lst-0069-book.cpp
bool operator==
(const error_code& lhs, const error_code& rhs) noexcept;
…(const error_code& lhs, const error_condition& rhs) noexcept;
…(const error_condition& lhs, const error_code& rhs) noexcept;
…(const error_condition& lhs, const error_condition& rhs) noexcept;