[fresh] Reset useMemoCache when size changes

packages/react-reconciler/src/ReactFiberHooks.js

@@ -1226,21 +1226,17 @@ function useMemoCache(size: number): Array<any> {
   updateQueue.memoCache = memoCache;
 
   let data = memoCache.data[memoCache.index];
-  if (data === undefined) {
+  // We reset the cache on init, or indiscriminately even in the event that the cache size requested
+  // differs from the previous render. In dev environments this is typically due to FastRefresh,
+  // where editing the code can change the cache size and should reset entirely to prevent stale
+  // values from being reused. In prod environments this is never expected to happen. However, in
+  // the unlikely case that it does vary between renders, we reset the cache anyway so behavior is
+  // consistent in both environments.
+  if (data === undefined || data.length !== size) {
     data = memoCache.data[memoCache.index] = new Array(size);
     for (let i = 0; i < size; i++) {
       data[i] = REACT_MEMO_CACHE_SENTINEL;
     }
-  } else if (data.length !== size) {
-    // TODO: consider warning or throwing here
-    if (__DEV__) {
-      console.error(
-        'Expected a constant size argument for each invocation of useMemoCache. ' +
-          'The previous cache was allocated with size %s but size %s was requested.',
-        data.length,
-        size,
-      );
-    }
   }
   memoCache.index++;
   return data;

packages/react-reconciler/src/__tests__/useMemoCache-test.js

@@ -621,4 +621,37 @@ describe('useMemoCache()', () => {
       </>,
     );
   });
+
+  // @gate enableUseMemoCacheHook
+  it('resets cache when cache size varies between renders', async () => {
+    const obj = {};
+    function Component(props) {
+      const $ = useMemoCache(props.cacheSize);
+      let cacheHit = 0;
+      if ($[0] === obj) {
+        cacheHit = 1;
+      } else {
+        $[0] = obj;
+      }
+      return cacheHit;
+    }
+    const root = ReactNoop.createRoot();
+    await act(() => {
+      root.render(<Component key="comp" cacheSize={1} />);
+    });
+    // Initial render, cache is empty
+    expect(root).toMatchRenderedOutput('0');
+
+    await act(() => {
+      root.render(<Component key="comp" cacheSize={1} />);
+    });
+    // Re-render, re-use cache since obj identity has not changed
+    expect(root).toMatchRenderedOutput('1');
+
+    await act(() => {
+      root.render(<Component key="comp" cacheSize={10} />);
+    });
+    // Cache size changed b/w renders, reset the whole cache
+    expect(root).toMatchRenderedOutput('0');
+  });
 });

packages/react-refresh/src/__tests__/ReactFreshIntegration-test.js

@@ -1637,8 +1637,7 @@ describe('ReactFreshIntegration', () => {
       }
     });
 
-    // eslint-disable-next-line jest/no-disabled-tests
-    it.skip('resets useMemoCache cache slots', async () => {
+    it('resets useMemoCache cache slots', async () => {
       if (__DEV__) {
         await render(`
           const useMemoCache = require('react/compiler-runtime').c;