#include <type_traits>
#include <cassert>
#include <string>

#include <iostream>

template <typename T1, typename T2>
struct common_type
{
  using type = std::remove_reference_t<decltype(true ? std::declval<T1>() : std::declval<T2>())>;
};

template <typename T>
struct common_type<T, void>
{
  using type = void;
};

template <typename T>
struct common_type<void, T>
{
  using type = void;
};

template <>
struct common_type<void, void>
{
  using type = void;
};

int main()
{
  // is_same_v is true if both types in the template-argument list are the same
  // and false otherwise.
  assert((std::is_same_v<
          common_type<int, int>::type,
          int>));
  
  assert((std::is_same_v<
          common_type<int const, int>::type,
          int const>));
  
  assert((std::is_same_v<
          common_type<int, double>::type,
          double>));
  
  assert((std::is_same_v<
          common_type<double, int>::type,
          double>));
  
  assert((std::is_same_v<
          common_type<std::string, char const*>::type,
          std::string>));

  assert((std::is_same_v<
          common_type<void, int>::type,
          void>));

  assert((std::is_same_v<
          common_type<double, void>::type,
          void>));

  assert((std::is_same_v<
          common_type<void, void>::type,
          void>));
}