Store strings or objects in Temporal objects' [[Calendar]] slot

In several tests involving custom calendars, we need to change the
implementation of dateFromFields/monthDayFromFields/yearMonthFromFields so
that the returned object gets the receiver as its calendar after chaining
up to the builtin implementation.

Normative PR: https://github.com/tc39/proposal-temporal/pull/2482

harness/temporalHelpers.js

@@ -840,6 +840,20 @@ var TemporalHelpers = {
         super("iso8601");
       }
 
+      dateFromFields(...args) {
+        return super.dateFromFields(...args).withCalendar(this);
+      }
+
+      monthDayFromFields(...args) {
+        const { isoYear, isoMonth, isoDay } = super.monthDayFromFields(...args).getISOFields();
+        return new Temporal.PlainMonthDay(isoMonth, isoDay, this, isoYear);
+      }
+
+      yearMonthFromFields(...args) {
+        const { isoYear, isoMonth, isoDay } = super.yearMonthFromFields(...args).getISOFields();
+        return new Temporal.PlainYearMonth(isoYear, isoMonth, this, isoDay);
+      }
+
       toString() {
         return "fast-path-check";
       }
@@ -1060,13 +1074,17 @@ var TemporalHelpers = {
         return "dateadd-plain-date-instance";
       }
 
+      dateFromFields(...args) {
+        return super.dateFromFields(...args).withCalendar(this);
+      }
+
       dateAdd(date, duration, options) {
         this.dateAddCallCount++;
         assert(date instanceof Temporal.PlainDate, "dateAdd() should be called with a PlainDate instance");
         if (this.dateAddCallCount === 1 && this.specificPlainDate) {
           assert.sameValue(date, this.specificPlainDate, `dateAdd() should be called first with the specific PlainDate instance ${this.specificPlainDate}`);
         }
-        return super.dateAdd(date, duration, options);
+        return super.dateAdd(date, duration, options).withCalendar(this);
       }
     }
     return new CalendarDateAddPlainDateInstance();

test/built-ins/Temporal/Calendar/prototype/dateUntil/argument-propertybag-calendar-string.js

@@ -9,6 +9,13 @@ features: [Temporal]
 ---*/
 
 const instance = new Temporal.Calendar("iso8601");
+Object.defineProperty(instance, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up on receiver");
+  },
+});
 
 const calendar = "iso8601";
 

test/built-ins/Temporal/Duration/compare/relativeto-propertybag-calendar-string.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.compare
+description: >
+    Builtin dateFromFields method is not observably called when the property bag
+    has a string-valued calendar property
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const relativeTo = { year: 2000, month: 5, day: 2, calendar: "iso8601" };
+Temporal.Duration.compare(new Temporal.Duration(), new Temporal.Duration(), { relativeTo });
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/Duration/prototype/add/relativeto-propertybag-calendar-string.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.add
+description: >
+    Builtin dateFromFields method is not observably called when the property bag
+    has a string-valued calendar property
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.Duration(1, 0, 0, 1);
+const relativeTo = { year: 2000, month: 5, day: 2, calendar: "iso8601" };
+instance.add(new Temporal.Duration(0, 0, 0, 0, -24), { relativeTo });
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/Duration/prototype/round/relativeto-propertybag-calendar-string.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.round
+description: >
+    Builtin dateFromFields method is not observably called when the property bag
+    has a string-valued calendar property
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.Duration(1, 0, 0, 0, 24);
+const relativeTo = { year: 2000, month: 5, day: 2, calendar: "iso8601" };
+instance.round({ largestUnit: "years", relativeTo });
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/Duration/prototype/subtract/relativeto-propertybag-calendar-string.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.subtract
+description: >
+    Builtin dateFromFields method is not observably called when the property bag
+    has a string-valued calendar property
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.Duration(1, 0, 0, 1);
+const relativeTo = { year: 2000, month: 5, day: 2, calendar: "iso8601" };
+instance.subtract(new Temporal.Duration(0, 0, 0, 0, 24), { relativeTo });
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/Duration/prototype/total/relativeto-propertybag-calendar-string.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.total
+description: >
+    Builtin dateFromFields method is not observably called when the property bag
+    has a string-valued calendar property
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.Duration(1, 0, 0, 0, 24);
+const relativeTo = { year: 2000, month: 5, day: 2, calendar: "iso8601" };
+instance.total({ unit: "days", relativeTo });
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/Instant/prototype/toZonedDateTimeISO/calendar-is-builtin.js

