unwrap.cpp

#include <vector>

template <typename T>
struct unwrap {
  using type = T;
};

template <template <typename> typename wrapper, typename T>
struct unwrap<wrapper<T>> {
  using type = T;
};

template <template <typename, typename> typename wrapper, typename T, typename R>
struct unwrap<wrapper<T, R>> {
  using type = T;
};

int main() {
  typename unwrap<std::vector<int>>::type a = 0;
}