Fail prerendering if a page is shadowed by an endpoint

Describe the bug

If you create a bob/index with a POST and bob/index.svelte --> posting will not work when deployed to Vercel.

Vercel throws error 405.

Reproduction

Deploy this repo to Vercel, and visit /bob and hit the submit button - and then the error will appear.

As long as it exists 😅, you can also try it here: https://sveltekit-p9hxntt45-jeddah.vercel.app/bob

Logs

No response

System Info

Latest version `next` of kit and vercel adapter.

Severity

serious, but I can work around it

Additional Information

I can simply work around by not enabling prerender=true on the sveltekit component.

Let me know if you need more information 😎

[Rich-Harris]

This is expected behaviour — prerendered pages always take priority over dynamic endpoints. The reason for that is that in most environments (Vercel included) there isn’t even an opportunity to run SvelteKit if a matching file exists on the filesystem. The 405 you’re seeing is coming from Vercel itself, which sees a request for /bob, sees that there’s a matching bob/index.html file already prerendered, then says ‘hang on a minute, you can’t POST to a prerendered file’.

So you’ll always need to disable prerendering for these pages. We could probably do a better job of diagnosing this at build time — if a page is shadowed by an endpoint, prerendering should fail. (When we implement #3532, we can change that to ‘if a page is shadowed by an endpoint with non-GET handlers’.)

[tcc-sejohnson]

I edited the PR description to include “fixes” before each issue number. You can see it linked the issues on the side of the PR so that they’ll be automatically closed if the PR is merged

Thanks! I was wondering why some of my PRs would auto-close issues and others wouldn’t… I edited the few I have open as well.