R"(x)" literals are neat not just because whitespaces matter, but also because they are tokenized before macro expansion. Thus you can write a C23 detector like this:

   #include<stdio.h>

   #define r(R) R"()"

   int main()
   {
      puts(r()[0] ? "C99"  /* r() evaluates to "()" */
                  : "C23"  /* r() evaluates to "" */);
   }

Output: https://gcc.godbolt.org/z/Wj3s6KEGK

I have used that trick here:

https://www.ioccc.org/years.html#2015_yang

(C23 wasn't a thing back then, but the same trick can be used to differentiate C++11 from C++98).