We updated our build servers compilers to VS2015 two weeks ago. we did not link the failures to compiler errors until today.
We are now in the process of reverting to VS2013 compiler, and reverting any C#6 code.
We have noticed at least 2 different occurrences of this problem, just in our code base... Any switch statement on a Nullable<long> will fail. The fix is apparently coming in VS2015 Update 2.