[time.cal.wd] - C++17 → C++20

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+### Class `weekday` <a id="time.cal.wd">[[time.cal.wd]]</a>
+#### Overview <a id="time.cal.wd.overview">[[time.cal.wd.overview]]</a>
+``` cpp
+namespace std::chrono {
+  class weekday {
+    unsigned char wd_;          // exposition only
+  public:
+    weekday() = default;
+    constexpr explicit weekday(unsigned wd) noexcept;
+    constexpr weekday(const sys_days& dp) noexcept;
+    constexpr explicit weekday(const local_days& dp) noexcept;
+    constexpr weekday& operator++()    noexcept;
+    constexpr weekday  operator++(int) noexcept;
+    constexpr weekday& operator--()    noexcept;
+    constexpr weekday  operator--(int) noexcept;
+    constexpr weekday& operator+=(const days& d) noexcept;
+    constexpr weekday& operator-=(const days& d) noexcept;
+    constexpr unsigned c_encoding() const noexcept;
+    constexpr unsigned iso_encoding() const noexcept;
+    constexpr bool ok() const noexcept;
+    constexpr weekday_indexed operator[](unsigned index) const noexcept;
+    constexpr weekday_last    operator[](last_spec) const noexcept;
+  };
+}
+```
+`weekday` represents a day of the week in the civil calendar. It
+normally holds values in the range `0` to `6`, corresponding to Sunday
+through Saturday, but it may hold non-negative values outside this
+range. It can be constructed with any `unsigned` value, which will be
+subsequently truncated to fit into `weekday`’s unspecified internal
+storage. `weekday` meets the *Cpp17EqualityComparable* (
+[[cpp17.equalitycomparable]]) requirements.
+[*Note 1*: `weekday` is not *Cpp17LessThanComparable* because there is
+no universal consensus on which day is the first day of the week.
+`weekday`’s arithmetic operations treat the days of the week as a
+circular range, with no beginning and no end. — *end note*]
+`weekday` is a trivially copyable and standard-layout class type.
+#### Member functions <a id="time.cal.wd.members">[[time.cal.wd.members]]</a>
+``` cpp
+constexpr explicit weekday(unsigned wd) noexcept;
+```
+*Effects:* Initializes `wd_` with `wd == 7 ? 0 : wd`. The value held is
+unspecified if `wd` is not in the range \[`0`, `255`\].
+``` cpp
+constexpr weekday(const sys_days& dp) noexcept;
+```
+*Effects:* Computes what day of the week corresponds to the `sys_days`
+`dp`, and initializes that day of the week in `wd_`.
+[*Example 1*: If `dp` represents 1970-01-01, the constructed `weekday`
+represents Thursday by storing `4` in `wd_`. — *end example*]
+``` cpp
+constexpr explicit weekday(const local_days& dp) noexcept;
+```
+*Effects:* Computes what day of the week corresponds to the `local_days`
+`dp`, and initializes that day of the week in `wd_`.
+*Ensures:* The value is identical to that constructed from
+`sys_days{dp.time_since_epoch()}`.
+``` cpp
+constexpr weekday& operator++() noexcept;
+```
+*Effects:* `*this += days{1}`.
+*Returns:* `*this`.
+``` cpp
+constexpr weekday operator++(int) noexcept;
+```
+*Effects:* `++(*this)`.
+*Returns:* A copy of `*this` as it existed on entry to this member
+function.
+``` cpp
+constexpr weekday& operator--() noexcept;
+```
+*Effects:* `*this -= days{1}`.
+*Returns:* `*this`.
+``` cpp
+constexpr weekday operator--(int) noexcept;
+```
+*Effects:* `–(*this)`.
+*Returns:* A copy of `*this` as it existed on entry to this member
+function.
+``` cpp
+constexpr weekday& operator+=(const days& d) noexcept;
+```
+*Effects:* `*this = *this + d`.
+*Returns:* `*this`.
+``` cpp
+constexpr weekday& operator-=(const days& d) noexcept;
+```
+*Effects:* `*this = *this - d`.
+*Returns:* `*this`.
+``` cpp
+constexpr unsigned c_encoding() const noexcept;
+```
+*Returns:* `wd_`.
+``` cpp
+constexpr unsigned iso_encoding() const noexcept;
+```
+*Returns:* `wd_ == 0u ? 7u : wd_`.
+``` cpp
+constexpr bool ok() const noexcept;
+```
+*Returns:* `wd_ <= 6`.
+``` cpp
+constexpr weekday_indexed operator[](unsigned index) const noexcept;
+```
+*Returns:* `{*this, index}`.
+``` cpp
+constexpr weekday_last operator[](last_spec) const noexcept;
+```
+*Returns:* `weekday_last{*this}`.
+#### Non-member functions <a id="time.cal.wd.nonmembers">[[time.cal.wd.nonmembers]]</a>
+``` cpp
+constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
+```
+*Returns:* `x.wd_ == y.wd_`.
+``` cpp
+constexpr weekday operator+(const weekday& x, const days& y) noexcept;
+```
+*Returns:*
+``` cpp
+weekday{modulo(static_cast<long long>(x.wd_) + y.count(), 7)}
+```
+where `modulo(n, 7)` computes the remainder of `n` divided by 7 using
+Euclidean division.
+[*Note 1*: Given a divisor of 7, Euclidean division truncates towards
+negative infinity and always produces a remainder in the range of \[`0`,
+`6`\]. Assuming no overflow in the signed summation, this operation
+results in a `weekday` holding a value in the range \[`0`, `6`\] even if
+`!x.ok()`. — *end note*]
+[*Example 1*: `Monday + days{6} == Sunday`. — *end example*]
+``` cpp
+constexpr weekday operator+(const days& x, const weekday& y) noexcept;
+```
+*Returns:* `y + x`.
+``` cpp
+constexpr weekday operator-(const weekday& x, const days& y) noexcept;
+```
+*Returns:* `x + -y`.
+``` cpp
+constexpr days operator-(const weekday& x, const weekday& y) noexcept;
+```
+*Returns:* If `x.ok() == true` and `y.ok() == true`, returns a value `d`
+in the range \[`days{0}`, `days{6}`\] satisfying `y + d == x`. Otherwise
+the value returned is unspecified.
+[*Example 2*: `Sunday - Monday == days{6}`. — *end example*]
+``` cpp
+template<class charT, class traits>
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const weekday& wd);
+```
+*Effects:* Equivalent to:
+``` cpp
+return os << (wd.ok() ?
+  format(os.getloc(), STATICALLY-WIDEN<charT>("{:%a}"), wd) :
+  format(os.getloc(), STATICALLY-WIDEN<charT>("{} is not a valid weekday"),
+         static_cast<unsigned>(wd.wd_)));
+```
+``` cpp
+template<class charT, class traits, class Alloc = allocator<charT>>
+  basic_istream<charT, traits>&
+    from_stream(basic_istream<charT, traits>& is, const charT* fmt,
+                weekday& wd, basic_string<charT, traits, Alloc>* abbrev = nullptr,
+                minutes* offset = nullptr);
+```
+*Effects:* Attempts to parse the input stream `is` into the `weekday`
+`wd` using the format flags given in the NTCTS `fmt` as specified in
+[[time.parse]]. If the parse fails to decode a valid weekday,
+`is.setstate(ios_base::failbit)` is called and `wd` is not modified. If
+`%Z` is used and successfully parsed, that value will be assigned to
+`*abbrev` if `abbrev` is non-null. If `%z` (or a modified variant) is
+used and successfully parsed, that value will be assigned to `*offset`
+if `offset` is non-null.
+*Returns:* `is`.

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