I wasn't giving C++ as an example of a language that maintains backwards compatibility through breaking changes using source upgrades, I was actually thinking o

I wasn't giving C++ as an example of a language that maintains backwards compatibility through breaking changes using source upgrades, I was actually thinking of Go when I said that. Obviously C++ does have a preprocessor, which means upgrades cannot be done automatically in general because e.g. how do you know which macros in an #ifdef are "supposed" to be defined and which are "supposed" to be undefined etc? Instead they have to maintain backwards compatibility in the spec.