@@ -0,0 +1,13 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tozoneddatetimeiso
+description: >
+  toZonedDateTimeISO() results in a ZonedDateTime with builtin ISO calendar
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(0n);
+const result = instance.toZonedDateTimeISO("UTC");
+assert.sameValue(result.getISOFields().calendar, "iso8601", "calendar slot stores a string");

test/built-ins/Temporal/PlainDate/compare/argument-propertybag-calendar-string.js

@@ -4,9 +4,19 @@
 /*---
 esid: sec-temporal.plaindate.compare
 description: A calendar ID is valid input for Calendar
+includes: [temporalHelpers.js]
 features: [Temporal]
 ---*/
 
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
 const calendar = "iso8601";
 
 const arg = { year: 1976, monthCode: "M11", day: 18, calendar };
@@ -16,3 +26,5 @@ assert.sameValue(result1, 0, `Calendar created from string "${arg}" (first argum
 
 const result2 = Temporal.PlainDate.compare(new Temporal.PlainDate(1976, 11, 18), arg);
 assert.sameValue(result2, 0, `Calendar created from string "${arg}" (second argument)`);
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainDate/from/argument-propertybag-calendar-string.js

@@ -13,3 +13,4 @@ const calendar = "iso8601";
 const arg = { year: 1976, monthCode: "M11", day: 18, calendar };
 const result = Temporal.PlainDate.from(arg);
 TemporalHelpers.assertPlainDate(result, 1976, 11, "M11", 18, `Calendar created from string "${calendar}"`);
+assert.sameValue(result.getISOFields().calendar, "iso8601", "calendar slot stores a string");

test/built-ins/Temporal/PlainDate/prototype/add/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.add
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.add(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainDate/prototype/calendarId/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.calendarid
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.calendarId;
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/day/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.day
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "day");
+Object.defineProperty(Temporal.Calendar.prototype, "day", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("day should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.day;
+
+Object.defineProperty(Temporal.Calendar.prototype, "day", dayOriginal);

test/built-ins/Temporal/PlainDate/prototype/dayOfWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.dayofweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOfWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dayOfWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dayOfWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.dayOfWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfWeek", dayOfWeekOriginal);

test/built-ins/Temporal/PlainDate/prototype/dayOfYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.dayofyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOfYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dayOfYear");
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dayOfYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.dayOfYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfYear", dayOfYearOriginal);

test/built-ins/Temporal/PlainDate/prototype/daysInMonth/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.daysinmonth
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInMonthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInMonth");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInMonth should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.daysInMonth;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", daysInMonthOriginal);

test/built-ins/Temporal/PlainDate/prototype/daysInWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.daysinweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.daysInWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInWeek", daysInWeekOriginal);

test/built-ins/Temporal/PlainDate/prototype/daysInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.daysinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.daysInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", daysInYearOriginal);

test/built-ins/Temporal/PlainDate/prototype/equals/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.equals
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.equals(new Temporal.PlainDate(2000, 5, 2));
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/inLeapYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.inleapyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const inLeapYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "inLeapYear");
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("inLeapYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.inLeapYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", inLeapYearOriginal);

test/built-ins/Temporal/PlainDate/prototype/month/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.month
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "month");
+Object.defineProperty(Temporal.Calendar.prototype, "month", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("month should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.month;
+
+Object.defineProperty(Temporal.Calendar.prototype, "month", monthOriginal);

test/built-ins/Temporal/PlainDate/prototype/monthCode/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.monthcode
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthCodeOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthCode");
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthCode should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.monthCode;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", monthCodeOriginal);

test/built-ins/Temporal/PlainDate/prototype/monthsInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.monthsinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthsInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthsInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthsInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.monthsInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", monthsInYearOriginal);

test/built-ins/Temporal/PlainDate/prototype/since/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.since
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateUntilOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateUntil");
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateUntil should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.since(new Temporal.PlainDate(1999, 4, 1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", dateUntilOriginal);

test/built-ins/Temporal/PlainDate/prototype/subtract/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.subtract
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.subtract(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainDate/prototype/toJSON/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.tojson
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.toJSON();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/toLocaleString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.tolocalestring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.toLocaleString();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/toPlainMonthDay/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.toplainmonthday
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthDayFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthDayFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthDayFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.toPlainMonthDay();
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", monthDayFromFieldsOriginal);

