Q:

Is it legit to specialize variadic template class inside other template class

Consider a code:
#include <iostream>

template <class T>
struct outer {
   template <class... Args>
   struct inner {
      static constexpr bool value = false;
   };

   template <class... Other>
   struct inner<T, Other...> {
      static constexpr bool value = true;
   };
};

int main() {
   std::cout << outer<int>::inner<int, void>::value << std::endl;
};

It does compile in both g++ and clang++ but I am not convinced it is legal. As far as I know one cannot for example specialize template method for template class if not explicit specializing the class itself. How come rules are different for inner classes?

A:

A partial specialization of a nested template class is OK:
template <class T>
struct outer {
    // template
   template <class... Args>
   struct inner {};

    // partial
   template <class... Other>
   struct inner<T, Other...> {};

    // error: explicit specialization in non-namespace scope ‘struct inner’
    // template <>
    // struct inner<char, int> {};
};

An explicit (full) specialization of the inner class only is not.
Explicit specializations of the outer class (without specialization of the inner class (which is actually a different class)) and explicit specializations of the outer and inner class are possible:
#include <iostream>
template <class T>
struct outer {
   template <class... Args>
   struct inner
   {
       static void print() { std::cout << "outer<T>::inner<Args...>\n"; }
   };
};

template <> // outer specialization
struct outer<int>
{
   template <class... Args>
   struct inner
   {
       static void print() { std::cout << "outer<int>::inner<Args...>\n"; }
   };
};

template <> // outer specialization
template <> // inner specialization
struct outer<int>::inner<int> // must be outside of the outer class
{
    static void print() { std::cout << "outer<int>::inner<int>\n"; }
};

int main() {
    outer<char>::inner<char>::print();
    outer<int>::inner<char>::print();
    outer<int>::inner<int>::print();
}

Note: The same applies to non variadic nested template classes.

