How can `sep` parameter be used? Is it broken?

I'm using a custom globber that internally uses only forward slashes and I also consistently specify sep='/' to iglob etc., and yet, on Windows I can still get paths joined by '\'.

I see that this sep is only being passed to _join_paths, and inside it all that happens is that / gets replaced with this character. What about replacing \, though?

https://github.com/miracle2k/python-glob2/blob/ef4b58f0d2a6eb0197446ab4b047cc45e4c60500/glob2/impl.py#L211-L215

There's also a suspicious backslash (that has no effect at all) inside the pattern 🤔

It's strange that os.path.join is being used for this library, which strives to fully abstract this operation -- and currently seems like I have no better option but to map all its outputs through pathlib.Path(p).to_posix(). This particular operation is not overridable.

So, my use case doesn't work, but I also find it difficult to understand how sep can be used at all, as on Windows it will always just be skipped.

Start with glob2/impl.py around lines 211-215 and reproduce the custom-globber case on Windows with sep='/'. Trace how _join_paths and os.path.join handle separators, then define and test the expected sep behavior, including the suspicious backslash noted in the issue.