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?