Variadic template deduction in variadic template template

Question

I am not sure if the title makes much sense, but the example is actually quite staightforward:

// A converter struct with a generic constructor.
template <template <typename ...> class TT>
struct converter
{
        template <typename ... Args>    
        converter(const TT<Args ...> &);
};

// A few class templates.
template <typename T>
struct foo {};

template <typename T, typename U>
struct foo2 {};

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

template <typename Arg0, typename ... Args>
struct foo_variadic2 {};

int main()
{
        // All these compile.
        converter<foo>(foo<int>{});
        converter<foo2>(foo2<int,double>{});
        converter<foo_variadic>(foo_variadic<>{});   
        converter<foo_variadic>(foo_variadic<int>{});
        converter<foo_variadic>(foo_variadic<int,double>{});
        // This will not compile.
        // converter<foo_variadic2>(foo_variadic2<int>{});                     
}

I have tried with GCC 4.8.1 and clang 3.3, the error messages vary a bit but they all point to some problem deducing Args around line 5 (and subsequent exclusion of the converter constructor from the candidate list).

How is foo_variadic2 special or different with respect to the other foos?

(For the record, I am trying to implement an is_instance_of type trait to detect instances of template classes)

UPDATE

Now both GCC 4.8.3 and 4.9.1 on my setup accept this. clang 3.4.2 still barking.

Answer 1

This is not the solution, but might help you or others to figure out what the problem is. The following compiles:

template <template <typename ...> class TT>
struct converter2
{
    template <typename Arg0, typename ... Args>    
    converter2(const TT<Arg0, Args ...> &);
};

// ...

converter2<foo_variadic2>(foo_variadic2<int>{});

I have to admit I don't understand why this is necessary and why your code doesn't work.