c++ - Resolution of function overloading about initializer_list

#include 
using namespace std;

class A
{
public:
    explicit A(const initializer_list & a) {}
};
void func(const vector& a)
{

}
void func(A a)
{

}
int main(void)
{
    func({ 1,2,3 });
}

This code fails to compile:

(19): error C2668: 'func': ambiguous call to overloaded function

(13): note: could be 'void func(A)'

(9): note: or 'void func(const std::vector> &)'
with[_Ty=int]
(19): note: while trying to match the argument list '(initializer list)'

Note that I specified 'explicit' on A's constructor.

In my view, func(A a) should not be considered as a candidate of {1,2,3}. And actually, it is not. If I remove func(const vector& a), then the code still fails, instead of succeeding by removing ambiguity.

In summary, in this code, the func(const vector& a) is the only callable function for {1,2,3}, so there is no ambiguity.

My question is..

1. How does C++ overloading resolution procedures come to conclusion of 'ambiguous'?




2. Why doesn't C++ just simply choose callable one?