Problem in Temporal.PlainDate.prototype.since/until

[FrankYFTang]

While I debug my v8 implementation test breakage I think I found a spec bug. Not 100% sure.
The test is built-ins/Temporal/PlainDate/prototype/since/roundingmode-undefined.js

const earlier = new Temporal.PlainDate(2000, 1, 1);

const later1 = new Temporal.PlainDate(2005, 2, 20);
const explicit1 = later1.since(earlier, { smallestUnit: "year", roundingMode: undefined });

I got a throw in the since line

look at
https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.since

6. Let disallowedUnits be « "hour", "minute", "second", "millisecond", "microsecond", "nanosecond" ».
7. Let smallestUnit be ? ToSmallestTemporalUnit(options, disallowedUnits, "day").
8. Let largestUnit be ? ToLargestTemporalUnit(options, disallowedUnits, "auto", "day").
9. Perform ? ValidateTemporalUnitRange(largestUnit, smallestUnit).

After step 7 smallestUnit will be "year".
After step 8 largestUnit will be "day"
And inside step 9 it will throw.

The spec look similar in until

should we change step 8 to

8. Let largestUnit be ? ToLargestTemporalUnit(options, disallowedUnits, "auto", smallestUnit).
   instead?

@ptomato @ljharb @Ms2ger @jugglinmike @ryzokuken

[ptomato]

At first glance it looks like PlainDate is missing something like

Let defaultLargestUnit be ! LargerOfTwoTemporalUnits("day", smallestUnit).

(we have this in other types such as PlainDateTime, I believe it was intended to have it everywhere as per #827 (comment))