Remove default export of a 404.html.html page

[mdubourg001]

fixes #36855

Bug

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• Integration tests added
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Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
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Documentation / Examples

• Make sure the linting passes by running yarn lint

[SukkaW]

IMHO it is not the correct fix.

The code here is designed to work like this:

• When you specify a custom 404 in exportPathMap:
  • Next.js shouldn't overwrite your custom 404 export alias.
• When you don't specify a custom 404 in exportPathMap (when, and only when neither /404 nor /404.html exists in exportPathMap):
  • When trailingSlash is disabled (which is by default):
    • only 404.html should be exported (which could be used to render https://example.com/404).
    • ['/404', '/404.html'] will be deduped to /404 after path normalization
  • When trailingSlash is enabled:
    • should export 404/index.html instead (which could be used to render https://example.com/404/).
    • A 404.html should also be exported for backward compatibility with GitHub Pages, GitLab Pages, Cloudflare Pages, Netlify, etc.
    • So, /404 would export /404/index.html and /404.html would export /404.html.

You could take a look at export/worker.ts to see how it handles the 404 (check the isBultinPaths part). Also, an integration test case to verify your fix would be appreciated.

[SukkaW]

The PR is superseded by #36910

[ijjk]

Closing as this should be resolved by #36910 as noted above. Thanks for the PR, though!