enum level {
  info,
  warning,
  error,
};

// Somewhere defined by the programmer.
constexpr auto MIN_LEVEL = warning;

template<level lvl>
void log(arg1, arg2, arg3, etc) {
    if (lvl >= MIN_LEVEL) real_log(arg1, arg2, arg3, etc);
}

void real_log(arg1, arg2, arg3, etc) {
  // do some logging
}