template<bool Condition, typename T = int>
struct EnableIf_c {};

template<typename T>
struct EnableIf_c<true, T>
{
    typedef T type;
};

template<int N, typename = void>
struct IsEven
{
    static const bool value = false;
};

template<int N>
struct IsEven<N, typename EnableIf_c<(N % 2) == 0>::type>
{
    static const bool value = true;
};

int main()
{
    static_assert(!IsEven<3>::value);   // works
    static_assert(IsEven<2>::value);  // fails as long as T = int in first line. When T = void, then it works
}