var_bug.cpp

#include <iostream>
#include <type_traits>
#include <typeinfo>

template <typename T, T... I>
struct integer_sequence {
  enum { size = sizeof...(I) };
  typedef T value_type;
};

template <typename... T>
struct type_sequence {
  enum { size = sizeof...(T) };
};

template <typename I, size_t J>
struct to;

template <typename T, T I0, T... I>
struct to<integer_sequence<T, I0, I...>, 1> {
  typedef integer_sequence<T, I0> type;
};

template <typename... S>
struct outer;

template <typename T0, T0 I0, typename T1, T1... I1>
struct outer<integer_sequence<T0, I0>, integer_sequence<T1, I1...>> {
  typedef type_sequence<integer_sequence<typename std::common_type<T0, T1>::type, I0, I1>...> type;
};

template <typename S0, typename S1>
struct outer<S0, S1> {
  typedef typename outer<typename to<S0, 1>::type, S1>::type type;
};

#define PRINT_TYPE(T) std::cout << typeid(std::declval<T>()).name() << std::endl;

int main() {
  // Commenting out these first two lines changes the output of the second two lines.
  typedef typename outer<integer_sequence<int, 0>, integer_sequence<int, 1, 2>>::type example1;
  PRINT_TYPE(example1);
  typedef typename outer<integer_sequence<int, 0, 1>, integer_sequence<int, 2, 3>>::type example2;
  PRINT_TYPE(example2);
}