Editorial: Add early return in CalendarDateUntil (#3218)

This change makes it possible to change a check to an assertion in the
polyfill.

Author: catamorphism

polyfill/lib/calendar.mjs

@@ -1055,10 +1055,8 @@ const nonIsoHelperBase = {
       case 'year': {
         // Sign is -1 if calendarTwo < calendarOne, 1 if calendarTwo > calendarOne
         const sign = this.compareCalendarDates(calendarTwo, calendarOne);
-        // If dates are equal, return 0 date duration
-        if (!sign) {
-          return { years: 0, months: 0, weeks: 0, days: 0 };
-        }
+        // If dates were equal, this would have been checked in CalendarDateUntil
+        assert(sign, 'equal dates should have been checked earlier');
         // Take the difference between the years of the two dates
         const diffYears = calendarTwo.year - calendarOne.year;
         // Take the difference between the days of the two dates

polyfill/lib/ecmascript.mjs

@@ -1724,6 +1724,9 @@ export function CalendarDateAdd(calendar, isoDate, dateDuration, overflow) {
 }
 
 export function CalendarDateUntil(calendar, isoDate, isoOtherDate, largestUnit) {
+  if (CompareISODate(isoDate, isoOtherDate) === 0) {
+    return { years: 0, months: 0, weeks: 0, days: 0 };
+  }
   return calendarImplForID(calendar).dateUntil(isoDate, isoOtherDate, largestUnit);
 }
 

spec/calendar.html

@@ -630,9 +630,11 @@ <h1>
         </dd>
       </dl>
       <emu-alg>
+        1. Let _sign_ be CompareISODate(_one_, _two_).
+        1. If _sign_ is 0, then
+          1. Return ZeroDateDuration().
         1. If _calendar_ is *"iso8601"*, then
-          1. Let _sign_ be -CompareISODate(_one_, _two_).
-          1. If _sign_ = 0, return ZeroDateDuration().
+          1. Set _sign_ to -sign.
           1. Let _years_ be 0.
           1. If _largestUnit_ is ~year~, then
             1. Let _candidateYears_ be _sign_.