test/built-ins/Temporal/PlainDate/prototype/toPlainYearMonth/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.toplainyearmonth
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const yearMonthFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "yearMonthFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "yearMonthFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("yearMonthFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.toPlainYearMonth();
+
+Object.defineProperty(Temporal.Calendar.prototype, "yearMonthFromFields", yearMonthFromFieldsOriginal);

test/built-ins/Temporal/PlainDate/prototype/toString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.tostring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.toString();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/until/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.until
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateUntilOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateUntil");
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateUntil should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.until(new Temporal.PlainDate(2001, 6, 13));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", dateUntilOriginal);

test/built-ins/Temporal/PlainDate/prototype/weekOfYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.weekofyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const weekOfYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "weekOfYear");
+Object.defineProperty(Temporal.Calendar.prototype, "weekOfYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("weekOfYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.weekOfYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "weekOfYear", weekOfYearOriginal);

test/built-ins/Temporal/PlainDate/prototype/with/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.with
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const mergeFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "mergeFields");
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("mergeFields should not be looked up");
+  },
+});
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.with({ year: 2001 });
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", mergeFieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainDate/prototype/withCalendar/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.withcalendar
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.withCalendar("iso8601");
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDate/prototype/year/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.year
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const yearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "year");
+Object.defineProperty(Temporal.Calendar.prototype, "year", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("year should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.year;
+
+Object.defineProperty(Temporal.Calendar.prototype, "year", yearOriginal);

test/built-ins/Temporal/PlainDate/prototype/yearOfWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.yearofweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const yearOfWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "yearOfWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "yearOfWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("yearOfWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDate(2000, 5, 2, "iso8601");
+instance.yearOfWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "yearOfWeek", yearOfWeekOriginal);

test/built-ins/Temporal/PlainDateTime/compare/argument-propertybag-calendar-string.js

@@ -4,9 +4,19 @@
 /*---
 esid: sec-temporal.plaindatetime.compare
 description: A calendar ID is valid input for Calendar
+includes: [temporalHelpers.js]
 features: [Temporal]
 ---*/
 
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
 const calendar = "iso8601";
 
 const arg = { year: 1976, monthCode: "M11", day: 18, calendar };
@@ -16,3 +26,5 @@ assert.sameValue(result1, 0, `Calendar created from string "${arg}" (first argum
 
 const result2 = Temporal.PlainDateTime.compare(new Temporal.PlainDateTime(1976, 11, 18), arg);
 assert.sameValue(result2, 0, `Calendar created from string "${arg}" (second argument)`);
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainDateTime/from/argument-propertybag-calendar-string.js

@@ -13,3 +13,4 @@ const calendar = "iso8601";
 const arg = { year: 1976, monthCode: "M11", day: 18, calendar };
 const result = Temporal.PlainDateTime.from(arg);
 TemporalHelpers.assertPlainDateTime(result, 1976, 11, "M11", 18, 0, 0, 0, 0, 0, 0, `Calendar created from string "${calendar}"`);
+assert.sameValue(result.getISOFields().calendar, "iso8601", "calendar slot stores a string");

