[9.5.6-canary.14][notFound] incorrect props in custom 404-page when using fallback

[rokinsky]

Bug report

Describe the bug

I use the experimental unstable_notFound feature on SSG pages with enabled fallback. The custom 404-page gets incorrect props from getStaticProps when notFound occurs.

To Reproduce

Repository: next.js-bug-not-found-props
Steps to reproduce the behavior:

1. Go to http://localhost:3000
2. See

{
  "statusCode": 404,
  "url": {
    "query": {},
    "pathname": "/[[...slug]]",
    "asPath": "/"
  }
}

Expected behavior

Displaying

{
  "is404": true
}

Screenshots

N/A

System information

• OS:macOS
• Browser (if applies): chrome
• Version of Next.js: 9.5.6-canary.14
• Version of Node.js: v10.22.1

Additional context

N/A

[balazsorban44]

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