std::chrono::operator+, std::chrono::operator- (std::chrono::year_month_weekday_last)

constexpr std::chrono::year_month_weekday_last 
operator+(const std::chrono::year_month_weekday_last& ymwdl,
          const std::chrono::months& dm) noexcept;
 (since C++20) 
constexpr std::chrono::year_month_weekday_last 
operator+(const std::chrono::months& dm, 
          const std::chrono::year_month_weekday_last& ymwdl) noexcept;
 (since C++20) 
constexpr std::chrono::year_month_weekday_last
operator+(const std::chrono::year_month_weekday_last& ymwdl, 
          const std::chrono::years& dy) noexcept;
 (since C++20) 
constexpr std::chrono::year_month_weekday_last
operator+(const std::chrono::years& dy, 
          const std::chrono::year_month_weekday_last& ymwdl) noexcept;
 (since C++20) 
constexpr std::chrono::year_month_weekday_last
operator-(const std::chrono::year_month_weekday_last& ymwdl, 
          const std::chrono::months& dm) noexcept;
 (since C++20) 
constexpr std::chrono::year_month_weekday_last 
operator-(const std::chrono::year_month_weekday_last& ymwdl,
          const std::chrono::years& dy) noexcept;
 (since C++20)
1-2) Adds dm.count() months to the date represented by ymwdl. The result has the same year() and month() as std::chrono::year_month(ymwdl.year(), ymwdl.month()) + dm and the same weekday() as ymwdl.
3-4) Adds dy.count() years to the date represented by ymwdl. The result is equivalent to std::chrono::year_month_weekday_last(ymwdl.year() + dy, ymwdl.month(), ymwd.weekday_last()).
5) Subtracts dm.count() months from the date represented by ymwdl. Equivalent to ymwdl+ -dm.
6) Subtracts dy.count() years from the date represented by ymwdl. Equivalent to ymwdl + -dy.

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