Normative: change ordering of balancing operations in Temporal.Durati…

…on.prototype.round

If we balance the bigger units before the smaller ones, then we never
properly balance the days that are added from balancing units up to day
length. Changing the order of operations ensures proper balancing of all
units.

Closes: tc39#2508

polyfill/lib/duration.mjs

@@ -310,10 +310,6 @@ export class Duration {
         roundingMode,
         relativeTo
       ));
-    ({ years, months, weeks, days } = ES.BalanceDurationRelative(years, months, weeks, days, largestUnit, relativeTo));
-    if (ES.IsTemporalZonedDateTime(relativeTo)) {
-      relativeTo = ES.MoveRelativeZonedDateTime(relativeTo, years, months, weeks, 0);
-    }
     ({ days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = ES.BalanceDuration(
       days,
       hours,
@@ -325,6 +321,10 @@ export class Duration {
       largestUnit,
       relativeTo
     ));
+    ({ years, months, weeks, days } = ES.BalanceDurationRelative(years, months, weeks, days, largestUnit, relativeTo));
+    if (ES.IsTemporalZonedDateTime(relativeTo)) {
+      relativeTo = ES.MoveRelativeZonedDateTime(relativeTo, years, months, weeks, 0);
+    }
 
     return new Duration(years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
   }

spec/duration.html

@@ -436,11 +436,11 @@ <h1>Temporal.Duration.prototype.round ( _roundTo_ )</h1>
         1. Let _unbalanceResult_ be ? UnbalanceDurationRelative(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _duration_.[[Days]], _largestUnit_, _relativeTo_).
         1. Let _roundResult_ be (? RoundDuration(_unbalanceResult_.[[Years]], _unbalanceResult_.[[Months]], _unbalanceResult_.[[Weeks]], _unbalanceResult_.[[Days]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]], _roundingIncrement_, _smallestUnit_, _roundingMode_, _relativeTo_)).[[DurationRecord]].
         1. Let _adjustResult_ be ? AdjustRoundedDurationDays(_roundResult_.[[Years]], _roundResult_.[[Months]], _roundResult_.[[Weeks]], _roundResult_.[[Days]], _roundResult_.[[Hours]], _roundResult_.[[Minutes]], _roundResult_.[[Seconds]], _roundResult_.[[Milliseconds]], _roundResult_.[[Microseconds]], _roundResult_.[[Nanoseconds]], _roundingIncrement_, _smallestUnit_, _roundingMode_, _relativeTo_).
-        1. Let _balanceResult_ be ? BalanceDurationRelative(_adjustResult_.[[Years]], _adjustResult_.[[Months]], _adjustResult_.[[Weeks]], _adjustResult_.[[Days]], _largestUnit_, _relativeTo_).
+        1. Let _balanceResult_ be ? BalanceDuration(_adjustResult_.[[Days]], _adjustResult_.[[Hours]], _adjustResult_.[[Minutes]], _adjustResult_.[[Seconds]], _adjustResult_.[[Milliseconds]], _adjustResult_.[[Microseconds]], _adjustResult_.[[Nanoseconds]], _largestUnit_, _relativeTo_).
+        1. Let _result_ be ? BalanceDurationRelative(_adjustResult_.[[Years]], _adjustResult_.[[Months]], _adjustResult_.[[Weeks]], _balanceResult_.[[Days]], _largestUnit_, _relativeTo_).
         1. If Type(_relativeTo_) is Object and _relativeTo_ has an [[InitializedTemporalZonedDateTime]] internal slot, then
-          1. Set _relativeTo_ to ? MoveRelativeZonedDateTime(_relativeTo_, _balanceResult_.[[Years]], _balanceResult_.[[Months]], _balanceResult_.[[Weeks]], 0).
-        1. Let _result_ be ? BalanceDuration(_balanceResult_.[[Days]], _adjustResult_.[[Hours]], _adjustResult_.[[Minutes]], _adjustResult_.[[Seconds]], _adjustResult_.[[Milliseconds]], _adjustResult_.[[Microseconds]], _adjustResult_.[[Nanoseconds]], _largestUnit_, _relativeTo_).
-        1. Return ! CreateTemporalDuration(_balanceResult_.[[Years]], _balanceResult_.[[Months]], _balanceResult_.[[Weeks]], _result_.[[Days]], _result_.[[Hours]], _result_.[[Minutes]], _result_.[[Seconds]], _result_.[[Milliseconds]], _result_.[[Microseconds]], _result_.[[Nanoseconds]]).
+          1. Set _relativeTo_ to ? MoveRelativeZonedDateTime(_relativeTo_, _result_.[[Years]], _result_.[[Months]], _result_.[[Weeks]], 0).
+        1. Return ! CreateTemporalDuration(_result_.[[Years]], _result_.[[Months]], _result_.[[Weeks]], _result_.[[Days]], _balanceResult_.[[Hours]], _balanceResult_.[[Minutes]], _balanceResult_.[[Seconds]], _balanceResult_.[[Milliseconds]], _balanceResult_.[[Microseconds]], _balanceResult_.[[Nanoseconds]]).
       </emu-alg>
     </emu-clause>