[spanbuf.assign]
31 Input/output library [input.output]
31.9 Span-based streams [span.streams]
31.9.3 Class template basic_spanbuf [spanbuf]
31.9.3.3 Assignment and swap [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).