Invalid build if chunkFileName has wrong path segment separator for OS

[jacksteamdev]

• Rollup Version: ^2.41.2
• Operating System (or Browser): Linux Mint 19.3 Cinnamon and Windows 10
• Node Version (if applicable): v14.15.1
• Link to reproduction (IMPORTANT, read below): https://repl.it/join/mumhzhkb-jacksteamdev

Expected Behavior

When the wrong path segment separator for the OS is used for output.chunkFileNames to put chunks in a subfolder, Rollup should error or generate a working bundle.

An alternative solution would be to change the docs to suggest using path.join('chunks','[name]-[hash].js'), instead of a forward slash.

Actual Behavior

Rollup outputs an invalid bundle if the POSIX separator is used on Windows. The chunks are not put into a folder, but rather the intended folder name is literally in the file name. The import paths have the intended structure, but those files do not exist.

I used an escaped backslash in the REPL to demonstrate the problem. The behavior is the same on Windows with a forward slash or on POSIX with a backslash.

This is a problem on shared boilerplates. Related issue: extend-chrome/js-react-boilerplate#2

[lukastaegert]

Actually the intention was to always use forward slashes in the bundle object on all environments to have consistent bundle objects across environments. Until now I always assumed that fs.writeFile would work correctly on Windows (actually I just tried it in a Windows REPL and fs.mkdirSync and fs.writeFileSync handle forward slashes correctly). So I wonder what the actual issue is.