This is a heads up.
I spoke with a couple of you (e.g. Chandler, Reid) at CppCon about this: the upcoming release of Visual Studio (VS “15”) has some changes to how _MSC_VER will be defined at every (future) update. These changes are already implemented in ‘VS “15” Preview 5’. The full details are here https://blogs.msdn.microsoft.com/vcblog/2016/10/05/visual-c-compiler-version/
Internal testing showed that Clang+LLVM source code needed a few changes to use inequalities instead of equalities with _MSC_VER.