Regular variadic arguments in general aren't used very often in C++ with exception of printf like functions. Not rare enough for majority of C++ programmers to not know about them, but definitely much more rare than their use in python. Main reason people know about it at all is printf. The "new" C compatible form has been supported since the first ISO standardized version of c++ if not longer. There haven't been a good reason to use the "old" form for a very long time. Which means that the amount of C++ code using deprecated form is very low.

Being deprecated means that most compilers and linters will likely add a warning/code fix suggestion. So any maintained project which was accidentally using C incompatible form will quickly fix it. No good reason not to.

As for the projects which for some reason are targeting ancient pre ISO standard c++ version they wouldn't have upgraded to newer standard anyway. So if new standard removed old form completely it wouldn't have helped with those projects.

So no you don't need to know the old form to read C++ code. And in the very unlikely case you encounter it, the way for accessing variadic arguments is the same for both forms through special va_list/va_arg calls. So if you only know the "new" form you should have a pretty good idea of whats going on there. You might lookup in references what's the deal with missing coma, but other than that it shouldn't be a major problem for reading code. This is hardly going to be the biggest obstacle when dealing with code bases that old.

Unless you have a massive legacy code base that is never updated, C++ has become much simpler over time. At a lot companies we made a point of slowly re-factoring old code to a more recent C++ standard (often a couple versions behind the bleeding edge) and it always made the code base smaller, safer, and more maintainable. It wasn’t much work to do this either.

To some extent with C++, complexity is a choice.

Not in the x86-64 SysV ABI they aren’t. The arguments will be passed in registers (yes, even the variadic ones), so how your compiler will interpret 1[&a] is anybody’s guess. (For me, x86_64-unknown-linux-gnu-g++ -O2 yields, essentially, return a+a+a+a; which is certainly an interpretation. I’m also getting strange results from i686-unknown-linux-gnu-g++ -O2, but my x87 assembly is rusty enough that I don’t really get what’s going on there.)

    double solve(double a,double b,double c,double d){return a+b+c+d;}
    double solve(double a...){return a+1[&a]+2[&a]+3[&a];}
    double solve(a,b,c,d)double a,c,b,d;{return a+b+c+d;}

Err no; https://gcc.godbolt.org/z/sW3ea58oc

They're equivalent on GCC.

    double solve(double a[]) {
      return 0[a] + 1[a] + 2[a] + 3[a];
    }

    solve((double[]){1, 2, 3, 4});

I'm also not totally convinced that someone not using a language regularly means their view on it being too complicated must be invalid; I would fully expect someone who views it as too complicated to try to avoid using it, and there are enough people doing that, it might be a cause for concern. That doesn't necessarily mean they're right, but without further context it also doesn't mean their opinion isn't relevant.

I have no idea where you get this idea from. I expect gcc v28 to be able to compile C++ from 2008, and I'm not alone in that.

I love C, but C++ has worthwhile advantages even if you heavily restrict which features you use.

The difference being that P1219R2 was actually a revised proposal from 2019 not 2021.

I've been using C++ for more than 30 years (I added thread_local to Cfront back in the early 90s), and while the language has grown dramatically in that time, there is fundamentally nothing that would prevent me from writing "C with classes" using the modern version.

I don't do that because I also like RAII, and polymorphism, and operator overloading and ...

I've never used .NET and could not imagine any scenario under which I would. The libraries that matter to me are mostly written in C or C++ and there are more of them than I'd ever need, mostly.