This is interesting.

`urlComponents.url(relativeTo: baseUrl)` uses `urlComponents.string` which evaluates to an empty string (on both Linux and Darwin) and passes it to `URL(string:relativeTo)` which is expected to return nil if `string` is empty.

So, the `URLComponents.url(relativeTo)` implementation is a composition of `urlComponents.string` and `URL(string:relativeTo)` both of which are behaving as expected. May be it needs an alternate implementation?

https://developer.apple.com/documentation/foundation/urlcomponents/1780048-url does not discuss the case of URLComponents value where nothing is defined. May be we should simply add a special case to make the behaviour same as Darwin.

I wonder whether there's anyone still looking at this issue.
This brings platform inconsistencies between macOS and other OSes.

For anyone who wants platform consistencies with this or similar API, I suggest using this one instead:

      #if canImport(Darwin)
      let filename = url.pathComponents.last
      #else
      let filename = url.pathComponents?.last
      #endif

The last is always nullable whatever the OS is.

Update: I found that the urls(forResourcesWithExtension returns NSURL in lieu of URL on non-Darwin platforms.

NSURL on non-Darwin platforms have API inconsitencies comparing to URL, hence manual conversion required:

let urls = nsURLs.compactMap { URL(string: $0.absoluteString) }