How to check if an object is an instance of template class of multiple template arguments and that one of said arguments fulfills some condition?

Question

From How to check if an object is an instance of a template class of multiple template arguments in C++? I got the following trait to check whether a type is a particular template instantiation of a templated type of several template arguments:

template <typename T1, typename T2>
struct A
{
};

template <typename Type>
struct IsA: std::false_type
{
};

template <typename T1, typename T2>
struct IsA<A<T1, T2>> : std::true_type
{
};

How would one additionally add the condition that the second type T2 fulfills some other condition? I tried doing

template <typename Type>
struct IsA: std::false_type
{
};

template <typename T1, typename T2, std::enable_if_t<SomeCondition<T2>::value>>
struct IsA<A<T1, T2>> : std::true_type
{
};

but am getting the error

error: template parameters not deducible in partial specialization:

Answer 1

You were on the right track:

#include <type_traits>
#include <iostream>

template <typename T1, typename T2>
struct A
{
};

template <typename Type, typename=void>
struct IsA: std::false_type
{
};

template <typename T1, typename T2>
struct IsA<A<T1, T2>, std::enable_if_t<std::is_same_v<T2, int>>>
    : std::true_type
{
};

int main()
{
    std::cout << IsA<int>::value << "\n";

    std::cout << IsA<A<char, char>>::value << "\n";

    std::cout << IsA<A<char, int>>::value << "\n";
    return 0;
}

In this trivial example the "some condition" is just a std::is_same_v<T2, int>, only for the sake of an example.