constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
constexpr strong_ordering operator<=>(const year_month& x, const year_month& y) noexcept;
if (auto c = x.year() <=> y.year(); c != 0) return c; return x.month() <=> y.month();
constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
constexpr months operator-(const year_month& x, const year_month& y) noexcept;
x.year() - y.year() + months{static_cast<int>(unsigned{x.month()}) - static_cast<int>(unsigned{y.month()})}
constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& os, const year_month& ym);
return os << format(os.getloc(), STATICALLY-WIDEN<charT>("{}/{}"), ym.year(), ym.month());
template<class charT, class traits, class Alloc = allocator<charT>>
basic_istream<charT, traits>&
from_stream(basic_istream<charT, traits>& is, const charT* fmt,
year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr,
minutes* offset = nullptr);