CalendarGregorian's date(from: components) does not honor the DateComponents time zone (swiftlang#421)

Use the date component's time zone if there is one.

Resolves 122918762

Author: itingliu

Sources/FoundationEssentials/Calendar/Calendar_Gregorian.swift

@@ -1475,9 +1475,10 @@ internal final class _CalendarGregorian: _CalendarProtocol, @unchecked Sendable
     // MARK:
 
     func date(from components: DateComponents) -> Date? {
+        // If the components specifies a new time zone, perform this calculation using the specified timezone
         // If the date falls into the skipped time frame when transitioning into DST (e.g. 1:00 - 3:00 AM for PDT), we want to treat it as if DST hasn't happened yet. So, use .former for dstRepeatedTimePolicy.
         // If the date falls into the repeated time frame when DST ends (e.g. 1:00 - 2:00 AM for PDT), we want the first instance, i.e. the instance before turning back the clock. So, use .former for dstSkippedTimePolicy.
-        date(from: components, inTimeZone: timeZone, dstRepeatedTimePolicy: .former, dstSkippedTimePolicy: .former)
+        date(from: components, inTimeZone: components.timeZone ?? timeZone, dstRepeatedTimePolicy: .former, dstSkippedTimePolicy: .former)
     }
 
     //  Returns the weekday with reference to `firstWeekday`, in the range of 0...6

Tests/FoundationEssentialsTests/GregorianCalendarTests.swift

@@ -145,6 +145,42 @@ final class GregorianCalendarTests : XCTestCase {
         test(.init(weekOfYear: 43, yearForWeekOfYear: 2935), expected: Date(timeIntervalSince1970: 30477945600.0))
     }
 
+    func testDateFromComponents_componentsTimeZone() {
+        let gregorianCalendar = _CalendarGregorian(identifier: .gregorian, timeZone: .gmt, locale: nil, firstWeekday: nil, minimumDaysInFirstWeek: nil, gregorianStartDate: nil)
+
+        let dcCalendar = Calendar(identifier: .japanese, locale: Locale(identifier: ""), timeZone: .init(secondsFromGMT: -25200), firstWeekday: 1, minimumDaysInFirstWeek: 1, gregorianStartDate: nil)
+        let dc = DateComponents(calendar: nil, timeZone: nil, era: 1, year: 2022, month: 7, day: 9, hour: 10, minute: 2, second: 55, nanosecond: 891000032, weekday: 7, weekdayOrdinal: 2, quarter: 0, weekOfMonth: 2, weekOfYear: 28, yearForWeekOfYear: 2022)
+
+        var dc_customCalendarAndTimeZone = dc
+        dc_customCalendarAndTimeZone.calendar = dcCalendar
+        dc_customCalendarAndTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        // calendar.timeZone = UTC+0, dc.calendar.timeZone = UTC-7, dc.timeZone = UTC+8
+        // expect local time in dc.timeZone (UTC+8)
+        XCTAssertEqual(gregorianCalendar.date(from: dc_customCalendarAndTimeZone)!, Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
+
+        var dc_customCalendar = dc
+        dc_customCalendar.calendar = dcCalendar
+        dc_customCalendar.timeZone = nil
+        // calendar.timeZone = UTC+0, dc.calendar.timeZone = UTC-7, dc.timeZone = nil
+        // expect local time in calendar.timeZone (UTC+0)
+        XCTAssertEqual(gregorianCalendar.date(from: dc_customCalendar)!, Date(timeIntervalSinceReferenceDate: 679053775.891)) // 2022-07-09T10:02:55Z
+
+        var dc_customTimeZone = dc_customCalendarAndTimeZone
+        dc_customTimeZone.calendar = nil
+        dc_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        // calendar.timeZone = UTC+0, dc.calendar = nil, dc.timeZone = UTC+8
+        // expect local time in dc.timeZone (UTC+8)
+        XCTAssertEqual(gregorianCalendar.date(from: dc_customTimeZone)!, Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
+
+        let dcCalendar_noTimeZone = Calendar(identifier: .japanese, locale: Locale(identifier: ""), timeZone: nil, firstWeekday: 1, minimumDaysInFirstWeek: 1, gregorianStartDate: nil)
+        var dc_customCalendarNoTimeZone_customTimeZone = dc
+        dc_customCalendarNoTimeZone_customTimeZone.calendar = dcCalendar_noTimeZone
+        dc_customCalendarNoTimeZone_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        // calendar.timeZone = UTC+0, dc.calendar.timeZone = nil, dc.timeZone = UTC+8
+        // expect local time in dc.timeZone (UTC+8)
+        XCTAssertEqual(gregorianCalendar.date(from: dc_customCalendarNoTimeZone_customTimeZone)!, Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
+    }
+
     // MARK: - DateComponents from date
     func testDateComponentsFromDate() {
         let calendar = _CalendarGregorian(identifier: .gregorian, timeZone: TimeZone(secondsFromGMT: 0)!, locale: nil, firstWeekday: 1, minimumDaysInFirstWeek: 5, gregorianStartDate: nil)

