Fast-path conversion of Temporal.PlainDate to Temporal.PlainDateTime

If passing a Temporal.PlainDate where a Temporal.PlainDateTime is
expected, then create a Temporal.PlainDateTime directly from the
PlainDate's [[ISOYear]], [[ISOMonth]], and [[ISODay]] internal slots, and
assume midnight as the time, just as if a property bag with year,
month/monthCode, and day properties had been passed.
Previously, the PlainDate's calendar, year, month, monthCode, and day
getters would have been called, and the time properties would have been
accessed as well.

See: #1428

polyfill/lib/ecmascript.mjs

@@ -1186,6 +1186,21 @@ export const ES = ObjectAssign({}, ES2020, {
     let year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar;
     if (ES.Type(item) === 'Object') {
       if (ES.IsTemporalDateTime(item)) return item;
+      if (ES.IsTemporalDate(item)) {
+        const TemporalPlainDateTime = GetIntrinsic('%Temporal.PlainDateTime%');
+        return new TemporalPlainDateTime(
+          GetSlot(item, ISO_YEAR),
+          GetSlot(item, ISO_MONTH),
+          GetSlot(item, ISO_DAY),
+          0,
+          0,
+          0,
+          0,
+          0,
+          0,
+          GetSlot(item, CALENDAR)
+        );
+      }
 
       calendar = item.calendar;
       if (calendar === undefined) calendar = ES.GetISO8601Calendar();

spec/plaindatetime.html

@@ -903,6 +903,8 @@ <h1>ToTemporalDateTime ( _item_ [ , _options_ ] )</h1>
         1. If Type(_item_) is Object, then
           1. If _item_ has an [[InitializedTemporalDateTime]] internal slot, then
             1. Return _item_.
+          1. If _item_ has an [[InitializedTemporalDate]] internal slot, then
+            1. Return ? CreateTemporalDateTime(_item_.[[ISOYear]], _item_.[[ISOMonth]], _item_.[[ISODay]], 0, 0, 0, 0, 0, 0, _item_.[[Calendar]]).
           1. Let _calendar_ be ? GetOptionalTemporalCalendar(_item_).
           1. Let _fieldNames_ be ? CalendarFields(_calendar_, « *"day"*, *"hour"*, *"microsecond"*, *"millisecond"*, *"minute"*, *"month"*, *"monthCode"*, *"nanosecond"*, *"second"*, *"year"* »).
           1. Let _fields_ be ? PrepareTemporalFields(_item_, _fieldNames_, «»).