Cannot use nested interfaces to type endpoint output

[flut1]

Describe the bug

This is a follow-up on #1997, which was recently fixed in #4897.

The new BodyValidator type seems to work well, but it doesn't seem to work with interfaces that have properties which are themselves interfaces.

Reproduction

If you prefer a repository: check out this file in flut1/sveltekit-endpoint-output-nested-interfaces-repro

import type { RequestHandler } from "@sveltejs/kit";

interface GetOutput {
    item: { message: string }
}

// No TS error
export const get: RequestHandler<any, GetOutput> = async () => {
    return {
        status: 200,
        body: {
            item: { message: 'hello' },
        },
    };
};

interface Item {
    message: string;
}

interface PostOutput {
    item: Item;
}

// TS error: Type '{ message: string; }' is not assignable to type 'never'.ts(2322)
export const post: RequestHandler<any, PostOutput> = async () => {
    return {
        status: 200,
        body: {
            item: { message: 'hello' },
        },
    };
};

Logs

Output of svelte-check:

sveltekit-endpoint-output-nested-interfaces-repro/src/routes/index.ts:26:14
Error: Type '() => Promise<{ status: number; body: { item: { message: string; }; }; }>' is not assignable to type 'RequestHandler<any, PostOutput>'.
  Type 'Promise<{ status: number; body: { item: { message: string; }; }; }>' is not assignable to type 'MaybePromise<RequestHandlerOutput<PostOutput>>'.
    Type 'Promise<{ status: number; body: { item: { message: string; }; }; }>' is not assignable to type 'Promise<RequestHandlerOutput<PostOutput>>'.
      Type '{ status: number; body: { item: { message: string; }; }; }' is not assignable to type 'RequestHandlerOutput<PostOutput>'.
        The types of 'body.item' are incompatible between these types.
          Type '{ message: string; }' is not assignable to type 'never'. 
export const post: RequestHandler<any, PostOutput> = async () => {

System Info

System:
  OS: macOS 12.3.1
  CPU: (8) arm64 Apple M1
  Memory: 111.95 MB / 8.00 GB
  Shell: 5.8 - /bin/zsh
Binaries:
  Node: 16.15.0 - ~/.nvs/default/bin/node
  npm: 8.5.5 - ~/.nvs/default/bin/npm
Browsers:
  Firefox: 100.0
  Safari: 15.4
npmPackages:
  @sveltejs/adapter-auto: next => 1.0.0-next.42 
  @sveltejs/kit: next => 1.0.0-next.330 
  svelte: ^3.44.0 => 3.48.0 

Severity

annoyance

Additional Information

No response

[Rich-Harris]

cc'ing @ignatiusmb for visibility

[ignatiusmb]

Thank you for the thorough report. Seems like the initial problem of interfaces not having implicit index signatures is still apparent when nested as properties. Changing Item from an interface to a type alias removes the error, so I guess something is not working with the recursive check.