I was wondering if anybody was going to point that out. It did occur to me that the CHECK was not technically valid due to that exact concern, but given that we control the compiler and the C++ library implementation and given that it's just debugging code (albeit debugging code that we ship to users) I'm fine with it.
In other words, I guess you shouldn't oughta do that generally, but I was fine with it being used there, and it did its job.