fix getSnapshot warning when a selector returns NaN

packages/react-reconciler/src/ReactFiberHooks.new.js

@@ -1290,7 +1290,8 @@ function mountSyncExternalStore<T>(
     nextSnapshot = getSnapshot();
     if (__DEV__) {
       if (!didWarnUncachedGetSnapshot) {
-        if (nextSnapshot !== getSnapshot()) {
+        const cachedSnapshot = getSnapshot();
+        if (!is(nextSnapshot, cachedSnapshot)) {
           console.error(
             'The result of getSnapshot should be cached to avoid an infinite loop',
           );
@@ -1362,7 +1363,8 @@ function updateSyncExternalStore<T>(
   const nextSnapshot = getSnapshot();
   if (__DEV__) {
     if (!didWarnUncachedGetSnapshot) {
-      if (nextSnapshot !== getSnapshot()) {
+      const cachedSnapshot = getSnapshot();
+      if (!is(nextSnapshot, cachedSnapshot)) {
         console.error(
           'The result of getSnapshot should be cached to avoid an infinite loop',
         );

packages/react-reconciler/src/ReactFiberHooks.old.js

@@ -1290,7 +1290,8 @@ function mountSyncExternalStore<T>(
     nextSnapshot = getSnapshot();
     if (__DEV__) {
       if (!didWarnUncachedGetSnapshot) {
-        if (nextSnapshot !== getSnapshot()) {
+        const cachedSnapshot = getSnapshot();
+        if (!is(nextSnapshot, cachedSnapshot)) {
           console.error(
             'The result of getSnapshot should be cached to avoid an infinite loop',
           );
@@ -1362,7 +1363,8 @@ function updateSyncExternalStore<T>(
   const nextSnapshot = getSnapshot();
   if (__DEV__) {
     if (!didWarnUncachedGetSnapshot) {
-      if (nextSnapshot !== getSnapshot()) {
+      const cachedSnapshot = getSnapshot();
+      if (!is(nextSnapshot, cachedSnapshot)) {
         console.error(
           'The result of getSnapshot should be cached to avoid an infinite loop',
         );

packages/use-sync-external-store/src/__tests__/useSyncExternalStoreShared-test.js

@@ -587,6 +587,33 @@ describe('Shared useSyncExternalStore behavior (shim and built-in)', () => {
     );
   });
 
+  test('getSnapshot can return NaN without infinite loop warning', async () => {
+    const store = createExternalStore('not a number');
+
+    function App() {
+      const value = useSyncExternalStore(store.subscribe, () =>
+        parseInt(store.getState(), 10),
+      );
+      return <Text text={value} />;
+    }
+
+    const container = document.createElement('div');
+    const root = createRoot(container);
+
+    // Initial render that reads a snapshot of NaN. This is OK because we use
+    // Object.is algorithm to compare values.
+    await act(() => root.render(<App />));
+    expect(container.textContent).toEqual('NaN');
+
+    // Update to real number
+    await act(() => store.set(123));
+    expect(container.textContent).toEqual('123');
+
+    // Update back to NaN
+    await act(() => store.set('not a number'));
+    expect(container.textContent).toEqual('NaN');
+  });
+
   describe('extra features implemented in user-space', () => {
     // The selector implementation uses the lazy ref initialization pattern
     // @gate !(enableUseRefAccessWarning && __DEV__)

packages/use-sync-external-store/src/useSyncExternalStoreShimClient.js

@@ -57,7 +57,8 @@ export function useSyncExternalStore<T>(
   const value = getSnapshot();
   if (__DEV__) {
     if (!didWarnUncachedGetSnapshot) {
-      if (value !== getSnapshot()) {
+      const cachedValue = getSnapshot();
+      if (!is(value, cachedValue)) {
         console.error(
           'The result of getSnapshot should be cached to avoid an infinite loop',
         );