From 1f7d10e2a11828db23e81e8e3eedfc2bbe177a2c Mon Sep 17 00:00:00 2001
From: eps1lon <silbermann.sebastian@gmail.com>
Date: Mon, 3 May 2021 23:20:18 +0200
Subject: [PATCH 1/2] test: Add current behavior for reducers

---
 .../ReactHooksWithNoopRenderer-test.js        | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
index fc5e61cd78ee6..a8940c781bd10 100644
--- a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.js
@@ -3784,4 +3784,49 @@ describe('ReactHooksWithNoopRenderer', () => {
 
     expect(Scheduler).toHaveYielded(['Render: 0']);
   });
+
+  it('reducer-closure', () => {
+    let setLimit;
+    let increment;
+    function Test() {
+      const [limit, _setLimit] = useState(5);
+      const [count, _increment] = useReducer((state, action) => {
+        return state < limit ? state + 1 : state;
+      }, 1);
+      setLimit = _setLimit;
+      increment = _increment;
+
+      return <Text text={`Render: ${count}`} />;
+    }
+
+    act(() => {
+      ReactNoop.render(<Test />);
+    });
+
+    expect(Scheduler).toHaveYielded(['Render: 1']);
+
+    act(() => {
+      increment();
+      increment();
+      increment();
+      increment();
+    });
+
+    expect(Scheduler).toHaveYielded(['Render: 5']);
+
+    act(() => {
+      increment();
+    });
+    act(() => {
+      increment();
+    });
+
+    expect(Scheduler).toHaveYielded([]);
+
+    act(() => {
+      setLimit(10);
+    });
+
+    expect(Scheduler).toHaveYielded(['Render: 5']);
+  });
 });

From 2a092aeb5dda444b93dc0eddc6229d5a7ebbd25b Mon Sep 17 00:00:00 2001
From: eps1lon <silbermann.sebastian@gmail.com>
Date: Mon, 3 May 2021 23:27:21 +0200
Subject: [PATCH 2/2] Naive fix

---
 packages/react-reconciler/src/ReactFiberHooks.new.js | 4 +++-
 packages/react-reconciler/src/ReactFiberHooks.old.js | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/packages/react-reconciler/src/ReactFiberHooks.new.js b/packages/react-reconciler/src/ReactFiberHooks.new.js
index b195a0cb57488..ea4312394c87b 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.new.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.new.js
@@ -821,7 +821,9 @@ function updateReducer<S, I, A>(
           newState = ((update.eagerState: any): S);
         } else {
           const action = update.action;
-          newState = reducer(newState, action);
+          const reducerImpl =
+            update.eagerReducer !== null ? update.eagerReducer : reducer;
+          newState = reducerImpl(newState, action);
         }
       }
       update = update.next;
diff --git a/packages/react-reconciler/src/ReactFiberHooks.old.js b/packages/react-reconciler/src/ReactFiberHooks.old.js
index 7c51c43ad2e7e..e2be5a96d5c97 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.old.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.old.js
@@ -821,7 +821,9 @@ function updateReducer<S, I, A>(
           newState = ((update.eagerState: any): S);
         } else {
           const action = update.action;
-          newState = reducer(newState, action);
+          const reducerImpl =
+            update.eagerReducer !== null ? update.eagerReducer : reducer;
+          newState = reducerImpl(newState, action);
         }
       }
       update = update.next;