correctly enforce meta and error types as returned from prepare

Author: phryneas

src/createSlice.ts

@@ -110,7 +110,7 @@ export type CaseReducerWithPrepare<State, Action extends PayloadAction> = {
 export type SliceCaseReducers<State> = {
   [K: string]:
     | CaseReducer<State, PayloadAction<any>>
-    | CaseReducerWithPrepare<State, PayloadAction<any>>
+    | CaseReducerWithPrepare<State, PayloadAction<any, string, any, any>>
 }
 
 /**
@@ -187,11 +187,13 @@ export type ValidateSliceCaseReducers<
   ACR extends SliceCaseReducers<S>
 > = ACR &
   {
-    [P in keyof ACR]: ACR[P] extends {
-      reducer(s: S, action?: { payload: infer O }): any
+    [T in keyof ACR]: ACR[T] extends {
+      prepare(
+        ...a: never[]
+      ): { payload: infer P; meta?: infer M; error?: infer E }
     }
       ? {
-          prepare(...a: never[]): { payload: O }
+          reducer(s: S, action: PayloadAction<P, string, M, E>): any
         }
       : {}
   }

type-tests/files/createSlice.typetest.ts

@@ -175,6 +175,65 @@ function expectType<T>(t: T) {
   expectType<string>(counter.actions.concatMetaStrLen('test').meta)
 }
 
+/**
+ * Test: access meta and error from reducer
+ */
+{
+  const counter = createSlice({
+    name: 'test',
+    initialState: { counter: 0, concat: '' },
+    reducers: {
+      // case: meta and error not used in reducer
+      testDefaultMetaAndError: {
+        reducer(_, action: PayloadAction<number, string>) {},
+        prepare: (payload: number) => ({
+          payload,
+          meta: 'meta' as 'meta',
+          error: 'error' as 'error'
+        })
+      },
+      // case: meta and error marked as "unknown" in reducer
+      testUnknownMetaAndError: {
+        reducer(_, action: PayloadAction<number, string, unknown, unknown>) {},
+        prepare: (payload: number) => ({
+          payload,
+          meta: 'meta' as 'meta',
+          error: 'error' as 'error'
+        })
+      },
+      // case: meta and error are typed in the reducer as returned by prepare
+      testMetaAndError: {
+        reducer(_, action: PayloadAction<number, string, 'meta', 'error'>) {},
+        prepare: (payload: number) => ({
+          payload,
+          meta: 'meta' as 'meta',
+          error: 'error' as 'error'
+        })
+      },
+      // case: meta is typed differently in the reducer than returned from prepare
+      testErroneousMeta: {
+        // typings:expect-error
+        reducer(_, action: PayloadAction<number, string, 'meta', 'error'>) {},
+        prepare: (payload: number) => ({
+          payload,
+          meta: 1,
+          error: 'error' as 'error'
+        })
+      },
+      // case: error is typed differently in the reducer than returned from prepare
+      testErroneousError: {
+        // typings:expect-error
+        reducer(_, action: PayloadAction<number, string, 'meta', 'error'>) {},
+        prepare: (payload: number) => ({
+          payload,
+          meta: 'meta' as 'meta',
+          error: 1
+        })
+      }
+    }
+  })
+}
+
 /*
  * Test: returned case reducer has the correct type
  */