[spanbuf.assign]

basic_spanbuf& operator=(basic_spanbuf&& rhs);

Effects: Equivalent to: basic_spanbuf tmp{std::move(rhs)}; this->swap(tmp); return *this;

void swap(basic_spanbuf& rhs);

Effects: Equivalent to: basic_streambuf<charT, traits>::swap(rhs); std::swap(mode, rhs.mode); std::swap(buf, rhs.buf);

template<class charT, class traits>
  void swap(basic_spanbuf<charT, traits>& x, basic_spanbuf<charT, traits>& y);

Effects: Equivalent to x.swap(y).

31 Input/output library [input.output]