27 Input/output library [input.output]

27.7 Formatting and manipulators [iostream.format]

27.7.3 Output streams [output.streams]

27.7.3.5 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.