Created attachment 243081 [details]
test case foo.c

It is clear from ISO C 23 (description of mbrtowc: § 7.31.6.3.2, description of mbrtoc32: § 7.30.1.5, description of mbrtoc16: § 7.30.1.3) that the notion of valid multibyte character is independent of which of these function a program uses. When a multibyte character is valid according to one of these functions, it should be valid according to the two others as well.

This is not the case in FreeBSD 13.2.

Test case:
=============================== foo.c ============================
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wchar.h>
#include <uchar.h>
int main ()
{
  if (setlocale (LC_ALL, "zh_CN.GB18030") != NULL)
    {
      mbstate_t state;
      wchar_t wc = (wchar_t) 0xBADFACE;
      memset (&state, '\0', sizeof (mbstate_t));
      if (mbrtowc (&wc, "\224\071\375\067", 4, &state) == 4)
        {
          printf ("mbrtowc return value = 4\n");
          {
            char32_t c32 = (char32_t) 0xBADFACE;
            memset (&state, '\0', sizeof (mbstate_t));
            size_t ret = mbrtoc32 (&c32, "\224\071\375\067", 4, &state);
            printf ("mbrtoc32 return value = %d\n", (int) ret);
          }
          {
            char16_t c16 = (char16_t) 0xBADFACE;
            memset (&state, '\0', sizeof (mbstate_t));
            size_t ret = mbrtoc16 (&c16, "\224\071\375\067", 4, &state);
            printf ("mbrtoc16 return value = %d\n", (int) ret);
          }
        }
    }
}
==========================================================================
$ cc -Wall foo.c
$ ./a.out

Expected result (e.g. as seen on glibc 2.35):
mbrtowc return value = 4
mbrtoc32 return value = 4
mbrtoc16 return value = 4

Actual result:
mbrtowc return value = 4
mbrtoc32 return value = -2
mbrtoc16 return value = -2

I think I've seen this effect also with other encodings than GB18030. But the test case above is with GB18030.