IssueTracker

The ISO C 23 standard specifies the return values of the function mbrtoc16 (§ 7.30.1.3).

"If the function determines that the next multibyte character is complete and valid, it determines the values of the corresponding wide characters and then, if pc16 is not a null pointer, stores the value of the first (or only) such character in the object pointed to by pc16. Subsequent calls will store successive wide characters without consuming any additional input until all the characters have been stored."

A small positive return value indicates that "the next n or fewer bytes complete a valid multibyte character (which is the value stored); the value returned is the number of bytes that complete the multibyte character."

The return value (size_t)(-3) indicates that "the next character resulting from a previous call has been stored (no bytes from the input have been consumed by this call)".

So, when the input is a Unicode character outside the BMP, the first mbrtoc16() call should return the number of bytes (= 4), whereas the second mbrtoc16() call should return (size_t)(-3).

Test case:
========================= foo.c =========================
#include <locale.h>
#include <stdio.h>
#include <string.h>
#include <wchar.h>
#include <uchar.h>
int main ()
{
  if (setlocale (LC_ALL, "en_US.UTF-8") == NULL)
    return 1;

  char input[] = "\360\237\230\213"; /* U+1F60B */
  mbstate_t state;
  char16_t wc;
  size_t ret;

  memset (&state, '\0', sizeof (mbstate_t));
  wc = (char16_t) 0xBADF;
  ret = mbrtoc16 (&wc, input, 4, &state);
  printf ("First  mbrtoc16 call: wc = 0x%04X, ret = %d\n", wc, (int) ret);
  ret = mbrtoc16 (&wc, input + 4, 0, &state);
  printf ("Second mbrtoc16 call: wc = 0x%04X, ret = %d\n", wc, (int) ret);
  return 0;
}
========================================================

Compile and run this program (e.g. under termux).

Expected output (seen e.g. in musl libc, which is generally very standards compliant, and GNU libc):
First  mbrtoc16 call: wc = 0xD83D, ret = 4
Second mbrtoc16 call: wc = 0xDE0B, ret = -3

Actual output (on Android 11):
First  mbrtoc16 call: wc = 0xD83D, ret = -3
Second mbrtoc16 call: wc = 0xDE0B, ret = 4

Returning (size_t)(-3) in the first call is wrong, because that is not "the next character from a previous call".