fix: provide better error message in case of missing +page.svelte

[dummdidumm]

closes #8270

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[dummdidumm]

Not wrapped with DEV because I noticed it's false when running vite dev in sites/kit.svelte.dev, similar to what Rich encountered. I'm wondering if this hints at a bigger problem? Is DEV not working as expected after all?

[benmccann]

I filed an issue so we can have a single place to investigate: #8492

[Rich-Harris]

Is this ssr specific? If you create src/routes/foo/+page.js with export const ssr = false and navigate to /foo, you'll see a blank page, which feels like a bug. I think we probably want to insist on there being a +page.svelte alongside any +page.js or +page.server.js regardless of page config (and we could do that check a single time, in create_manifest_data)

[Rich-Harris]

Hmm, except then again I've written +page(.server).js like this without a +page.svelte file before, and it seems like a reasonable thing to be able to do:

import { redirect } from '@sveltejs/kit';

export function load() {
  throw redirect(308, '/docs/introduction');
}

[dummdidumm]

Yes, because of that we can't just have a static check. The linked issue is handled by the implemented check, your edge case isn't thought, but I doubt that someone having only export const ssr = false and no load function or +page.svelte occurs that often that we should try to catch this, too. (theoretically it's possible to catch this case at runtime, too, but would require more code gymnastics than I think it's worth)

[Rich-Harris]

Yeah I had a quick go at adding logic to client.js but couldn't immediately see an easy way to make it work, so I'm happy to leave that as a TODO for another day