Editorial: Include ISO 8601 algorithm in CalendarDateToISO

Move the ISO 8601 code from CalendarDateFromFields and
CalendarYearMonthFromFields into CalendarDateToISO. At this point it just
consists of RegulateISODate anyway.

To achieve this, we also drop the requirement that CalendarDateToISO
checks its return value is in the representable range, instead moving that
to CalendarDateFromFields and CalendarYearMonthFromFields since it is
different in both cases.

See: #2948
Closes: #2998

Author: ptomato

polyfill/lib/calendar.mjs

@@ -138,16 +138,12 @@ impl['iso8601'] = {
   dateFromFields(fields, overflow) {
     requireFields(fields, ['year', 'day']);
     let { year, month, day } = resolveNonLunisolarMonth(fields);
-    ({ year, month, day } = ES.RegulateISODate(year, month, day, overflow));
-    ES.RejectDateRange(year, month, day);
-    return { year, month, day };
+    return ES.RegulateISODate(year, month, day, overflow);
   },
   yearMonthFromFields(fields, overflow) {
     requireFields(fields, ['year']);
     let { year, month } = resolveNonLunisolarMonth(fields);
-    ({ year, month } = ES.RegulateISODate(year, month, 1, overflow));
-    ES.RejectYearMonthRange(year, month);
-    return { year, month, /* reference */ day: 1 };
+    return ES.RegulateISODate(year, month, 1, overflow);
   },
   monthDayFromFields(fields, overflow) {
     requireFields(fields, ['day']);

polyfill/lib/ecmascript.mjs

@@ -1956,11 +1956,15 @@ export function CalendarEquals(one, two) {
 }
 
 export function CalendarDateFromFields(calendar, fields, overflow) {
-  return GetIntrinsic('%calendarImpl%')(calendar).dateFromFields(fields, overflow);
+  const result = GetIntrinsic('%calendarImpl%')(calendar).dateFromFields(fields, overflow);
+  RejectDateRange(result.year, result.month, result.day);
+  return result;
 }
 
 export function CalendarYearMonthFromFields(calendar, fields, overflow) {
-  return GetIntrinsic('%calendarImpl%')(calendar).yearMonthFromFields(fields, overflow);
+  const result = GetIntrinsic('%calendarImpl%')(calendar).yearMonthFromFields(fields, overflow);
+  RejectYearMonthRange(result.year, result.month);
+  return result;
 }
 
 export function CalendarMonthDayFromFields(calendar, fields, overflow) {

spec/calendar.html

@@ -830,12 +830,11 @@ <h1>
         1. If _calendar_ is *"iso8601"*, then
           1. If _fields_.[[Year]] is ~unset~ or _fields_.[[Day]] is ~unset~, throw a *TypeError* exception.
           1. Perform ? ISOResolveMonth(_fields_).
-          1. Assert: _fields_.[[Month]] is not ~unset~.
-          1. Let _result_ be ? RegulateISODate(_fields_.[[Year]], _fields_.[[Month]], _fields_.[[Day]], _overflow_).
-          1. If ISODateWithinLimits(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *false*, throw a *RangeError* exception.
-          1. Return _result_.
-        1. Perform ? CalendarResolveFields(_calendar_, _fields_, ~date~).
-        1. Return ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
+        1. Else,
+          1. Perform ? CalendarResolveFields(_calendar_, _fields_, ~date~).
+        1. Let _result_ be ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
+        1. If ISODateWithinLimits(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *false*, throw a *RangeError* exception.
+        1. Return _result_.
       </emu-alg>
     </emu-clause>
 
@@ -855,18 +854,13 @@ <h1>
         1. If _calendar_ is *"iso8601"*, then
           1. If _fields_.[[Year]] is ~unset~, throw a *TypeError* exception.
           1. Perform ? ISOResolveMonth(_fields_).
-          1. Assert: _fields_.[[Month]] is not ~unset~.
-          1. Let _result_ be ? RegulateISODate(_fields_.[[Year], _fields_.[[Month]], 1, _overflow_).
-          1. If ISOYearMonthWithinLimits(_result_.[[Year]], _result_.[[Month]]) is *false*, throw a *RangeError* exception.
-          1. Return ISO Date Record {
-              [[Year]]: _result_.[[Year]],
-              [[Month]]: _result_.[[Month]],
-              [[Day]]: 1
-            }.
+        1. Else,
+          1. Perform ? CalendarResolveFields(_calendar_, _fields_, ~year-month~).
         1. Let _firstDayIndex_ be the 1-based index of the first day of the month described by _fields_ (i.e., 1 unless the month's first day is skipped by this calendar.)
         1. Set _fields_.[[Day]] to _firstDayIndex_.
-        1. Perform ? CalendarResolveFields(_calendar_, _fields_, ~year-month~).
-        1. Return ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
+        1. Let _result_ be ? CalendarDateToISO(_calendar_, _fields_, _overflow_).
+        1. If ISOYearMonthWithinLimits(_result_.[[Year]], _result_.[[Month]]) is *false*, throw a *RangeError* exception.
+        1. Return _result_.
       </emu-alg>
     </emu-clause>
 
@@ -1116,7 +1110,7 @@ <h1>
     <emu-clause id="sec-temporal-calendardatetoiso" type="implementation-defined abstract operation">
       <h1>
         CalendarDateToISO (
-          _calendar_: a calendar type that is not *"iso8601"*,
+          _calendar_: a calendar type,
           _fields_: a Calendar Fields Record,
           _overflow_: ~constrain~ or ~reject~,
         ): either a normal completion containing an ISO Date Record or a throw completion
@@ -1129,11 +1123,14 @@ <h1>
           For ~constrain~, values that do not form a valid date are clamped to their respective valid range.
         </dd>
       </dl>
-      <p>
-        Like RegulateISODate, the operation throws a *RangeError* exception when _overflow_ is ~reject~ and the date described by _fields_ does not exist.
-        It also throws a *RangeError* exception if the date described by _fields_ is outside the range allowed by ISODateWithinLimits.
-      </p>
-      <p>Clamping an invalid date to the correct range when _overflow_ is ~constrain~ is a behaviour specific to each built-in calendar, but all built-in calendars follow this guideline:</p>
+      <emu-alg>
+        1. If _calendar_ is *"iso8601"*, then
+          1. Assert: _fields_.[[Year]], _fields_.[[Month]], and _fields_.[[Day]] are not ~unset~.
+          1. Return ? RegulateISODate(_fields_.[[Year]], _fields_.[[Month]], _fields_.[[Day]], _overflow_).
+        1. Return an implementation-defined ISO Date Record, or throw a *RangeError* exception, as described below.
+      </emu-alg>
+      <p>Like RegulateISODate, the operation throws a *RangeError* exception when _overflow_ is ~reject~ and the date described by _fields_ does not exist.</p>
+      <p>Clamping an invalid date to the correct range when _overflow_ is ~constrain~ is a behaviour specific to each calendar other than *"iso8601"*, but all calendars follow this guideline:</p>
       <ul>
         <li>Pick the closest day in the same month. If there are two equally-close dates in that month, pick the later one.</li>
         <li>If the month is a leap month that doesn't exist in the year, pick another date according to the cultural conventions of that calendar's users. Usually this will result in the same day in the month before or after where that month would normally fall in a leap year.</li>