fix(types): assume getState and setState will always exist on the store (#1371)

* fix(types): assume `getState` and `setState` will always exist on the store

* run prettier

* make fallback in `Get` required

src/vanilla.ts

@@ -12,7 +12,7 @@ export interface StoreApi<T> {
   destroy: () => void
 }
 
-type Get<T, K, F = never> = K extends keyof T ? T[K] : F
+type Get<T, K, F> = K extends keyof T ? T[K] : F
 
 export type Mutate<S, Ms> = number extends Ms['length' & keyof Ms]
   ? S
@@ -28,8 +28,8 @@ export type StateCreator<
   Mos extends [StoreMutatorIdentifier, unknown][] = [],
   U = T
 > = ((
-  setState: Get<Mutate<StoreApi<T>, Mis>, 'setState', undefined>,
-  getState: Get<Mutate<StoreApi<T>, Mis>, 'getState', undefined>,
+  setState: Get<Mutate<StoreApi<T>, Mis>, 'setState', never>,
+  getState: Get<Mutate<StoreApi<T>, Mis>, 'getState', never>,
   store: Mutate<StoreApi<T>, Mis>,
   $$storeMutations: Mis
 ) => U) & { $$storeMutators?: Mos }

tests/types.test.tsx

@@ -1,4 +1,10 @@
-import create, { StateCreator, StoreApi, UseBoundStore } from 'zustand'
+import create, {
+  StateCreator,
+  StoreApi,
+  StoreMutatorIdentifier,
+  UseBoundStore,
+} from 'zustand'
+import { persist } from 'zustand/middleware'
 
 it('can use exposed types', () => {
   type ExampleState = {
@@ -188,3 +194,22 @@ it('state is covariant', () => {
     baz: string
   }> = store
 })
+
+it('StateCreator<T, [StoreMutatorIdentfier, unknown][]> is StateCreator<T, []>', () => {
+  interface State {
+    count: number
+    increment: () => void
+  }
+
+  const foo: <M extends [StoreMutatorIdentifier, unknown][]>() => StateCreator<
+    State,
+    M
+  > = () => (set, get) => ({
+    count: 0,
+    increment: () => {
+      set({ count: get().count + 1 })
+    },
+  })
+
+  create<State>()(persist(foo()))
+})