30 Input/output library [input.output]

30.7 Formatting and manipulators [iostream.format]

30.7.5 Output streams [output.streams] Rvalue stream insertion [ostream.rvalue]

template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);
Effects: As if by: os << x;
Returns: os.
Remarks: This function shall not participate in overload resolution unless the expression os << x is well-formed.