Tests/FoundationInternationalizationTests/CalendarTests.swift

@@ -1032,6 +1032,46 @@ final class GregorianCalendarCompatibilityTests: XCTestCase {
         test(.init(year: 2023, month: 11, day: 5, hour: 3, minute: 34, second: 52))
     }
 
+    func testDateFromComponents_componentsTimeZone() {
+        let timeZone = TimeZone.gmt
+        let icuCalendar = _CalendarICU(identifier: .gregorian, timeZone: timeZone, locale: nil, firstWeekday: nil, minimumDaysInFirstWeek: nil, gregorianStartDate: nil)
+        let gregorianCalendar = _CalendarGregorian(identifier: .gregorian, timeZone: timeZone, locale: nil, firstWeekday: nil, minimumDaysInFirstWeek: nil, gregorianStartDate: nil)
+
+
+        func test(_ dateComponents: DateComponents, file: StaticString = #file, line: UInt = #line) {
+            let date_new = gregorianCalendar.date(from: dateComponents)!
+            let date_old = icuCalendar.date(from: dateComponents)!
+            expectEqual(date_new, date_old, "dateComponents: \(dateComponents)")
+            print("""
+
+XCTAssertEqual(gregorianCalendar.date(from: dateComponents)!, Date(timeIntervalSinceReferenceDate: \(date_old.timeIntervalSinceReferenceDate))) // \(date_old.formatted(.iso8601))
+""")
+        }
+
+        let dcCalendar = Calendar(identifier: .japanese, locale: Locale(identifier: ""), timeZone: .init(secondsFromGMT: -25200), firstWeekday: 1, minimumDaysInFirstWeek: 1, gregorianStartDate: nil)
+        let dc = DateComponents(calendar: nil, timeZone: nil, era: 1, year: 2022, month: 7, day: 9, hour: 10, minute: 2, second: 55, nanosecond: 891000032, weekday: 7, weekdayOrdinal: 2, quarter: 0, weekOfMonth: 2, weekOfYear: 28, yearForWeekOfYear: 2022)
+        var dc_customCalendarAndTimeZone = dc
+        dc_customCalendarAndTimeZone.calendar = dcCalendar
+        dc_customCalendarAndTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        test(dc_customCalendarAndTimeZone) // calendar.timeZone = .gmt, dc.calendar.timeZone = UTC-7, dc.timeZone = UTC+8
+
+        var dc_customCalendar = dc
+        dc_customCalendar.calendar = dcCalendar
+        dc_customCalendar.timeZone = nil
+        test(dc_customCalendar) // calendar.timeZone = .gmt, dc.calendar.timeZone = UTC-7, dc.timeZone = nil
+
+        var dc_customTimeZone = dc_customCalendarAndTimeZone
+        dc_customTimeZone.calendar = nil
+        dc_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        test(dc_customTimeZone) // calendar.timeZone = .gmt, dc.calendar = nil, dc.timeZone = UTC+8
+
+        let dcCalendar_noTimeZone = Calendar(identifier: .japanese, locale: Locale(identifier: ""), timeZone: nil, firstWeekday: 1, minimumDaysInFirstWeek: 1, gregorianStartDate: nil)
+        var dc_customCalendarNoTimeZone_customTimeZone = dc
+        dc_customCalendarNoTimeZone_customTimeZone.calendar = dcCalendar_noTimeZone
+        dc_customCalendarNoTimeZone_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
+        test(dc_customCalendarNoTimeZone_customTimeZone) // calendar.timeZone = .gmt, dc.calendar.timeZone = nil, dc.timeZone = UTC+8
+    }
+
     func testDateComponentsFromDateCompatibility() {
         let componentSet = Calendar.ComponentSet([.era, .year, .month, .day, .hour, .minute, .second, .nanosecond, .weekday, .weekdayOrdinal, .quarter, .weekOfMonth, .weekOfYear, .yearForWeekOfYear, .calendar])