From 8beef87572e6955a2b42ac0d582cb623041a8b4a Mon Sep 17 00:00:00 2001
From: Lenz Weber <mail@lenzw.de>
Date: Mon, 2 Jan 2023 23:23:44 +0100
Subject: [PATCH] fix CaseReducer to infer from argument, not return value

---
 packages/toolkit/src/createReducer.ts             |  2 +-
 .../toolkit/src/tests/createSlice.typetest.ts     | 15 ++++++++-------
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/packages/toolkit/src/createReducer.ts b/packages/toolkit/src/createReducer.ts
index debd4d2381..2ccb730864 100644
--- a/packages/toolkit/src/createReducer.ts
+++ b/packages/toolkit/src/createReducer.ts
@@ -55,7 +55,7 @@ export type ActionMatcherDescriptionCollection<S> = Array<
 export type CaseReducer<S = any, A extends Action = AnyAction> = (
   state: Draft<S>,
   action: A
-) => S | void | Draft<S>
+) => NoInfer<S> | void | Draft<NoInfer<S>>
 
 /**
  * A mapping from action types to case reducers for `createReducer()`.
diff --git a/packages/toolkit/src/tests/createSlice.typetest.ts b/packages/toolkit/src/tests/createSlice.typetest.ts
index 287187c32a..64c33078c5 100644
--- a/packages/toolkit/src/tests/createSlice.typetest.ts
+++ b/packages/toolkit/src/tests/createSlice.typetest.ts
@@ -59,21 +59,22 @@ const value = actionCreators.anyKey
       increment: (state: number, action) => state + action.payload,
       decrement: (state: number, action) => state - action.payload,
     },
-    extraReducers: {
-      [firstAction.type]: (state: number, action) =>
-        state + action.payload.count,
+    extraReducers: (builder) => {
+      builder.addCase(
+        firstAction,
+        (state, action) => state + action.payload.count
+      )
     },
   })
 
   /* Reducer */
 
-  const reducer: Reducer<number, PayloadAction> = slice.reducer
+  expectType<Reducer<number, PayloadAction>>(slice.reducer)
 
   // @ts-expect-error
-  const stringReducer: Reducer<string, PayloadAction> = slice.reducer
+  expectType<Reducer<string, PayloadAction>>(slice.reducer)
   // @ts-expect-error
-  const anyActionReducer: Reducer<string, AnyAction> = slice.reducer
-
+  expectType<Reducer<string, AnyAction>>(slice.reducer)
   /* Actions */
 
   slice.actions.increment(1)