What's the syntax to partially specialize a parmeter pack argument for void type?

Question

I can't find a way to get this to work. Is it even possible? I don't see why it wouldn't be.

template <auto id, typename FirstField, typename... OtherFields>
struct FieldTypeById {
    using Type = int;
};

template <auto id>
struct FieldTypeById<id, void> {
    using Type = void;
};


int main()
{
   using t1 = FieldTypeById<0, int>::Type;
   using t2 = FieldTypeById<1>::Type;

    return 0;
}

https://godbolt.org/z/AggnDq

Answer 1

The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:

template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
    using Type = int;
};

Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.

See it live