) are partial.
  */
-export type PreloadedState<S> = Required<S> extends {
-  [$CombinedState]: undefined
-}
+export type PreloadedState<S> = Required<S> extends EmptyObject
   ? S extends CombinedState<infer S1>
     ? {
         [K in keyof S1]?: S1[K] extends object ? PreloadedState<S1[K]> : S1[K]
       }
-    : never
+    : S
   : {
       [K in keyof S]: S[K] extends string | number | boolean | symbol
         ? S[K]
@@ -90,6 +91,12 @@ export interface Unsubscribe {
   (): void
 }
 
+declare global {
+  interface SymbolConstructor {
+    readonly observable: symbol
+  }
+}
+
 /**
  * A minimal observable of state changes.
  * For more information, see the observable proposal: