31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

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

template<class Ostream, class T> Ostream&& operator<<(Ostream&& os, const T& x);
Constraints: The expression os << x is well-formed when treated as an unevaluated operand and Ostream is publicly and unambiguously derived from ios_base.
Effects: As if by: os << x;
Returns: std​::​move(os).