Fix a return value type of combine (#395)

Co-authored-by: Jaga Apple <jagaapple@uniboar.com>

readme.md

@@ -233,7 +233,7 @@ function Component() {
   const scratchRef = useRef(useStore.getState().scratches)
   // Connect to the store on mount, disconnect on unmount, catch state-changes in a reference
   useEffect(() => useStore.subscribe(
-    scratches => (scratchRef.current = scratches), 
+    scratches => (scratchRef.current = scratches),
     state => state.scratches
   ), [])
 ```
@@ -457,14 +457,14 @@ const useStore = create<BearState>(set => ({
 }))
 ```
 
-Or, use `combine` and let tsc infer types.
+Or, use `combine` and let tsc infer types. This merges two states shallowly.
 
 ```tsx
 import { combine } from 'zustand/middleware'
 
 const useStore = create(
   combine(
-    { bears: 0 }, 
+    { bears: 0 },
     (set) => ({ increase: (by: number) => set((state) => ({ bears: state.bears + by })) })
   ),
 )

src/middleware.ts

@@ -121,6 +121,7 @@ export const devtools = <S extends State>(
   return initialState
 }
 
+type Combine<T, U> = Omit<T, keyof U> & U
 export const combine = <
   PrimaryState extends State,
   SecondaryState extends State
@@ -131,13 +132,13 @@ export const combine = <
     get: GetState<PrimaryState>,
     api: StoreApi<PrimaryState>
   ) => SecondaryState
-): StateCreator<PrimaryState & SecondaryState> => (set, get, api) =>
+): StateCreator<Combine<PrimaryState, SecondaryState>> => (set, get, api) =>
   Object.assign(
     {},
     initialState,
     create(
       (set as unknown) as SetState<PrimaryState>,
-      get as GetState<PrimaryState>,
+      (get as unknown) as GetState<PrimaryState>,
       (api as unknown) as StoreApi<PrimaryState>
     )
   )