@ldionne This one is also an early dependency of the rest of the patchset; it doesn't yet have a formal approval of any others, but seems to be mostly ok, with just a small style preference question left (whether to explicitly use '\\' and '/' literals, or unconditionally add something like __posix_preferred_separator and __windows_preferred_separator).
It's covered by existing tests, more or less, I haven't singled out which specific bit covers this.
The thing is, currently, libc++'s tests (in general, not specific to std::filesystem) on windows are in a severely broken state (with around >100 tests failing out of the box). And libc++'s tests for std::filesystem assume lots of posix-specific behaviours. I have a huge branch with 50+ patches that make libc++'s tests pass with MS STL (as I'd consider the reference for how std::filesystem should behave on windows), and in that state of the testsuite, all those same tests pass with libc++ when this whole patchset is applied.
And it's hard to single out specific test changes among the patches in the beginning of this patchset, as you need up to around patch 17 or 18 before libc++ with filesystem enabled actually compiles successfully for windows.
I tried reverting this particular patch on top of the otherwise fully patched branch with all tests passing, and that broke 80 out of 137 testcases - so I think it's fairly covered. I think all bits in the patch has been written to make some aspect of the existing testset work.