I never got this. Can't you just decide to use subset of the language? No-one forces people to use every single feature. It's okay to use C++ like "C with classes" and occasionally cool new thing, when it is right tool for the job. Only people where this argument is truly valid are compiler/tools people.
No, you can't. Not in the long run, at least. You will have to use libraries that have chosen a different subset, new people will join your team, things will get sloppy as the company grows, etc. Committing to a subset of the language is not free, it's hard and it's a lot of work. And for what?
That is harder than it sounds. First you have to select which subset to use, it is almost impossible to get any two engineers to agree on that - if you do get agreement that is a bad sign - generally it means the people in the room don't know C++ well enough to make the decision and so you choose the wrong one. The best you can hope for is a compromise where nobody is happy - and thus everyone will look for an excuse to bring in their pet part of C++ not in the subset because there is a "good reason" for an exception.
Even if you select a subset it is almost impossible to enforce whatever subset because even if you don't allow your people to use it directly odds are you bring in a third party library that does use it (the C++ standard library being the big one!)
There are a few exceptions to the above. No exceptions/no RTTI are commonly disabled exceptions and so you will get some compiler and library support. Game companies commonly write their own standard library. Both of these are done for performance reasons and have specific domain specific reasons that can be validated in a profiler to set their rules.
Not related to reflection, but C++26 is also likely to get profiles which will disable specific old constructs, (new/delete) which are proven to be error prone and thus result in security issues. These are a good subset to use, but it is about security and so mostly doesn't get the types of subsets you are talking about.
Plenty of the modern C++ people already do this by enforcing things like "no raw loops", "no raw pointers", or "no raw synchronization primitives" during code review.
The issue is that it's a lot harder to justify avoiding new features than it is to justify avoiding old features unless you have a tooling specific reason (ex lack of support) to do so.
There is the problem - code reviews are done by humans and so it is really easy to read some code and not think "wait this is new code it can't do that anymore". I read a lot of old code as part of my job and it often isn't worth the bother to update working code to new standards.
> it's a lot harder to justify avoiding new features than it is to justify avoiding old features
The problem is the opposite - people keep using old features when the new is better. I realize not everything new is better, but most of them are, yet people keep refusing to learn and use the new thing without good justification.
I think we are saying the same thing here. Often the new is better so it's hard to really justify sticking to the old outside of project specific reasons (i.e. toolchain doesn't support new std). That people do it has less to do with justification and more to do with time commitment or laziness and the excuses given tend to fall away once pressed.
However that opinion is kind of a minority. There are a lot of people in the community who don't want to have to learn new features just because a dependency happens to use/expose them. I don't personally see the issue with it.
I'd rather learn std features any day over non-std features. It's just a better use of my time because they work everywhere and someday I might need them. However again not everyone shares this opinion.
If you're writing library code that someone else might use, you don't have much need to understand the features you don't use, unless you have to handle them at the interface. If you're debugging your own code, you really shouldn't have to understand features that you didn't use. (Mostly - see the next paragraph.)
You did say "intentionally". You could wind up using a feature unintentionally, but it's not very common, because most of the new features are either in a new library (which you have to explicitly call), or a new syntax. There are definitely exceptions - I could easily see you using a move constructor without meaning to.
Maintaining someone else's code... yeah. You have to understand whatever they used, whether or not it made any sense for them to use.
