fix(queryObserver): always structurally share placeholderData (#4402)

Author: TkDodo

packages/query-core/src/queryObserver.ts

@@ -517,11 +517,6 @@ export class QueryObserver<
         if (options.select && typeof placeholderData !== 'undefined') {
           try {
             placeholderData = options.select(placeholderData)
-            placeholderData = replaceData(
-              prevResult?.data,
-              placeholderData,
-              options,
-            )
             this.selectError = null
           } catch (selectError) {
             if (process.env.NODE_ENV !== 'production') {
@@ -534,7 +529,7 @@ export class QueryObserver<
 
       if (typeof placeholderData !== 'undefined') {
         status = 'success'
-        data = placeholderData as TData
+        data = replaceData(prevResult?.data, placeholderData, options) as TData
         isPlaceholderData = true
       }
     }

packages/query-core/src/tests/queryObserver.test.tsx

@@ -505,6 +505,24 @@ describe('queryObserver', () => {
     expect(results[1]).toMatchObject({ status: 'success', data: 'data' })
   })
 
+  test('should structurally share placeholder data', async () => {
+    const key = queryKey()
+    const observer = new QueryObserver(queryClient, {
+      queryKey: key,
+      enabled: false,
+      queryFn: () => 'data',
+      placeholderData: {},
+    })
+
+    const firstData = observer.getCurrentResult().data
+
+    observer.setOptions({ placeholderData: {} })
+
+    const secondData = observer.getCurrentResult().data
+
+    expect(firstData).toBe(secondData)
+  })
+
   test('the retrier should not throw an error when reject if the retrier is already resolved', async () => {
     const key = queryKey()
     let count = 0