Ambiguous creation of a template template with the nontype parameter package

Question

I tried to specialize Expr:

#include <tuple>
#include <type_traits>
#include <iostream>

template<class Tp, class List> 
struct Expr {
    Expr(){std::cout << "0"<< std::endl;};
};

//specialization #1
template<class Tp, int...i>
struct Expr<Tp,std::tuple<std::integral_constant<int,i>...>> {

    Expr(){std::cout << "1"<< std::endl;};
};

//specialization #2
template<int...i>
struct Expr<double,std::tuple<std::integral_constant<int,i>...>> {

    Expr(){std::cout << "2"<< std::endl;};
};

int main() {

    typedef std::tuple<std::integral_constant<int,1>> mylist;

    Expr<double,mylist> test{};

    return 0;
}

However, I received the following compiler errors:

[x86-64 gcc 6.3] error: ambiguous template instantiation for 'struct Expr<double, std::tuple<std::integral_constant<int, 1> > >'
[x86-64 gcc 6.3] error: variable 'Expr<double, std::tuple<std::integral_constant<int, 1> > > test' has initializer but incomplete type

Here, especially the first mistake bothers me. I tried to understand why this is an ambiguous creation.

Shouldn't I choose a specialization #2compiler?

If I manage without a package of packages int...iin std::integral_constantthat does not execute, it compiles without any problems and the second specialization is selected. The following example works:

#include <tuple>
#include <type_traits>
#include <iostream>

template<class Tp, class List> 
struct Expr {
    Expr(){std::cout << "0"<< std::endl;};
};

//specialization #1
template<class Tp, class...args>
struct Expr<Tp,std::tuple<args...>> {

    Expr(){std::cout << "1"<< std::endl;};
};

//specialization #2
template<class...args>
struct Expr<double,std::tuple<args...>> {

    Expr(){std::cout << "2"<< std::endl;};
};

int main() {

    typedef std::tuple<std::integral_constant<int,1>> mylist;

    Expr<double,mylist> test{};

    return 0;
}

Answer 1

This is not true. This is a gcc error (I cannot find the error report, maybe it has not been sent yet?).

, №2 . 2 , , , # 2 (double, , ).

, clang - .