Allow alternative implementations for Chinese lunar calendar

Author: anba

test/intl402/Temporal/Calendar/prototype/yearMonthFromFields/reference-day-chinese.js

@@ -17,6 +17,11 @@ const chinese = new Temporal.Calendar("chinese");
 const months2022TestData = [
   // TODO: Sources conflict over whether M01L and M12L exist in _any_ year.
   // Clarify this, and delete if appropriate. ICU has them, but may be wrong.
+  //
+  // - ICU4C and ICU4X both have M01L in Gregorian year 1651.
+  // - ICU4C has M12L in Gregorian year 1889, but ICU4X doesn't.
+  // - ICU4X has M12L in Gregorian year 1403, but ICU4C doesn't.
+
   ["M01", 1, 1],
   ["M02", 2, 3],
   ["M03", 3, 1],
@@ -31,13 +36,15 @@ const months2022TestData = [
   ["M12", 12, 23],
 ];
 for (const [nonLeapMonthCode, month, referenceISODay] of months2022TestData) {
+  // Allow implementation-defined "epoch year" for the Chinese calendar.
+  const year = new Temporal.PlainDate(2022, 3, 1).withCalendar(chinese).year;
   const leapMonthCode = nonLeapMonthCode + "L";
-  const fields = { year: 2022, monthCode: leapMonthCode };
+  const fields = { year, monthCode: leapMonthCode };
 
   const result = chinese.yearMonthFromFields(fields, { overflow: "constrain" });
   TemporalHelpers.assertPlainYearMonth(
     result,
-    2022, month, nonLeapMonthCode,
+    year, month, nonLeapMonthCode,
     `Chinese intercalary month ${leapMonthCode} does not exist in year 2022 (overflow constrain)`,
     /* era = */ undefined, /* era year = */ undefined, referenceISODay
   );
@@ -49,9 +56,9 @@ for (const [nonLeapMonthCode, month, referenceISODay] of months2022TestData) {
   );
 }
 
-// Years in which leap months exist according to ICU
+// Years in which leap months exist according to ICU4C/ICU4X.
 const leapMonthsTestData = [
-  ["M01L", 2148, 2, 20],
+  ["M01L", 1651, 2, 20],
   ["M02L", 2023, 3, 22],
   ["M03L", 1993, 4, 22],
   ["M04L", 2020, 5, 23],
@@ -61,10 +68,11 @@ const leapMonthsTestData = [
   ["M08L", 1995, 9, 25],
   ["M09L", 2014, 10, 24],
   ["M10L", 1984, 11, 23],
-  ["M11L", 2033, 12, 22],
-  ["M12L", 1889, 13, 21, 1890, 1],
+  ["M11L", 1517, 12, 23],
 ];
-for (const [monthCode, year, month, referenceISODay, isoYear = year, isoMonth = month] of leapMonthsTestData) {
+for (const [monthCode, relatedYear, month, referenceISODay, isoYear = relatedYear, isoMonth = month] of leapMonthsTestData) {
+  // Allow implementation-defined "epoch year" for the Chinese calendar.
+  const year = new Temporal.PlainDate(relatedYear, 3, 1).withCalendar(chinese).year;
   const result = chinese.yearMonthFromFields({ year, monthCode });
   TemporalHelpers.assertPlainYearMonth(
     result,