The documentation page for QTextStream contains examples of type (stream << left). However, neither that page nor any other page provides any explanation why such syntax is valid.

These operators should be documented:
Ok, the problem is that these operators are undocumented in QTextStream:

inline QTextStream &operator>>(QTextStream &s, QTextStreamFunction f)
{ return (*f)(s); }

inline QTextStream &operator<<(QTextStream &s, QTextStreamFunction f)
{ return (*f)(s); }

inline QTextStream &operator<<(QTextStream &s, QTextStreamManipulator m)
{ m.exec(s); return s; }