if (byte < 0xC0) {
	/*
	 * Handles properly formed UTF-8 characters between 0x01 and 0x7F.
	 * Also treats \0 and naked trail bytes 0x80 to 0xBF as valid
	 * characters representing themselves.
	 */

#if TCL_UTF_MAX <= 4
#if TCL_UTF_MAX == 4
	/* If *chPtr contains a high surrogate (produced by a previous
	 * Tcl_UtfToUniChar() call) and the next 3 bytes are UTF-8 continuation
	 * bytes, then we must produce a follow-up low surrogate. We only
	 * do that if the high surrogate matches the bits we encounter.
	 */
	if (((byte & 0xC0) == 0x80)
		&& ((src[1] & 0xC0) == 0x80) && ((src[2] & 0xC0) == 0x80)

    }
    else if (byte < 0xF5) {
	if (((src[1] & 0xC0) == 0x80) && ((src[2] & 0xC0) == 0x80)) {
	    /*
	     * Four-byte-character lead byte followed by at least two trail bytes.
	     * We don't test the validity of 3th trail byte, see [ed29806ba]
	     */
#if TCL_UTF_MAX <= 4
#if TCL_UTF_MAX == 4
	    Tcl_UniChar high = (((byte & 0x07) << 8) | ((src[1] & 0x3F) << 2)
		    | ((src[2] & 0x3F) >> 4)) - 0x40;
	    if (high < 0x400) {
		/* produce high surrogate, advance source pointer */
		*chPtr = 0xD800 + high;
		return 1;
	    }
	    /* out of range, < 0x10000 or > 0x10FFFF */
#else
#elif TCL_UTF_MAX > 4
	    if ((src[3] & 0xC0) == 0x80) {
		*chPtr = (((byte & 0x07) << 18) | ((src[1] & 0x3F) << 12)
			| ((src[2] & 0x3F) << 6) | (src[3] & 0x3F));
		if ((unsigned)(*chPtr - 0x10000) <= 0xFFFFF) {
		    return 4;
		}
	    }

	 * when (src <= optPtr), (endPtr - src) >= (endPtr - optPtr)
	 * By initialization above (endPtr - optPtr) = TCL_UTF_MAX
	 * So (endPtr - src) >= TCL_UTF_MAX, and passing that to
	 * Tcl_UtfCharComplete we know will cause return of 1.
	 */
	while (src <= optPtr
		/* && Tcl_UtfCharComplete(src, endPtr - src) */ ) {
#if TCL_UTF_MAX < 4
	    if (((unsigned)UCHAR(*src) - 0xF0) < 5) {
		/* treat F0 - F4 as single character */
		ch = 0;
		src++;
	    } else
#endif
	    src += TclUtfToUniChar(src, &ch);
	    i++;
	}
	/* Loop over the remaining string where call must happen */
	while ((src < endPtr) && Tcl_UtfCharComplete(src, endPtr - src)) {
#if TCL_UTF_MAX < 4
	    if (((unsigned)UCHAR(*src) - 0xF0) < 5) {
		/* treat F0 - F4 as single character */
		ch = 0;
		src++;
	    } else
#endif
	    src += TclUtfToUniChar(src, &ch);
	    i++;
	}
	if (src < endPtr) {
	    /*
	     * String ends in an incomplete UTF-8 sequence.
	     * Count every byte in it.