C++ map strongly typed enum to type (C++17)

enum_to_type.cpp

#include <iostream>
#include <string>
#include <vector>
#include <utility>
#include <type_traits>

enum class Foo {
  A1, A2
};

enum class Bar {
  B1, B2
};

template <Foo> struct foo_enum_to_type;
template <Bar> struct bar_enum_to_type;
template <> struct foo_enum_to_type<Foo::A1> {using type = int;};
template <> struct foo_enum_to_type<Foo::A2> {using type = double;};
template <> struct bar_enum_to_type<Bar::B1> {using type = std::string;};
template <> struct bar_enum_to_type<Bar::B2> {using type = std::vector<int>;};

template <auto EnumVal>
constexpr auto enum_to_type_impl() {
  if constexpr (std::is_same_v<decltype(EnumVal), Foo>) {
    return foo_enum_to_type<EnumVal>{};
  } else {
    return bar_enum_to_type<EnumVal>{};
  }
}

template <auto EnumVal>
struct enum_to_type {
  using type = typename decltype(enum_to_type_impl<EnumVal>())::type;
};

int main() {
  enum_to_type<Bar::B2>::type x;
  x.resize(10);
  enum_to_type<Bar::B1>::type s{"Hello"};
  std::cout << "Size of x: " << x.size() << std::endl;
  std::cout << s << std::endl;
  return 0;
}