As others have said, you can’t have an implicit conversion, and that’s by-design.
If you want you can avoid the need to specify the underlying type in the cast.
template <typename E>
constexpr typename std::underlying_type<E>::type to_underlying(E e) noexcept {
return static_cast<typename std::underlying_type<E>::type>(e);
}
std::cout << foo(to_underlying(b::B2)) << std::endl;