Considere el siguiente código:

#include <iostream>
#include <type_traits>

int main(int argc, char* argv[])
{
    std::cout<<"std::is_same<int, int>::value = "<<std::is_same<int, int>::value<<std::endl;
    std::cout<<"std::is_same<int, signed int>::value = "<<std::is_same<int, signed int>::value<<std::endl;
    std::cout<<"std::is_same<int, unsigned int>::value = "<<std::is_same<int, unsigned int>::value<<std::endl;
    std::cout<<"std::is_same<signed int, int>::value = "<<std::is_same<signed int, int>::value<<std::endl;
    std::cout<<"std::is_same<signed int, signed int>::value = "<<std::is_same<signed int, signed int>::value<<std::endl;
    std::cout<<"std::is_same<signed int, unsigned int>::value = "<<std::is_same<signed int, unsigned int>::value<<std::endl;
    std::cout<<"std::is_same<unsigned int, int>::value = "<<std::is_same<unsigned int, int>::value<<std::endl;
    std::cout<<"std::is_same<unsigned int, signed int>::value = "<<std::is_same<unsigned int, signed int>::value<<std::endl;
    std::cout<<"std::is_same<unsigned int, unsigned int>::value = "<<std::is_same<unsigned int, unsigned int>::value<<std::endl;
    std::cout<<"----"<<std::endl;
    std::cout<<"std::is_same<char, char>::value = "<<std::is_same<char, char>::value<<std::endl;
    std::cout<<"std::is_same<char, signed char>::value = "<<std::is_same<char, signed char>::value<<std::endl;
    std::cout<<"std::is_same<char, unsigned char>::value = "<<std::is_same<char, unsigned char>::value<<std::endl;
    std::cout<<"std::is_same<signed char, char>::value = "<<std::is_same<signed char, char>::value<<std::endl;
    std::cout<<"std::is_same<signed char, signed char>::value = "<<std::is_same<signed char, signed char>::value<<std::endl;
    std::cout<<"std::is_same<signed char, unsigned char>::value = "<<std::is_same<signed char, unsigned char>::value<<std::endl;
    std::cout<<"std::is_same<unsigned char, char>::value = "<<std::is_same<unsigned char, char>::value<<std::endl;
    std::cout<<"std::is_same<unsigned char, signed char>::value = "<<std::is_same<unsigned char, signed char>::value<<std::endl;
    std::cout<<"std::is_same<unsigned char, unsigned char>::value = "<<std::is_same<unsigned char, unsigned char>::value<<std::endl;
    return 0;
}

El resultado es :

std::is_same<int, int>::value = 1
std::is_same<int, signed int>::value = 1
std::is_same<int, unsigned int>::value = 0
std::is_same<signed int, int>::value = 1
std::is_same<signed int, signed int>::value = 1
std::is_same<signed int, unsigned int>::value = 0
std::is_same<unsigned int, int>::value = 0
std::is_same<unsigned int, signed int>::value = 0
std::is_same<unsigned int, unsigned int>::value = 1
----
std::is_same<char, char>::value = 1
std::is_same<char, signed char>::value = 0
std::is_same<char, unsigned char>::value = 0
std::is_same<signed char, char>::value = 0
std::is_same<signed char, signed char>::value = 1
std::is_same<signed char, unsigned char>::value = 0
std::is_same<unsigned char, char>::value = 0
std::is_same<unsigned char, signed char>::value = 0
std::is_same<unsigned char, unsigned char>::value = 1 

Lo que significa queint ysigned int se consideran del mismo tipo, pero nochar ysigned char. Porqué es eso ?

Y si puedo transformar unchar dentrosigned char utilizandomake_signed, como hacer lo contrario (transformar unsigned char a unchar)?