Ah, Victor, that helps.  I was having trouble reproducing the problem on a different system.  I was suspecting a small difference in compiler version, but I hadn't considered it being because I started fresh.

From what I can see, a particular build tree can be successful if it remains on either side of the instruction commit, but you can't cross the boundary - in either direction.  There's clearly some build artifact not being reset properly that gets out of sync.  I've been able to create odd name errors even in older commits as long as the first one I build is at or after the instruction commit.

But a pristine checkout on the buildbot of the latest master works, as does a git clean on the tree. 

I always use the buildbot scripts for building and they invoke a full clean first.  So either the clean process on Windows is missing something, or there's an artifact that is supposed to be kept in sync in the source tree itself that isn't.  I'm not familiar enough with the internals to guess at which yet.  I suppose given that it's not a problem on other buildbots argues for the clean issue, although I suppose it could also be something that is only kept in the tree to benefit a subset of systems, like Windows.

(While resetting the checkouts on the buildbot should therefore fix the current exceptions, I'm going to leave that alone for the moment, since that leaves it positioned to confirm any subsequent fix)