cpp17-sfinae.cpp

template<class L, class R>
auto fold(L l, R r) {
  using lTag = typename L::tag;
  using rTag = typename R::tag;
  if constexpr (is_base_of<rTag, BarTag>::value) {
    if constexpr (is_same<lTag, FooTag>::value) {
      return foldFB(l, r);
    } else {
      return foldBB(l, r);
    }
  } else {
    return foldFF();
  }
}