How to implement is_enum_class type trait? [duplicate]

Answer 1

Based on your +T{} test:

Option #1:

Expression SFINAE in trailing return type:

#include 

template 
auto test(int) -> decltype((void)+T{}, std::false_type{});

template 
auto test(...) -> std::true_type;

template 
using is_enum_class = std::integral_constant(0))::value && std::is_enum::value>;

DEMO

Option #2:

In void_t-fashion:

template 
struct test : std::false_type {};

template 
struct test : std::true_type {};

template 
using is_enum_class = std::integral_constant::value && std::is_enum::value>;

DEMO 2

Tests:

enum class EC { a, b };
enum E { c, d };

int main()
{
    static_assert(is_enum_class::value, "!");
    static_assert(!is_enum_class::value, "!");
    static_assert(!is_enum_class::value, "!");
}