meta.cpp

#include<iostream>
#include<type_traits>

struct Wrong1 {};

template<typename T1, typename T2>
struct Wrong2 {
    T1 f;
    T2 s;
};

template<typename T1, typename T2>
struct First {
    T1 f;
    T2 s;
};

template <typename T>
struct IsFirst : std::false_type {};

template <typename T1, typename T2>
struct IsFirst<First<T1, T2>> : std::true_type {};

int main() {
    std::cout << std::boolalpha;
    std::cout << IsFirst<Wrong1>::value << std::endl;
    std::cout << IsFirst<Wrong2<int, int>>::value << std::endl;
    std::cout << IsFirst<First<int, int>>::value << std::endl;
}

false
false
true