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std::chrono::weekday::operator++, std::chrono::weekday::operator--

constexpr std::chrono::weekday& operator++() noexcept;	(1)	(since C++20)
constexpr std::chrono::weekday operator++(int) noexcept;	(2)	(since C++20)
constexpr std::chrono::weekday& operator--() noexcept;	(3)	(since C++20)
constexpr std::chrono::weekday operator--(int) noexcept;	(4)	(since C++20)

Adds or subtracts 1 from the weekday value, reducing the result modulo 7 to an integer in the range [0, 6].

1-2) Performs *this += std::chrono::days{1};

3-4) Performs *this -= std::chrono::days{1};

(none).

1,3) a reference to this weekday after modification

2,4) a copy of the weekday made before modification

After a call to one of these functions, ok() is always true.

operator+=operator-=	adds or subtracts a number of days (public member function)
operator+operator-	performs arithmetic on `weekday`s (function)