test/built-ins/Temporal/PlainDateTime/prototype/add/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.add
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.add(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/calendarId/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.calendarid
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.calendarId;
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/day/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.day
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "day");
+Object.defineProperty(Temporal.Calendar.prototype, "day", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("day should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.day;
+
+Object.defineProperty(Temporal.Calendar.prototype, "day", dayOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/dayOfWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.dayofweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOfWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dayOfWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dayOfWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.dayOfWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfWeek", dayOfWeekOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/dayOfYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.dayofyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOfYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dayOfYear");
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dayOfYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.dayOfYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "dayOfYear", dayOfYearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/daysInMonth/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.daysinmonth
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInMonthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInMonth");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInMonth should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.daysInMonth;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", daysInMonthOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/daysInWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.daysinweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.daysInWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInWeek", daysInWeekOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/daysInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.daysinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.daysInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", daysInYearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/equals/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.equals
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.equals(new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321));
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/inLeapYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.inleapyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const inLeapYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "inLeapYear");
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("inLeapYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.inLeapYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", inLeapYearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/month/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.month
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "month");
+Object.defineProperty(Temporal.Calendar.prototype, "month", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("month should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.month;
+
+Object.defineProperty(Temporal.Calendar.prototype, "month", monthOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/monthCode/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.monthcode
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthCodeOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthCode");
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthCode should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.monthCode;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", monthCodeOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/monthsInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.monthsinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthsInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthsInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthsInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.monthsInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", monthsInYearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/since/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateUntilOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateUntil");
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateUntil should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.since(new Temporal.PlainDateTime(1999, 4, 1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", dateUntilOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/subtract/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.subtract
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.subtract(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/toJSON/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.tojson
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.toJSON();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/toLocaleString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.tolocalestring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.toLocaleString();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/toPlainMonthDay/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,34 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.toplainmonthday
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const monthDayFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthDayFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthDayFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.toPlainMonthDay();
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", monthDayFromFieldsOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/toPlainYearMonth/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,34 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.toplainyearmonth
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const yearMonthFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "yearMonthFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "yearMonthFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("yearMonthFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.toPlainYearMonth();
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "yearMonthFromFields", yearMonthFromFieldsOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/toString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.tostring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.toString();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/until/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateUntilOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateUntil");
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateUntil should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.until(new Temporal.PlainDateTime(2001, 6, 13));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", dateUntilOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/weekOfYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.weekofyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const weekOfYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "weekOfYear");
+Object.defineProperty(Temporal.Calendar.prototype, "weekOfYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("weekOfYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.weekOfYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "weekOfYear", weekOfYearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/with/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.with
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const mergeFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "mergeFields");
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("mergeFields should not be looked up");
+  },
+});
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.with({ year: 2001 });
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", mergeFieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/withCalendar/argument-string.js

@@ -20,16 +20,12 @@ TemporalHelpers.assertPlainDateTime(
   "'iso8601' is a recognizable calendar"
 );
 
-const resultCalendar = result.calendar;
-
 assert.sameValue(
-  resultCalendar instanceof Temporal.Calendar,
-  true,
-  "underlying calendar is no longer a plain object"
+  result.getISOFields().calendar,
+  "iso8601",
+  "underlying calendar has changed and calendar slot stores a string"
 );
 
-assert.sameValue(resultCalendar.toString(), "iso8601", "underlying calendar has changed");
-
 assert.throws(
   RangeError,
   () => dt.withCalendar("this will fail"),

test/built-ins/Temporal/PlainDateTime/prototype/withCalendar/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.withcalendar
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.withCalendar("iso8601");
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/withPlainDate/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.withplaindate
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.withPlainDate(new Temporal.PlainDate(2001, 6, 13));
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/year/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.year
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const yearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "year");
+Object.defineProperty(Temporal.Calendar.prototype, "year", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("year should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.year;
+
+Object.defineProperty(Temporal.Calendar.prototype, "year", yearOriginal);

test/built-ins/Temporal/PlainDateTime/prototype/yearOfWeek/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.yearofweek
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const yearOfWeekOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "yearOfWeek");
+Object.defineProperty(Temporal.Calendar.prototype, "yearOfWeek", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("yearOfWeek should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 654, 321, "iso8601");
+instance.yearOfWeek;
+
+Object.defineProperty(Temporal.Calendar.prototype, "yearOfWeek", yearOfWeekOriginal);

test/built-ins/Temporal/PlainMonthDay/from/argument-propertybag-calendar-string.js

@@ -13,3 +13,4 @@ const calendar = "iso8601";
 const arg = { monthCode: "M11", day: 18, calendar };
 const result = Temporal.PlainMonthDay.from(arg);
 TemporalHelpers.assertPlainMonthDay(result, "M11", 18, `Calendar created from string "${calendar}"`);
+assert.sameValue(result.getISOFields().calendar, "iso8601", "calendar slot stores a string");

test/built-ins/Temporal/PlainMonthDay/from/calendar-monthdayfromfields-called-with-options-undefined.js

