#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
template <typename E>
std::false_type has_a_impl(...);
 
template <typename E> auto has_a_impl(int) -> decltype(E::a, std::true_type{});
 
template <typename E>
using has_a = decltype(has_a_impl<E>(0));
 
 
template <typename E>
std::enable_if_t<has_a<E>::value, int>
get_a_int_val() { return static_cast<int>(E::a); }
 
template <typename E>
std::enable_if_t<!has_a<E>::value, int>
get_a_int_val() { return 42; }
 
enum class E
{
    a = 0
};
 
int main()
{
    std::cout << get_a_int_val<E>() << std::endl;
    std::cout << get_a_int_val<int>() << std::endl;
}
 

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