We need to document that assigning QStringBuilder expressions to auto variables or returning QStringBuilder expressions from functions with deduced return types (in particular, lambdas) doesn't work. This was the original report:

Usages of QStringBuilder seem to raise `std::bad_alloc` when used in the reducer of `std::transform_reduce`. For example, the following: 

#define QT_USE_QSTRINGBUILDER 1

#include <QString>
#include <QStringList>

int main(int argc, char *argv[])
{
	QStringList a{"abv", "def"};
	std::transform_reduce(
            a.begin(),
            a.end(),
            QString{},
            [](QString accumulator, QString element) { return accumulator + ", " + element; },
            [](auto a){ return a; }
	).toStdString();
}

Reports, when executed:

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc
fish: Job 1, 'build/exampleme' terminated by signal SIGABRT (Abort)

Disabling QStringBuilder removes the issue and completes the call correctly.

Similarly, specifying the return type of the reducer as `QString` or casting/constructing to `QString` in the reducer will remove the issue, such that all of the following complete correctly:

            [](QString accumulator, QString element) -> QString { return accumulator + ", " + element; },

            [](QString accumulator, QString element) { return QString{accumulator + ", " + element}; },

            [](QString accumulator, QString element) { return (QString)(accumulator + ", " + element); },

            [](QString accumulator, QString element) { return static_cast<QString>(accumulator + ", " + element); },

Removing the middle element will solve the issue too:

            [](QString accumulator, QString element) { return accumulator + element; },

But making it a `QString` will instead fail with a SIGSEGV:

            [](QString accumulator, QString element) { return accumulator + QString{", "} + element; },

fish: Job 1, 'build/exampleme' terminated by signal SIGSEGV (Address boundary error)