What are different ways of specifying a C++ template types in a structure declaration?

Question

So I was learning template meta programming and came across a declaration like this.

template<class> struct S;
template<class R, class...A> struct S<R(A...)>;

So I was wondering what are the different ways to specify the typenames in angular brackets after identifier. So I tried different combinations of declaring a structure. I tried googling but that didn't solve this issue as I didn't know what to google exactly. Or if there's a resource(blog, SO Answer) for this, please point me to it.

template<class> struct S;
// template<class A> struct S<A>; // 1. does not work
// template<class A, class B> struct S<A, B>; // 2. does not work
template<class A, class B> struct S<A(B)>;
template<class A, class ... B> struct S<A(B...)>;
template<class A, class ... B, class C> struct S<A(B..., C)>;
template<class A, class B, class ... C> struct S<A(B, C...)>;
template<class A, class ... B, class ... C> struct S<A(B..., C...)>;
template<class A, class ... B, class ... C, class D> struct S<A(B..., D, C...)>;

// template<class A, class ... B> struct S<B..., A>; // 3. does not work
// template<class A, class ... B> struct S<A, B...>; // 4. does not work
template<class A, class B, class C, class D> struct S<A(B(C(D)))>;
template<class A, class B, class C, class D> struct S<A(B(C, D))>;
template<class A, class B, class C, class D> struct S<A(B, C(D))>;
template<class A, class B, class C, class D> struct S<A(B(D), C)>;
template<class A, class B, class C, class D> struct S<A(B, C, D)>;

int main() {
    return 0;
}

Answer 1

Good sources to learn C++ are learncpp.com which is a tutorial collection and of course cppreference.com, the full language and standard library reference.

template<class> struct S;
template<class R, class...A> struct S<R(A...)>;

The first line is a template class declaration. The second line is a specialization of this template, in case the type S is a function that receives any number of arguments of any type, and returns any type R.

#include <iostream>

// standard implementation
template<class>
struct S {
    static constexpr auto value = 1;
};

// full specialization
template <>
struct S<bool> {
    static constexpr auto value = 2;
};

// partitial specialization for all functions
template<class R, class ... A>
struct S<R(A...)> {
    static constexpr auto value = 3;
};

int main(){
    std::cout << S<int>::value << " S<int>::value\n";
    std::cout << S<float>::value << " S<float>::value\n";
    std::cout << S<bool>::value << " S<bool>::value\n";
    std::cout << S<void()>::value << " S<void()>::value\n";
    std::cout << S<int()>::value << " S<int()>::value\n";
    std::cout << S<int(float, char, bool)>::value << " S<int(float, char, bool)>::value\n";
}

does print

1 S<int>::value
1 S<float>::value
2 S<bool>::value
3 S<void()>::value
3 S<int()>::value
3 S<int(float, char, bool)>::value