Recent clang versions optimize more aggressively which leads to runtime errors using valid code, see e.g.,
MESOS-5745, due to code exposing undefined behavior in libcxx-3.8 and earlier. This was fixed with upstream libcxx-3.9. See https://reviews.llvm.org/D20786 for the patch and https://llvm.org/bugs/show_bug.cgi?id=28469 for the code example extracted from our code base.
We should consider rejecting builds if libcxx-3.8 or older is detected since not all users compiling Mesos might run the test suite. In our decision to reject we could possibly also take the used clang versions into account (which would just ensure we don't run into the known problems from the UB in libcxx).