C++17 split constexpr string on comma and have the number of elements at compile time?

I would like to convert the char array to string, then split on "," and have the number of elements available at compile time.

If for "string" do you mean "std::string", it isn't constexpr so it's incompatible with a compile time computation.

If for "string" you accept the C-style string, a char const *, and if you're interested in separators of a single char, you can try something as follows

#include <iostream>

constexpr static char arrayStr[] = "a,b,c";

constexpr std::size_t SPLIT (char const * str, char sep)
   std::size_t  ret { 1u };

   while ( *str )
      if ( sep == *str++ )
         ++ ret;

   return ret;

int main ()
   constexpr auto numFields = SPLIT(arrayStr, ',');

   std::cout << numFields << std::endl;  // print 3