Start RC - `lazyFn` for `loader` with `head` results in type error

[checkerschaf]

Which project does this relate to?

Router

Describe the bug

After upgrading to RC I've seen this type error that was working fine before the upgrade.

Error:

Type 'Promise<{ id: string; title: string; body: string; }>' is not assignable to type 'never'

Your Example Website or App

https://github.com/checkerschaf/tanstack-start-lazyfn-loader-head-bug/blob/main/src/routes/posts.%24postId.tsx#L10

Steps to Reproduce the Bug or Issue

1. Clone the reproduction repo
2. Open the posts.$postId.ts file
3. See the type error

When removing the params from the head function the error is gone.

Functionally the behaviour seems correct.

Expected behavior

No type error. The same code was working before the RC release.

Screenshots or Videos

No response

Platform

• Router / Start Version: 1.132.6
• OS: macOS
• Browser: -
• Browser Version: -
• Bundler: vite

Additional context

No response

[schiller-manuel]

it works if you dont use it inline:

const l = lazyFn(
    () => import("../lazy-fn-test"),
    "loader",
  )
export const Route = createFileRoute('/posts/$postId')({
  loader: l,
  head: ({ params: { postId } }) => {
    return {};
  },
  errorComponent: PostErrorComponent,
  component: PostComponent,
  notFoundComponent: () => {
    return <NotFound>Post not found</NotFound>
  },
})

[checkerschaf]

Okay, that's good enough for me. Thank you @schiller-manuel 👍

[arackaf]

fwiw here's another repro - nothing to do with lazyFn - dunno if this is helpful or useful - apologies if not

export const Route = createFileRoute("/blog/$slug")({
  head: ({ params }) => {
    return {
      meta: [
        {
          title: `${params.slug} | Adam Rackis's blog`,
        },
      ],
    };
  },
  // @ts-ignore
  loader: async ({ params }) => {
    const post = getPostBySlug(params.slug, ["title", "date", "slug", "author", "content", "ogImage", "coverImage"]);
    const content = await markdownToHtml(post.content || "");

    return {
      post: {
        ...post,
        content,
      },
    };
  },
  component: RouteComponent,
});

[arackaf]

Edit - the above fails because the order of the config properties is wrong - head needs to come last because of how the TS inference works

https://tanstack.com/router/latest/docs/eslint/create-route-property-order