The code in PyUnicode_Join() checks on overflow after making an addition.

        sz += PyUnicode_GET_LENGTH(item);
        item_maxchar = PyUnicode_MAX_CHAR_VALUE(item);
        maxchar = Py_MAX(maxchar, item_maxchar);
        if (i != 0)
            sz += seplen;
        if (sz < old_sz || sz > PY_SSIZE_T_MAX) {
            PyErr_SetString(PyExc_OverflowError,
                            "join() result is too long for a Python string");
            goto onError;
        }

Maybe there are other cases.