Xcode Version 9.2 (9C40b)
commit c5c6cc861d6bfcb74035a07f493fab9617b1c36f on swift-corelibs-foundation, master as this bug is filed.
The code in question:
On macOS, the result is ".." while on Linux, the result is ".". I think the former makes more sense since ".." represents parent directory on unix and treating part of it as file extension is a bit of a stretch. Another argument for aligning behavior to macOS is the fact that it existed longer therefore has more potential users.
This bug manifests here: https://travis-ci.org/kylef/PathKit/builds/289637066
The PathKit project CI is failing because it relies on this behavior and the example value is being used in tests on both macOS and Linux.