Can I partially specialize a template with a pattern like foo<T..., int, U...>?

Question

If this is possible, one can index into a variadic template parameter pack without recursion. However, GCC is refusing to pick up my partial specialization here:

template <int I, typename List>
struct element_impl;

template <typename... TL, int... IL, typename T, int I, typename... TR, int... IR>
struct element_impl<I, typelist<pair<TL,IL>..., pair<T,I>, pair<TR,IR>...>> {
    typedef T type;
};

prog.cpp: In instantiation of 'element<0, typelist<int, double, char, float, long int> >':
prog.cpp:52:34: instantiated from here
prog.cpp:47:79: error: invalid use of incomplete type 'struct element_impl<0, typelist<pair<int, 0>, pair<double, 1>, pair<char, 2>, pair<float, 3>, pair<long int, 4> >'

Is GCC buggy, or am I ignoring some limitation of variadic templates?

Answer 1

The spec says at 14.8.2.5p9

If P has a form that contains <T> or <i>, then each argument Pi of the respective template argument list P is compared with the corresponding argument Ai of the corresponding template argument list of A. If the template argument list of P contains a pack expansion that is not the last template argument, the entire template argument list is a non-deduced context.

Your typelist<T> unfortunately matches that pattern.

Answer 2

AFAICT, the rules for matching partial specializations are the same as the rules type deduction of function parameters. And §14.8.2.1/1 says the following:

For a function parameter pack that occurs at the end of the parameter-declaration-list, the type A of each remaining argument of the call is compared with the type P of the declarator-id of the function parameter pack. Each comparison deduces template arguments for subsequent positions in the template parameter packs expanded by the function parameter pack. For a function parameter pack that does not occur at the end of the parameter-declaration-list, the type of the parameter pack is a non-deduced context.

So the packs TL and IL cannot be deduced in this case, and the partial specialization is not picked.