fix(query-core): setQueryData updater keeps discriminated-union members when spreading a narrowed value

.changeset/tidy-cougars-serve.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+fix(query-core): setQueryData updater no longer loses discriminated-union members when spreading a narrowed value

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

@@ -157,6 +157,27 @@ describe('queryClient', () => {
       expectTypeOf(data).toEqualTypeOf<number | undefined>()
     })
 
+    it('should narrow a discriminated union when spreading the updater value', () => {
+      type ProbeData =
+        | { kind: 'a'; value: number; requiredA: number }
+        | { kind: 'b'; value: number; requiredB: number }
+      const key = ['probe'] as DataTag<Array<string>, ProbeData>
+      const queryClient = new QueryClient()
+
+      // Regression test for #11795: this call must compile without an
+      // explicit annotation on `prev`
+      queryClient.setQueryData(key, (prev) => {
+        expectTypeOf(prev).toEqualTypeOf<ProbeData | undefined>()
+        if (!prev || prev.kind !== 'a') return prev
+        expectTypeOf(prev).toEqualTypeOf<{
+          kind: 'a'
+          value: number
+          requiredA: number
+        }>()
+        return { ...prev, value: prev.value + 1 }
+      })
+    })
+
     it('should type the value if the key is tagged', () => {
       const key = ['key'] as DataTag<Array<string>, number>
       const queryClient = new QueryClient()

packages/query-core/src/queryClient.ts

@@ -289,7 +289,7 @@ export class QueryClient {
   >(
     queryKey: TTaggedQueryKey,
     updater: Updater<
-      NoInfer<TInferredQueryFnData> | undefined,
+      TInferredQueryFnData | undefined,
       NoInfer<TInferredQueryFnData> | undefined
     >,
     options?: SetDataOptions,