StefanKssmr StefanKssmr - 3 years ago 129
C++ Question

Ambiguous class template instantiation with nontype parameter packs

I was trying 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 got 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 error bothers me. I have tried to figure out why this is an ambiguous instantiation.

Shouldn't
specialization #2
be chosen by the compiler?

If I avoid wrapping the non-type parameter pack
int...i
in
std::integral_constant
it compiles without any problem and the second specialization is chosen. 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 Source

That can't be right. This is a gcc bug (I can't find a bug report to it, maybe it hasn't been reported yet?).

You are right, specialization #2 has to be chosen. Because there are 2 specializations that match, partial ordering selects the most specialized one, which in your case is #2 (double as first parameter is more specialized than any type as first parameter).

Additionally, clang compiles your code without any problems.

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