From 0669f4bfb6cb9769ae46e23339476373dea2f555 Mon Sep 17 00:00:00 2001
From: Richard Gibson <richard.gibson@gmail.com>
Date: Mon, 21 Aug 2023 16:11:02 -0400
Subject: [PATCH] Polyfill: Align MonthDay code with spec

---
 polyfill/lib/calendar.mjs   | 3 ---
 polyfill/lib/ecmascript.mjs | 8 ++------
 2 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index bafdcf8210..d01f7871d2 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -322,9 +322,6 @@ impl['iso8601'] = {
   monthDayFromFields(fields, options, calendarSlotValue) {
     fields = ES.PrepareTemporalFields(fields, ['day', 'month', 'monthCode', 'year'], ['day']);
     const overflow = ES.ToTemporalOverflow(options);
-    if (fields.month !== undefined && fields.year === undefined && fields.monthCode === undefined) {
-      throw new TypeError('either year or monthCode required with month');
-    }
     const referenceISOYear = 1972;
     fields = resolveNonLunisolarMonth(fields);
     let { month, day, year } = fields;
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
index c7f231517c..b823f3267a 100644
--- a/polyfill/lib/ecmascript.mjs
+++ b/polyfill/lib/ecmascript.mjs
@@ -1323,13 +1323,11 @@ export function ToTemporalMonthDay(item, options) {
   if (options !== undefined) options = SnapshotOwnProperties(GetOptionsObject(options), null);
   if (Type(item) === 'Object') {
     if (IsTemporalMonthDay(item)) return item;
-    let calendar, calendarAbsent;
+    let calendar;
     if (HasSlot(item, CALENDAR)) {
       calendar = GetSlot(item, CALENDAR);
-      calendarAbsent = false;
     } else {
       calendar = item.calendar;
-      calendarAbsent = calendar === undefined;
       if (calendar === undefined) calendar = 'iso8601';
       calendar = ToTemporalCalendarSlotValue(calendar);
     }
@@ -1338,9 +1336,6 @@ export function ToTemporalMonthDay(item, options) {
     // Callers who omit the calendar are not writing calendar-independent
     // code. In that case, `monthCode`/`year` can be omitted; `month` and
     // `day` are sufficient. Add a `year` to satisfy calendar validation.
-    if (calendarAbsent && fields.month !== undefined && fields.monthCode === undefined && fields.year === undefined) {
-      fields.year = 1972;
-    }
     return CalendarMonthDayFromFields(calendar, fields, options);
   }
 
@@ -1351,6 +1346,7 @@ export function ToTemporalMonthDay(item, options) {
   ToTemporalOverflow(options); // validate and ignore
 
   if (referenceISOYear === undefined) {
+    if (calendar !== 'iso8601') throw new Error(`assertion failed: missing year with non-"iso8601" calendar identifier ${calendar}`);
     RejectISODate(1972, month, day);
     return CreateTemporalMonthDay(month, day, calendar);
   }