template<class T, class U> bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
template<class T> bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
template<class T, class U> strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
template<class T> strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;