Sum types can actually be implemented quite effectively using X-macros in both C and C++. In fact, I feel like they're simpler and more intuitive than this variant stuff.

Edit: Let's use the same example.

  #define SETTINGS \
    X(string, str) \
    X(int, num) \
    X(bool, b)

  struct Setting {
    union {
      #define X(type, name) type name;
      SETTINGS
      #undef X
    };
    enum Type {
      #define X(type, ...) t_ ## type,
      SETTINGS
      #undef X
    };
    Type tag;
  };
  

Printing settings like in the example becomes this:

  void printSettings(const Setting& s) {
    switch(s.tag) {
    case t_string: printf("A string: %s\n", s.str.c_str()); break;
    case t_int: printf("An integer: %d\n", s.num); break;
    case t_bool: printf("A boolean: %d\n", s.b); break;
    }
  }

We can also load more things into the x-macro, so it's possible to define the switch cases above just like in the structure definition. We could add a third parameter called full_name:

  void printSettings(const Setting& s) {
    switch(s.tag) {
    #define X(type, name, full_name) \
      case t_ ## type: std::cout << "A " full_name ":" << s.name << "\n";
    #undef X
    }
  }

Disclaimer: I have not compiled or run any of this code.