From c2544ad16b228cf89f2993ce7e96763118d44ddd Mon Sep 17 00:00:00 2001
From: Oliver Joseph Ash <oliverjash@gmail.com>
Date: Fri, 17 Jan 2025 10:20:31 +0000
Subject: [PATCH 1/3] Enable `exactOptionalPropertyTypes`

---
 tsconfig.base.json | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tsconfig.base.json b/tsconfig.base.json
index 8f209a52e9..f53b7ca935 100644
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -20,6 +20,7 @@
     "resolveJsonModule": true,
     "skipLibCheck": true,
     "strict": true,
+    "exactOptionalPropertyTypes": true,
     "target": "ESnext",
     "types": ["vitest/globals", "vitest/importMeta"]
   },

From 824d44daef1bf2571e79f3d8c019d2791447f388 Mon Sep 17 00:00:00 2001
From: Oliver Joseph Ash <oliverjash@gmail.com>
Date: Fri, 17 Jan 2025 10:20:42 +0000
Subject: [PATCH 2/3] Add failing tests

---
 test/typescript/reducers.test-d.ts | 34 +++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/test/typescript/reducers.test-d.ts b/test/typescript/reducers.test-d.ts
index 46266aea92..26e0e17977 100644
--- a/test/typescript/reducers.test-d.ts
+++ b/test/typescript/reducers.test-d.ts
@@ -1,4 +1,10 @@
-import type { Action, AnyAction, Reducer, ReducersMapObject } from 'redux'
+import type {
+  Action,
+  AnyAction,
+  PreloadedStateShapeFromReducersMapObject,
+  Reducer,
+  ReducersMapObject
+} from 'redux'
 import { combineReducers } from 'redux'
 
 describe('type tests', () => {
@@ -265,4 +271,30 @@ describe('type tests', () => {
       >()
     }
   })
+
+  test('`PreloadedStateShapeFromReducersMapObject` has correct type when given a custom action', () => {
+    type MyAction = { type: 'foo' }
+
+    // TODO: not sure how to write this test??
+    // Expect this to match type `{ nested: string | undefined; }`
+    type P = PreloadedStateShapeFromReducersMapObject<{
+      nested: Reducer<string, MyAction>
+    }>
+  })
+
+  test('`combineReducer` has correct return type when given a custom action', () => {
+    type MyAction = { type: 'foo' }
+
+    type State = string
+    const nested: Reducer<State, MyAction> = (state = 'foo') => state
+
+    type Combined = { nested: State }
+
+    // Expect no error
+    const combined: Reducer<
+      Combined,
+      MyAction,
+      Partial<Combined>
+    > = combineReducers({ nested })
+  })
 })

From bf0a41827136837398bcaae52aeeb719af06ccda Mon Sep 17 00:00:00 2001
From: Oliver Joseph Ash <oliverjash@gmail.com>
Date: Fri, 17 Jan 2025 10:20:48 +0000
Subject: [PATCH 3/3] Fix failing test

---
 src/types/reducers.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/types/reducers.ts b/src/types/reducers.ts
index 146fc137f6..ca234898f2 100644
--- a/src/types/reducers.ts
+++ b/src/types/reducers.ts
@@ -106,7 +106,7 @@ export type PreloadedStateShapeFromReducersMapObject<M> = M[keyof M] extends
   ? {
       [P in keyof M]: M[P] extends (
         inputState: infer InputState,
-        action: UnknownAction
+        action: ActionFromReducersMapObject<M>
       ) => any
         ? InputState
         : never