C++ variadic templates deduction logic

Question

I have simple code:

#include <iostream>

template<class... Types>
class A;

template<>
class A<> {
public:
    explicit A() {
        std::cout << "empty" << std::endl;
    }
};

template<class Head, class... Tail>
class A<Head, Tail...> {
public:
    explicit A(Head head, Tail ...tail) : tail_(tail...) {
        std::cout << head << std::endl;
    }

private:
    A<Tail...> tail_;
};


int main() {
    auto a = A(1, 2);
    return 0;
}

It doesn't compile:

test2.cpp:27:20: error: class template argument deduction failed:
     auto a = A(1, 2);
                    ^
test2.cpp:27: confused by earlier errors, bailing out

It could be fixed by adding deduction guide like:

template<class... T>
A(T...) -> A<T...>;

My question is why compiler can't resolve such simple case?

Answer 1

Implicit deduction guides are only generated from the constructors of the primary class template. (C++17 [over.match.class.deduct]/1.1) You have not defined the primary class template.

Therefore, the class template partial specialization template<class Head, class... Tail> class A<Head, Tail...> ... is ignored when performing class template argument deduction. Writing an explicit deduction guide is a possible solution to this problem, as you have observed.