fix(core): never revert to undefined data

packages/query-core/src/__tests__/queryClient.test.tsx

@@ -1,6 +1,7 @@
 import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest'
 import { queryKey, sleep } from '@tanstack/query-test-utils'
 import {
+  CancelledError,
   MutationObserver,
   QueryClient,
   QueryObserver,
@@ -1060,6 +1061,24 @@ describe('queryClient', () => {
         status: 'error',
       })
     })
+
+    test('should throw CancelledError when initial fetch is cancelled', async () => {
+      const key = queryKey()
+
+      const promise = queryClient.fetchQuery({
+        queryKey: key,
+        queryFn: async () => {
+          await sleep(50)
+          return 25
+        },
+      })
+
+      await vi.advanceTimersByTimeAsync(10)
+
+      await queryClient.cancelQueries({ queryKey: key })
+
+      await expect(promise).rejects.toBeInstanceOf(CancelledError)
+    })
   })
 
   describe('refetchQueries', () => {

packages/query-core/src/query.ts

@@ -558,7 +558,11 @@ export class Query<
             fetchStatus: 'idle' as const,
           })
           // transform error into reverted state data
-          return this.state.data!
+          // if the initial fetch was cancelled, we have no data, so we have
+          // to get into error state with the CancelledError
+          if (this.state.data !== undefined) {
+            return this.state.data
+          }
         }
       }
       this.#dispatch({