Add test about negative extended years (#5087)

components/calendar/src/gregorian.rs

@@ -54,7 +54,7 @@ pub struct Gregorian;
 
 #[derive(Copy, Clone, Debug, Hash, Eq, PartialEq, PartialOrd, Ord)]
 /// The inner date type used for representing [`Date`]s of [`Gregorian`]. See [`Date`] and [`Gregorian`] for more details.
-pub struct GregorianDateInner(IsoDateInner);
+pub struct GregorianDateInner(pub(crate) IsoDateInner);
 
 impl Calendar for Gregorian {
     type DateInner = GregorianDateInner;

components/calendar/src/hebrew.rs

@@ -523,6 +523,23 @@ mod tests {
         assert_eq!(yi.keviyah.year_length(), 383);
     }
 
+    #[test]
+    fn test_negative_era_years() {
+        let greg_date = Date::try_new_gregorian_date(-5000, 1, 1).unwrap();
+        // Extended year is accessible via the inner value.
+        // Era year is accessible via the public getter.
+        // TODO(#3962): Make extended year publicly accessible.
+        assert_eq!(greg_date.inner.0 .0.year, -5000);
+        assert_eq!(greg_date.year().era.0, "bce");
+        // In Gregorian, era year is 1 - extended year
+        assert_eq!(greg_date.year().number, 5001);
+        let hebr_date = greg_date.to_calendar(Hebrew);
+        assert_eq!(hebr_date.inner.0.year, -1240);
+        assert_eq!(hebr_date.year().era.0, "hebrew");
+        // In Hebrew, there is no inverse era, so negative extended years are negative era years
+        assert_eq!(hebr_date.year().number, -1240);
+    }
+
     #[test]
     fn test_weekdays() {
         // https://github.com/unicode-org/icu4x/issues/4893