don't auto deduce QStringBuilder
see 'auto QString(Builder) considered VERY HARMFUL' thread on kde-devel https://markmail.org/message/mbrp5ypyhcfk7zjn
What happens is that the type of str ends up being QStringBuilder instead of QString. The QStringBuilder is destroyed after the semicolon, and all access to "str" produces undefined behavior.
Never use auto for QString concatenation!