是否允许在类模板的函数签名模板参数中使用未扩展的参数包？ （可能的VS2013错误）

Question

Is an unexpanded parameter pack allowed in a function signature that is a template parameter to another template?

I expect that it is, but I get different result from different compilers. The following minimal example works with Apple LLVM version 6.0 and gcc 4.8.1 but not VS2013.

Example in Ideone: http://ideone.com/xuyhRb

From my searching, this may be a bug in VS2013, possibly related to: Parameter pack expansion fails However, the solutions given there do not work, the problem seems to be specific to having the unexpanded parameter pack in the function signature. If we replace this in VS2013 with another template to create the function type, it compiles.

#include <functional>
#include <string>
#include <tuple>
#include <iostream>

using namespace std;

template<typename ...Args>
struct TypeList {};

typedef TypeList<int, float, string> Types;

template <typename ...Args>
struct MakeTupleOfTypes;

template <typename ...Args>
struct MakeTupleOfTypes<TypeList<Args...>>
{
    typedef tuple<Args...> type;
};

template <typename ...Args>
struct MakeTupleOfFunction;

template <typename ...Args>
struct MakeTupleOfFunction<TypeList<Args...>>
{
    typedef tuple<function<void(Args)>...> type; // <-- VS2013 error here
};


typedef TypeList<int, float, string> Types;
typedef MakeTupleOfTypes<Types>::type TupleOfTypes;
typedef MakeTupleOfFunction<Types>::type TupleOfFunctions;

void f1(int x) { cout << x << endl; }
void f2(float x) { cout << x << endl; }
void f3(string x) { cout << x << endl; }

int main()
{
    TupleOfTypes values;
    get<0>(values) = 42;
    get<1>(values) = 3.14159;
    get<2>(values) = "Hello, World!";

    TupleOfFunctions functions;
    get<0>(functions) = &f1;
    get<1>(functions) = &f2;
    get<2>(functions) = &f3;

    get<0>(functions)(get<0>(values));
    get<1>(functions)(get<1>(values));
    get<2>(functions)(get<2>(values));

    return 0;
}

Visual Studio fails with:

error C3520: 'Args': parameter pack must be expanded in this context.

at the line

Fix for VS2013: use this template to avoid having an unexpanded parameter pack in the function signature.

template <typename Arg>
struct MakeFunction
{
    typedef function<void(Arg)> type;
};

and

template <typename ...Args>
struct MakeTupleOfFunction<TypeList<Args...>>
{
    typedef tuple<typename MakeFunction<Args>::type...> type;
};

Answer 1

Looks very much like a VS bug to me, since that's a perfectly valid pattern expansion to use. VS's variadics implementation is extremely flaky, in my experience.

That said, this primary template:

template <typename ...Args>
struct MakeTupleOfTypes;

should really not be varadic, since the template only accepts one TypeList :

template <typename ArgsList>
struct MakeTupleOfTypes;