Following How to make ICU stop early when meeting 65533?, I would like to use a self-defined callback to stop early when it meets PUA characters.

For example, 0xA8 0xC1 is not defined in GB2312, and the conversion by ICU will be 57465 (U+E079), which belongs to private user area.

The code below is generated by ChatGPT, but it fails to work:

bool isPrivateUse(UChar32 ch) {
    return (ch >= 0xE000 && ch <= 0xF8FF) ||
           (ch >= 0xF0000 && ch <= 0xFFFFD) ||
           (ch >= 0x100000 && ch <= 0x10FFFD);
}

void U_CALLCONV stopOnPUACallBack(const void *context, UConverterToUnicodeArgs *toUArgs,
                                  const char *codeUnits, int32_t length, UConverterCallbackReason reason,
                                  UErrorCode *pErrorCode) {
    if (reason <= UCNV_IRREGULAR) {
        UChar32 ch;
        int32_t i = 0;
        U8_NEXT(codeUnits, i, length, ch);

        if (isPrivateUse(ch)) {
            *pErrorCode = U_ILLEGAL_CHAR_FOUND;
            return;
        }
    }

    UCNV_TO_U_CALLBACK_STOP(context, toUArgs, codeUnits, length, reason, pErrorCode);
}

ucnv_setToUCallBack(conv, stopOnPUACallBack, nullptr, nullptr, nullptr, &status);

I tried to debug the code, and I found that its reason is UCNV_CLOSE.