@@ -1,23 +0,0 @@
-// Copyright (C) 2022 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: >
-    Calendar.monthDayFromFields method is called with undefined as the options
-    value when call originates internally
-features: [Temporal]
----*/
-
-const realMonthDayFromFields = Temporal.Calendar.prototype.monthDayFromFields;
-let monthDayFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.monthDayFromFields = function (fields, options) {
-  monthDayFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "monthDayFromFields shouldn't be called with options");
-  return realMonthDayFromFields.call(this, fields, options);
-}
-
-Temporal.PlainMonthDay.from("2000-05-02");
-assert.sameValue(monthDayFromFieldsCallCount, 1);
-
-Temporal.Calendar.prototype.monthDayFromFields = realMonthDayFromFields;

test/built-ins/Temporal/PlainMonthDay/prototype/calendarId/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.calendarid
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.calendarId;
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/day/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.day
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dayOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "day");
+Object.defineProperty(Temporal.Calendar.prototype, "day", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("day should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.day;
+
+Object.defineProperty(Temporal.Calendar.prototype, "day", dayOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/equals/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.equals
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.equals(new Temporal.PlainMonthDay(5, 2));
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/equals/calendar-monthdayfromfields-called-with-options-undefined.js

@@ -14,20 +14,3 @@ const calendar = TemporalHelpers.calendarFromFieldsUndefinedOptions();
 const instance = new Temporal.PlainMonthDay(5, 2, calendar);
 instance.equals({ monthCode: "M05", day: 3, calendar });
 assert.sameValue(calendar.monthDayFromFieldsCallCount, 1);
-
-// Test again, but overriding the global Temporal.Calendar.prototype method so
-// we can observe the call to monthDayFromFields() on the ISO8601 calendar
-// that occurs when we parse the string
-
-const realMonthDayFromFields = Temporal.Calendar.prototype.monthDayFromFields;
-let monthDayFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.monthDayFromFields = function (fields, options) {
-  monthDayFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "monthDayFromFields shouldn't be called with options");
-  return realMonthDayFromFields.call(this, fields, options);
-}
-
-instance.equals("2000-05-03");
-assert.sameValue(monthDayFromFieldsCallCount, 1);
-
-Temporal.Calendar.prototype.monthDayFromFields = realMonthDayFromFields;

test/built-ins/Temporal/PlainMonthDay/prototype/monthCode/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.monthcode
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthCodeOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthCode");
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthCode should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.monthCode;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", monthCodeOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/toJSON/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.tojson
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.toJSON();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/toLocaleString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.tolocalestring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.toLocaleString(undefined, { calendar: "iso8601" });
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/toPlainDate/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.toplaindate
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const mergeFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "mergeFields");
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("mergeFields should not be looked up");
+  },
+});
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.toPlainDate({ year: 2002 });
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", mergeFieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/toString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainmonthday.prototype.tostring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.toString();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainMonthDay/prototype/with/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2023 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: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const mergeFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "mergeFields");
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("mergeFields should not be looked up");
+  },
+});
+const monthDayFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthDayFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthDayFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainMonthDay(5, 2, "iso8601", 1972);
+instance.with({ monthCode: "M06" });
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", mergeFieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "monthDayFromFields", monthDayFromFieldsOriginal);

test/built-ins/Temporal/PlainYearMonth/compare/argument-propertybag-calendar-string.js

@@ -4,9 +4,19 @@
 /*---
 esid: sec-temporal.plainyearmonth.compare
 description: A calendar ID is valid input for Calendar
+includes: [temporalHelpers.js]
 features: [Temporal]
 ---*/
 
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
 const calendar = "iso8601";
 
 const arg = { year: 2019, monthCode: "M06", calendar };
@@ -16,3 +26,5 @@ assert.sameValue(result1, 0, `Calendar created from string "${arg}" (first argum
 
 const result2 = Temporal.PlainYearMonth.compare(new Temporal.PlainYearMonth(2019, 6), arg);
 assert.sameValue(result2, 0, `Calendar created from string "${arg}" (second argument)`);
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);

test/built-ins/Temporal/PlainYearMonth/compare/calendar-yearmonthfromfields-called-with-options-undefined.js

@@ -13,20 +13,3 @@ features: [Temporal]
 const calendar = TemporalHelpers.calendarFromFieldsUndefinedOptions();
 Temporal.PlainYearMonth.compare({ year: 2000, month: 5, calendar }, { year: 2000, month: 6, calendar });
 assert.sameValue(calendar.yearMonthFromFieldsCallCount, 2);
-
-// Test again, but overriding the global Temporal.Calendar.prototype method so
-// we can observe the call to yearMonthFromFields() on the ISO8601 calendar
-// that occurs when we parse the string
-
-const realYearMonthFromFields = Temporal.Calendar.prototype.yearMonthFromFields;
-let yearMonthFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.yearMonthFromFields = function (fields, options) {
-  yearMonthFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "yearMonthFromFields shouldn't be called with options");
-  return realYearMonthFromFields.call(this, fields, options);
-}
-
-Temporal.PlainYearMonth.compare("2000-05-01", "2000-06-01");
-assert.sameValue(yearMonthFromFieldsCallCount, 2);
-
-Temporal.Calendar.prototype.yearMonthFromFields = realYearMonthFromFields;

test/built-ins/Temporal/PlainYearMonth/from/argument-propertybag-calendar-string.js

