Temporal: Ensure year field in PlainMonthDay not used for range check

When constructing PlainMonthDay from a property bag, or applying partial
fields to PlainMonthDay, as a special case for the iso8601 calendar, the
year field should only be used for applying the overflow parameter, not
for range checking.

Discovered using snapshot testing.
V8/Boa bug: boa-dev/temporal#688

Author: ptomato

test/built-ins/Temporal/PlainMonthDay/from/iso-year-used-only-for-overflow.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.from
+description: >
+  As a special case when constructing PlainMonthDay with iso8601 calendar, the
+  year property is only used for applying the overflow option, not a range check
+features: [Temporal]
+includes: [temporalHelpers.js]
+---*/
+
+var outOfRangeCommonYear = -999999;
+var outOfRangeLeapYear = -1000000;
+
+var result = Temporal.PlainMonthDay.from({
+  year: outOfRangeCommonYear,
+  month: 1,
+  day: 1,
+});
+TemporalHelpers.assertPlainMonthDay(result, "M01", 1, "ISO year is not checked for range");
+
+var commonResult = Temporal.PlainMonthDay.from({
+  year: outOfRangeCommonYear,
+  monthCode: "M02",
+  day: 29
+});
+TemporalHelpers.assertPlainMonthDay(commonResult, "M02", 28, "ISO year is used to apply overflow");
+
+assert.throws(RangeError, function () {
+  Temporal.PlainMonthDay.from({
+    year: outOfRangeCommonYear,
+    monthCode: "M02",
+    day: 29
+  }, { overflow: "reject" });
+}, "ISO year is used to apply overflow");
+
+var leapResult = Temporal.PlainMonthDay.from({
+  year: outOfRangeLeapYear,
+  monthCode: "M02",
+  day: 29
+});
+TemporalHelpers.assertPlainMonthDay(leapResult, "M02", 29, "ISO year is used to apply overflow");

test/built-ins/Temporal/PlainMonthDay/prototype/with/iso-year-used-only-for-overflow.js

@@ -0,0 +1,30 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.with
+description: >
+  As a special case applying partial fields to PlainMonthDay with iso8601
+  calendar, the year property is only used for applying the overflow option, not
+  a range check
+features: [Temporal]
+includes: [temporalHelpers.js]
+---*/
+
+var outOfRangeCommonYear = -999999;
+var outOfRangeLeapYear = -1000000;
+
+var md = new Temporal.PlainMonthDay(1, 1, "iso8601", 1972);
+var result = md.with({ year: outOfRangeCommonYear });
+TemporalHelpers.assertPlainMonthDay(result, "M01", 1, "ISO year is not checked for range");
+
+var leap = new Temporal.PlainMonthDay(2, 29, "iso8601", 1972);
+var commonResult = leap.with({ year: outOfRangeCommonYear });
+TemporalHelpers.assertPlainMonthDay(commonResult, "M02", 28, "ISO year is used to apply overflow");
+
+assert.throws(RangeError, function () {
+  leap.with({ year: outOfRangeCommonYear }, { overflow: "reject" });
+}, "ISO year is used to apply overflow");
+
+var leapResult = leap.with({ year: outOfRangeLeapYear });
+TemporalHelpers.assertPlainMonthDay(leapResult, "M02", 29, "ISO year is used to apply overflow");