[ispanstream.swap]
31 Input/output library [input.output]
31.9 Span-based streams [span.streams]
31.9.4 Class template basic_ispanstream [ispanstream]
31.9.4.3 Swap [ispanstream.swap]
void swap(basic_ispanstream& rhs);
template<class charT, class traits>
void swap(basic_ispanstream<charT, traits>& x, basic_ispanstream<charT, traits>& y);