@@ -13,3 +13,4 @@ const calendar = "iso8601";
 const arg = { year: 2019, monthCode: "M06", calendar };
 const result = Temporal.PlainYearMonth.from(arg);
 TemporalHelpers.assertPlainYearMonth(result, 2019, 6, "M06", `Calendar created from string "${calendar}"`);
+assert.sameValue(result.getISOFields().calendar, "iso8601", "calendar slot stores a string");

test/built-ins/Temporal/PlainYearMonth/from/calendar-yearmonthfromfields-called-with-options-undefined.js

@@ -1,23 +0,0 @@
-// Copyright (C) 2022 Igalia, S.L. All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-esid: sec-temporal.plainyearmonth.from
-description: >
-    Calendar.yearMonthFromFields method is called with undefined as the options
-    value when call originates internally
-features: [Temporal]
----*/
-
-const realYearMonthFromFields = Temporal.Calendar.prototype.yearMonthFromFields;
-let yearMonthFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.yearMonthFromFields = function (fields, options) {
-  yearMonthFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "yearMonthFromFields shouldn't be called with options");
-  return realYearMonthFromFields.call(this, fields, options);
-}
-
-Temporal.PlainYearMonth.from("2000-05-01");
-assert.sameValue(yearMonthFromFieldsCallCount, 1);
-
-Temporal.Calendar.prototype.yearMonthFromFields = realYearMonthFromFields;

test/built-ins/Temporal/PlainYearMonth/prototype/add/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.add
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.add(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/add/calendar-datefromfields-called.js

@@ -78,7 +78,7 @@ class CustomCalendar extends Temporal.Calendar {
   }
   dateAdd(date, duration, options) {
     if (duration.months) throw new Error("adding months not implemented in this test");
-    return super.dateAdd(date, duration, options);
+    return super.dateAdd(date, duration, options).withCalendar(this);
   }
   toString() {
     return "thirty-six";

test/built-ins/Temporal/PlainYearMonth/prototype/calendarId/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.calendarid
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.calendarId;
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/daysInMonth/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.daysinmonth
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInMonthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInMonth");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInMonth should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.daysInMonth;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInMonth", daysInMonthOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/daysInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.daysinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const daysInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "daysInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("daysInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.daysInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "daysInYear", daysInYearOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/equals/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.equals
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.equals(new Temporal.PlainYearMonth(2000, 5));
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/equals/calendar-yearmonthfromfields-called-with-options-undefined.js

@@ -14,22 +14,3 @@ let calendar = TemporalHelpers.calendarFromFieldsUndefinedOptions();
 let instance = new Temporal.PlainYearMonth(2000, 5, calendar);
 instance.equals({ year: 2000, month: 6, calendar });
 assert.sameValue(calendar.yearMonthFromFieldsCallCount, 1);
-
-// Test again, but overriding the global Temporal.Calendar.prototype method so
-// we can observe the call to yearMonthFromFields() on the ISO8601 calendar
-// that occurs when we parse the string
-
-const realYearMonthFromFields = Temporal.Calendar.prototype.yearMonthFromFields;
-let yearMonthFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.yearMonthFromFields = function (fields, options) {
-  yearMonthFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "yearMonthFromFields shouldn't be called with options");
-  return realYearMonthFromFields.call(this, fields, options);
-}
-
-calendar = new Temporal.Calendar("iso8601");
-instance = new Temporal.PlainYearMonth(2000, 5, calendar);
-instance.equals("2000-06-01");
-assert.sameValue(yearMonthFromFieldsCallCount, 1);
-
-Temporal.Calendar.prototype.yearMonthFromFields = realYearMonthFromFields;

