if (length < 0) {
	while (*src != '\0') {
	    src += TclUtfToUniChar(src, &ch);
	    i++;
	}
	if (i < 0) i = INT_MAX; /* Bug [2738427] */
    } else {
	const char *endPtr = src + length - TCL_UTF_MAX;

	while (src < endPtr) {
	    src += TclUtfToUniChar(src, &ch);
	    i++;
	}
	endPtr += TCL_UTF_MAX;
	while ((src < endPtr) && Tcl_UtfCharComplete(src, endPtr - src)) {
	    src += TclUtfToUniChar(src, &ch);
	    i++;
	}
	if (src < endPtr) {
	    i += endPtr - src;
	}

	     * and exit this loop before we run past the start of the string.
	     */
	    return fallback;
	}

	/* Continue the search backwards... */
	look--;
    } while (trailBytesSeen < TCL_UTF_MAX);

    /*
     * We've seen TCL_UTF_MAX trail bytes, so we know there will not be a
     * properly formed byte sequence to find, and we can stop looking,
     * accepting the fallback (for TCL_UTF_MAX > 3) or just go back as
     * far as we can.
     */
#if TCL_UTF_MAX > 3
    return fallback;
#else
    return src - TCL_UTF_MAX;
#endif
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_UniCharAtIndex --
 *