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#include <iostream>
#include <type_traits>
#include <functional>
 
using namespace std;
 
 
enum test
{
    TEST = 2
};
 
/*template <typename T, typename std::enable_if<std::is_enum<T>::value>::type* = nullptr>
static std::size_t Hash(T const & t)
{
    std::cout << "Calling Hash enum" << std::endl;
    return std::hash<typename std::underlying_type<T>::type>()(t);
}
 
template <typename T, typename std::enable_if<!std::is_enum<T>::value>::type* = nullptr>
static std::size_t Hash(T const & t)
{
    std::cout << "Calling Hash other type" << std::endl;
    return std::hash<T>()(t);
}*/
 
template <typename T>
static std::size_t Hash(T const & t)
{
    typedef typename std::conditional<std::is_enum<T>::value, std::hash<typename std::underlying_type<T>::type>, std::hash<T>>::type Hasher;
    return Hasher()(t);
}
 
int main() {
    Hash<test>(TEST);
    Hash<int>(5);
    std::cin.get();
    return 0;
}

In file included from /usr/include/c++/5/bits/move.h:57:0,
                 from /usr/include/c++/5/bits/stl_pair.h:59,
                 from /usr/include/c++/5/bits/stl_algobase.h:64,
                 from /usr/include/c++/5/bits/char_traits.h:39,
                 from /usr/include/c++/5/ios:40,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/5/type_traits: In instantiation of 'struct std::underlying_type<int>':
prog.cpp:30:134:   required from 'std::size_t Hash(const T&) [with T = int; std::size_t = unsigned int]'
prog.cpp:36:16:   required from here
/usr/include/c++/5/type_traits:2190:38: error: 'int' is not an enumeration type
       typedef __underlying_type(_Tp) type;
                                      ^

Standard output is empty