test/built-ins/Temporal/PlainYearMonth/prototype/inLeapYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.inleapyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const inLeapYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "inLeapYear");
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("inLeapYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.inLeapYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "inLeapYear", inLeapYearOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/month/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.month
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "month");
+Object.defineProperty(Temporal.Calendar.prototype, "month", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("month should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.month;
+
+Object.defineProperty(Temporal.Calendar.prototype, "month", monthOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/monthCode/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.monthcode
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthCodeOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthCode");
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthCode should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.monthCode;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthCode", monthCodeOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/monthsInYear/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.monthsinyear
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const monthsInYearOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "monthsInYear");
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("monthsInYear should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.monthsInYear;
+
+Object.defineProperty(Temporal.Calendar.prototype, "monthsInYear", monthsInYearOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/since/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.since
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateUntilOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateUntil");
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateUntil should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.since(new Temporal.PlainYearMonth(1999, 4));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateUntil", dateUntilOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/since/calendar-yearmonthfromfields-called-with-options-undefined.js

@@ -14,22 +14,3 @@ let calendar = TemporalHelpers.calendarFromFieldsUndefinedOptions();
 let instance = new Temporal.PlainYearMonth(2000, 5, calendar);
 instance.since({ year: 2000, month: 6, calendar });
 assert.sameValue(calendar.yearMonthFromFieldsCallCount, 1);
-
-// Test again, but overriding the global Temporal.Calendar.prototype method so
-// we can observe the call to yearMonthFromFields() on the ISO8601 calendar
-// that occurs when we parse the string
-
-const realYearMonthFromFields = Temporal.Calendar.prototype.yearMonthFromFields;
-let yearMonthFromFieldsCallCount = 0;
-Temporal.Calendar.prototype.yearMonthFromFields = function (fields, options) {
-  yearMonthFromFieldsCallCount++;
-  assert.sameValue(options, undefined, "yearMonthFromFields shouldn't be called with options");
-  return realYearMonthFromFields.call(this, fields, options);
-}
-
-calendar = new Temporal.Calendar("iso8601");
-instance = new Temporal.PlainYearMonth(2000, 5, calendar);
-instance.since("2000-06-01");
-assert.sameValue(yearMonthFromFieldsCallCount, 1);
-
-Temporal.Calendar.prototype.yearMonthFromFields = realYearMonthFromFields;

test/built-ins/Temporal/PlainYearMonth/prototype/subtract/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.subtract
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const dateAddOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateAdd");
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateAdd should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.subtract(new Temporal.Duration(1));
+
+Object.defineProperty(Temporal.Calendar.prototype, "dateAdd", dateAddOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/subtract/calendar-datefromfields-called.js

@@ -78,7 +78,7 @@ class CustomCalendar extends Temporal.Calendar {
   }
   dateAdd(date, duration, options) {
     if (duration.months) throw new Error("adding months not implemented in this test");
-    return super.dateAdd(date, duration, options);
+    return super.dateAdd(date, duration, options).withCalendar(this);
   }
   toString() {
     return "thirty-six";

test/built-ins/Temporal/PlainYearMonth/prototype/toJSON/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.tojson
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.toJSON();
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/toLocaleString/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.tolocalestring
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const idOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "id");
+Object.defineProperty(Temporal.Calendar.prototype, "id", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("id should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.toLocaleString(undefined, { calendar: "iso8601" });
+
+Object.defineProperty(Temporal.Calendar.prototype, "id", idOriginal);

test/built-ins/Temporal/PlainYearMonth/prototype/toPlainDate/builtin-calendar-no-observable-calls.js

@@ -0,0 +1,43 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.toplaindate
+description: >
+  Calling the method on an instance constructed with a builtin calendar causes
+  no observable lookups or calls to calendar methods.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const fieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "fields");
+Object.defineProperty(Temporal.Calendar.prototype, "fields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("fields should not be looked up");
+  },
+});
+const mergeFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "mergeFields");
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("mergeFields should not be looked up");
+  },
+});
+const dateFromFieldsOriginal = Object.getOwnPropertyDescriptor(Temporal.Calendar.prototype, "dateFromFields");
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", {
+  configurable: true,
+  enumerable: false,
+  get() {
+    TemporalHelpers.assertUnreachable("dateFromFields should not be looked up");
+  },
+});
+
+const instance = new Temporal.PlainYearMonth(2000, 5, "iso8601", 1);
+instance.toPlainDate({ day: 12 });
+
+Object.defineProperty(Temporal.Calendar.prototype, "fields", fieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "mergeFields", mergeFieldsOriginal);
+Object.defineProperty(Temporal.Calendar.prototype, "dateFromFields", dateFromFieldsOriginal);