Fast-path conversion of Temporal.PlainDate to relativeTo object

The value of a relativeTo property in a property bag is already allowed to
be either a Temporal.PlainDateTime or Temporal.ZonedDateTime. If passing a
Temporal.PlainDate, then create a Temporal.PlainDateTime from its slots as
we do in ToTemporalDateTime.

See: #1428

polyfill/lib/ecmascript.mjs

@@ -861,6 +861,21 @@ export const ES = ObjectAssign({}, ES2020, {
     let year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar, timeZone, offset;
     if (ES.Type(relativeTo) === 'Object') {
       if (ES.IsTemporalZonedDateTime(relativeTo) || ES.IsTemporalDateTime(relativeTo)) return relativeTo;
+      if (ES.IsTemporalDate(relativeTo)) {
+        const TemporalDateTime = GetIntrinsic('%Temporal.PlainDateTime%');
+        return new TemporalDateTime(
+          GetSlot(relativeTo, ISO_YEAR),
+          GetSlot(relativeTo, ISO_MONTH),
+          GetSlot(relativeTo, ISO_DAY),
+          0,
+          0,
+          0,
+          0,
+          0,
+          0,
+          GetSlot(relativeTo, CALENDAR)
+        );
+      }
       calendar = relativeTo.calendar;
       if (calendar === undefined) calendar = ES.GetISO8601Calendar();
       calendar = ES.ToTemporalCalendar(calendar);

spec/abstractops.html

@@ -555,6 +555,8 @@ <h1>ToRelativeTemporalObject ( _options_ )</h1>
       1. If Type(_value_) is Object, then
         1. If _value_ has either an [[InitializedTemporalDateTime]] or [[InitializedTemporalZonedDateTime]] internal slot, then
           1. Return _value_.
+        1. If _value_ has an [[InitializedTemporalDate]] internal slot, then
+          1. Return ? CreateTemporalDateTime(_value_.[[ISOYear]], _value_.[[ISOMonth]], _value_.[[ISODay]], 0, 0, 0, 0, 0, 0, _value_.[[Calendar]]).
         1. Let _calendar_ be ? GetOptionalTemporalCalendar(_value_).
         1. Let _fieldNames_ be ? CalendarFields(_calendar_, « *"day"*, *"month"*, *"monthCode"*, *"year"* »).
         1. Let _fields_ be ? PrepareTemporalFields(_value_, _fieldNames_, «»).