C++ 将int_least8_t转换为char时如何发出警告

I am building a string library to support both ascii and utf8.
I create two typedef for t_ascii and t_utf8. ascii is safe to be read as utf8, but utf8 is not safe to be read as ascii.
Do I have any way to issue a warning when implicitely casting from t_utf8 to t_ascii, but not when implicitely casting t_ascii to t_utf8 ?

理想情况下，我希望发布以下警告(而且只有这些警告):

#include <stdint.h>

typedef char           t_ascii;
typedef uint_least8_t  t_utf8;

int main()
{
    t_ascii const* asciistr = "Hello world"; // Ok
    t_utf8 const*   utf8str = "你好世界";    // Ok

    asciistr = utf8str; // Warning: utf8 to ascii is not safe
    utf8str = asciistr; // Ok: ascii to utf8 is safe

    t_ascii asciichar = 'A';
    t_utf8   utf8char = 'B';

    asciichar = utf8char; // Warning: utf8 to ascii is not safe
    utf8char = asciichar; // Ok: ascii to utf8 is safe
}

目前，当使用-Wall(甚至-funsigned-char)构建时，我会收到以下警告:

gcc main.c -Wall -Wextra                          
main.c: In function ‘main’:
main.c:10:35: warning: pointer targets in initialization of ‘const t_utf8 *’ {aka ‘const unsigned char *’} from ‘char *’ differ in signedness [-Wpointer-sign]
   10 |         t_utf8 const*   utf8str = "你好世界";    // Ok
      |                                   ^~~~~~~~~~
main.c:12:18: warning: pointer targets in assignment from ‘const t_utf8 *’ {aka ‘const unsigned char *’} to ‘const t_ascii *’ {aka ‘const char *’} differ in signedness [-Wpointer-sign]
   12 |         asciistr = utf8str; // Warning: utf8 to ascii is not safe
      |                  ^
main.c:16:17: warning: pointer targets in assignment from ‘const t_ascii *’ {aka ‘const char *’} to ‘const t_utf8 *’ {aka ‘const unsigned char *’} differ in signedness [-Wpointer-sign]
   16 |         utf8str = asciistr; // Ok: ascii to utf8 is safe
      |                 ^