#include <type_traits>

template<typename T, typename U = void>
struct is_defined {
	static constexpr const bool value = false;
};

template<typename T>
struct is_defined<T, std::enable_if_t<sizeof(T)>>{
	static constexpr const bool value = true;
};

template<typename T, typename U>
using choose = std::conditional<is_defined<T>::value, T, U>;

int main(){
	choose<char*, void*>::type i;
	i = 9;
}