TS error when createServerFn returns a nested unknown type

[rhyek]

Which project does this relate to?

Start

Describe the bug

The following:

const testUnknown = createServerFn().handler(() => {
  return {} as unknown as { a: unknown };
});

produces this error:

Argument of type '() => { a: unknown; }' is not assignable to parameter of type 'ServerFn<Method, "data", undefined, undefined, { a: unknown; }>'.
  Type '{ a: unknown; }' is not assignable to type 'Promise<{ a: {}; }> | { a: {}; }'.
    Type '{ a: unknown; }' is not assignable to type '{ a: {}; }'.
      Types of property 'a' are incompatible.
        Type 'unknown' is not assignable to type '{}'.ts(2345)

Your Example Website or App

https://stackblitz.com/edit/github-jwt7xpxn?file=src%2Froutes%2Findex.tsx

Steps to Reproduce the Bug or Issue

1. Go to https://stackblitz.com/edit/github-jwt7xpxn?file=src%2Froutes%2Findex.tsx
2. Go to index.tsx
3. See TS error in server fn

Expected behavior

The return type of the server fn should include the never type and not {}.

Screenshots or Videos

No response

Platform

• OS: macOS
• TS: 5.8.3
• Tanstack Router: 1.121.2
• Tanstack Start: 1.121.7

Additional context

My api endpoint is returning data that might include the unknown type and I want to be able to handle that in my components.

[schiller-manuel]

we check whether the returned type can be serialized. as this is undecidable for unknown, a TS error is raised.
If you don't want to properly type the return value and are sure the data is serializable you need to suppress the error.

[rhyek]

That makes sense.