Using basePath breaks SPA when using getServerSideProps or getStaticProps

Bug report

Describe the bug

Setting basePath breaks SPA fetches and causes full re-render.

To Reproduce

1. Set basePath to project in next.config.js
2. Add either getServerSideProps or getStaticProps and getStaticPaths method to a page.

Repo for testing: https://github.com/IikkaWinter/next-basepath-demo

Deployments with basePath: https://nextjs-blog-d3av2acke.vercel.app/docs/foo

without basePath (everything works): https://nextjs-blog-2o56gnpjr.vercel.app/foo

Everything works as expected when run locally with vc dev. Deploy to Vercel and every _next/data//.json path returns 404.

Expected behavior

Expected to run serverside method and return 200.

Screenshots

OBS! Prerendered page1.json and page2.json work.

foo.json missing. Also see how Lambda function is named instead of /foo.

System information

• OS: macOS
• Browser (if applies) Chrome
• Version of Next.js: 9.5.5
• Version of Node.js: 12.13.0

Additional context

Additionally, when using basePath (/docs) with rewrites and parameters in sources, e.g. /:locale/:path, the route/docs/en will pass the following parameters to the getServerSideProps:

{
  locale: 'doc',
  path: 'en'
}

[ijjk]

Hi, this should be corrected now, please re-deploy your applications and give it a try!

[joiglifberg]

Confirmed fixed!