#include <type_traits>

enum class a_t { a1, a2, a3, a4 };
enum class b_t { };
enum class c_t { };

// Make a collection of types we want to define an operator| for.
enum typename or_types_t {
  a_t, c_t
};

// Loop over each enumerator in or_types_t.
@meta for enum(auto e : or_types_t) {
  // Define the function that takes two parameters with the associated type
  // and returns a result of the associated type.
  @enum_type(e) operator|(@enum_type(e) lhs, @enum_type(e) rhs) {
    typedef @enum_type(e) type_t;
    static_assert(std::is_enum<type_t>::value);

    typedef std::underlying_type<type_t>::type underlying_t;

    return (type_t)((underlying_t)lhs | (underlying_t)rhs);
  }
}

int main() {
  a_t a = a_t::a2 | a_t::a4;
  return 0;
}