First, thanks to @ldionne for making this possible! Upon request a few months ago, Louis added the libcxx CI build (Builds · libcxx CI) to clang code reviews in Phabricator to help us avoid both breaking libc++ and from causing problems with some commonly used template instantiations, code patterns, and build configurations (including many C++ versions, modules/etc).
While our Windows/Debian CI builds are historically not particularly reliable, Louis and the libc++ developers have done a fantastic job going out of their way to make sure that this CI is reliable and trustworthy. Because of this, code reviewers and patch authors should not be quick to dismiss failing libc++ CI notifications in Phabricator. If the libc++ CI is red for a given patch, it should be triaged to determine whether the issue is with the patch under review or with libc++ itself. If there is a clear issue in libc++ (due to diagnostic wording changes or other conforming changes to Clang) or if it’s unclear where the problem lies, you should include the libc++ code review group for further discussion. For more immediate feedback, you can also include @ldionne or @mordante as reviewers.
Either way, we want to do our best to ensure that the libc++ CI bots stay green, so please ensure they build on your review before committing your review. Thanks!