SimpleString::ToLower()

This line is giving me grief on Travis still. I tried an explicit cast

    return isUpper(ch) ? ch +(char) ('a' - 'A') : ch;

but that won't make any difference. The compiler does a conversion to int internally, and there is a Werror that the value might change when cast to char. I know this calculation is okay, but how to avoid the implicit cast to int?