Zero-config type-safety doesn't work properly with `PageData`

[aradalvand]

Describe the bug

The PageData interface is meant to define a set of properties that every page's load function must return as part of its "data" object.

With the new zero-config type-safety, however, it seems like this is not working as expected. The return types of the load functions aren't typed according to PageData, and there will be no errors even if you haven't provided a required property, there's no typing at all:

With this app.d.ts:

declare global {
    namespace App {
        interface PageData {
            meta: {
                title: string;
            };
        }
    }
}

export {};

This +page.ts yields no errors:

export function load() {
    return {};
}

And there are no relevant suggestions, etc:

The return type is typed without regard to PageData:

It may be worth noting the classic explicitly-typed load functions still work properly in this respect:

Reproduction

Described above.

Logs

No response

System Info

System:
    OS: Linux 5.10 Ubuntu 22.04.1 LTS 22.04.1 LTS (Jammy Jellyfish)
    CPU: (4) x64 Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
    Memory: 1.65 GB / 3.84 GB
    Container: Yes
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.12.1 - /usr/bin/node
    Yarn: 1.22.19 - /usr/bin/yarn
    npm: 9.3.0 - /usr/bin/npm
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.0.1
    @sveltejs/kit: ^1.5.0 => 1.15.9
    svelte: ^3.54.0 => 3.58.0
    vite: ^4.3.0 => 4.3.3

Severity

blocking an upgrade

Additional Information

No response

[aradalvand]

@dummdidumm Sorry to tag but could you please confirm this?

[dummdidumm]

For these only the event type is added automatically, not the outgoing type. I'm not sure if it's possible to add it without losing the infered proper shape on the consumer side in +page.svelte. We could do a satisfies transformation under the hood but that's brittle because more code modification is required and it would only work for when the load function is defined as an arrow function.

[aradalvand]

@dummdidumm How was the outgoing type inferred before satisfies was added to TS? I remember it used to be inferred successfully back then too. Couldn't we do the same here?