fix(nuxt): prevent getCachedData from shaping type of useAsyncData (#25946)

packages/nuxt/src/app/composables/asyncData.ts

@@ -36,6 +36,8 @@ export type KeyOfRes<Transform extends _Transform> = KeysOf<ReturnType<Transform
 
 export type MultiWatchSources = (WatchSource<unknown> | object)[]
 
+export type NoInfer<T> = [T][T extends any ? 0 : never]
+
 export interface AsyncDataOptions<
   ResT,
   DataT = ResT,
@@ -61,7 +63,7 @@ export interface AsyncDataOptions<
    * A `null` or `undefined` return value will trigger a fetch.
    * Default is `key => nuxt.isHydrating ? nuxt.payload.data[key] : nuxt.static.data[key]` which only caches data when payloadExtraction is enabled.
    */
-  getCachedData?: (key: string, nuxtApp: NuxtApp) => DataT
+  getCachedData?: (key: string, nuxtApp: NuxtApp) => NoInfer<DataT>
   /**
    * A function that can be used to alter handler function result after resolving.
    * Do not use it along with the `pick` option.

test/fixtures/basic-types/types.ts

@@ -512,6 +512,20 @@ describe('composables', () => {
     expectTypeOf(notTypedData.value!.content).toEqualTypeOf<string[]>()
     expectTypeOf(notTypedData.value!.untypedKey).toEqualTypeOf<any>()
   })
+
+  it('correctly types returns when using with getCachedData', () => {
+    expectTypeOf(useAsyncData('test', () => Promise.resolve({ foo: 1 }), {
+      getCachedData: key => useNuxtApp().payload.data[key],
+    }).data).toEqualTypeOf<Ref<{ foo: number } | null>>()
+    useAsyncData('test', () => Promise.resolve({ foo: 1 }), {
+      // @ts-expect-error cached data should return the same as value of fetcher
+      getCachedData: () => ({ bar: 2 }),
+    })
+    useAsyncData<{ foo: number }, unknown, { foo: number }>('test', () => Promise.resolve({ foo: 1 }), {
+      // @ts-expect-error cached data should return the same as asserted type of `useAsyncData`
+      getCachedData: () => ({ bar: 2 }),
+    })
+  })
 })
 
 describe('app config', () => {