参数包被推导忽略

Question

Why can't my gcc-5.4.0 not deduce parameter packs if they do not appear in the end of the argument list of a function? While the call to works is deduced in a correct way way to works<int,int,int> , the call to fails is not deduced but instead only an empty parameter pack is assumed. Leading to an error message about too many provided arguments for the function.

#include <iostream>

template <typename...args_t>
void works (int first, args_t...args) {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

template <typename...args_t, typename last_t, typename=void>
void fails (args_t...args, last_t last) {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

int main () {
    works (0, 1, 2, 3);
    fails (0, 1, 2, 3);
    return 0;
}

EDIT: as the answers explained, it is not allowed to have typenames after the parameter packs. But according to cppreference.com , it should be valid to have other template parameters after it, if they may be deduced. Apparently the given example does not compile with my gcc . Instead it stays with the same error about too many given arguments.

#include <iostream>
template <typename...args_t, typename U, typename=void>
static int valid (args_t...args, U u) {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    return u;
}

int main () {
    return valid(0, 0.0, -1, 3u);
}

Answer 1

Why can't my gcc-5.4.0 not deduce parameter packs if they do not appear in the end of the argument list of a function?

Because you cannot specify any types beyond the variadic parameter pack like that

 void fails (args_...args, int last)
                      // ^^^^^^^^^^

It's merely the same problem as with default parameter values, or plain ellipsis ( ... ) that those need to be open towards the end of the parameter list (or say they're required to be the last element):

 void fails(int x = 0, int last);
 void fails(int x, ..., int last);