The modern codebase has grown from a legacy one. The legacy one with parts of the codebase that were C, then got partially turned into object oriented C++, then

The modern codebase has grown from a legacy one. The legacy one with parts of the codebase that were C, then got partially turned into object oriented C++, then partially turned into template abstractions. The parts least likely to have comprehensive test coverage. That place is indeed where a compiler upgrade is most likely to change the behaviour of your application.