Since it seems that there are a bunch of folks interested in Foundation on Windows, I figure that it makes some sense to disseminate some information around the current state of Foundation on Windows.
Foundation is possible to currently build on Windows in Debug and RelWithDebInfo configurations. Release mode exposes a bug in the swift compiler (SR-10192 ) which needs to be investigated. A temporary workaround is possible adding
-Xlinker -force:multiple to the link invocation.
In order to support Foundation builds on the nightlies, unfortunately, a set of changes are needed to improve the Foundation build (which I had not expected). Most of the work for that has preliminary work completed for it and has patches on GitHub , , , . This should pretty much enable the nightlies to build Foundation properly .
I had started improving the implementation of Foundation on Windows such that it could pass the associated test suite, but found that there is a lot of work to be done still. There were a number of issues, including TimeZone support being completely broken , bundle loading being broken , amongst other smaller issues. There are still many issues left to investigate. One of the problems is that the test suite is a monolithic blob, and many of these test failures will cause the test harness itself to crash! XCTest has been absolved of its involvement in this - there is one known issue left there  which is unrelated to the crashes.
There are issues left within libdispatch (needing support for file/socket sources) and Foundation's curl integration to support HTTPServer  for the test suite. As an example: the socket handle is currently truncated at both levels. Additionally, the
MultiHandle is not correctly being constructed IIRC.
Removing some portion of the tests (~25 or so IIRC), it is possible to get through a full run of the test suite, although there are still failures to look into. However, even then, about 80% or so of the test suite passes on Windows, so its not like the library is not usable as it currently stands (hey, it is used to run XCTest which is needed to run the test suite!). However, there is still a pretty large surface area to cover, so, others are welcome to join the bug hunting :-).