From cd616a11793a37b14b0fca23973c78ce2c40b4ba Mon Sep 17 00:00:00 2001
From: Ms2ger <Ms2ger@gmail.com>
Date: Mon, 28 Jun 2021 14:48:50 +0200
Subject: [PATCH] Editorial: Avoid dividing by zero in ZonedDateTime#round.

Ref. #1502.
---
 polyfill/lib/zoneddatetime.mjs                |  3 +++
 .../ZonedDateTime/prototype/round/div-zero.js | 23 +++++++++++++++++++
 spec/zoneddatetime.html                       |  2 ++
 3 files changed, 28 insertions(+)
 create mode 100644 polyfill/test/ZonedDateTime/prototype/round/div-zero.js

diff --git a/polyfill/lib/zoneddatetime.mjs b/polyfill/lib/zoneddatetime.mjs
index f001f78db1..e344afd96c 100644
--- a/polyfill/lib/zoneddatetime.mjs
+++ b/polyfill/lib/zoneddatetime.mjs
@@ -644,6 +644,9 @@ export class ZonedDateTime {
     const instantStart = ES.BuiltinTimeZoneGetInstantFor(timeZone, dtStart, 'compatible');
     const endNs = ES.AddZonedDateTime(instantStart, timeZone, calendar, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0);
     const dayLengthNs = endNs.subtract(GetSlot(instantStart, EPOCHNANOSECONDS));
+    if (dayLengthNs.isZero()) {
+      throw new RangeError('can not round a ZonedDateTime in a calendar with zero-length days');
+    }
     ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = ES.RoundISODateTime(
       year,
       month,
diff --git a/polyfill/test/ZonedDateTime/prototype/round/div-zero.js b/polyfill/test/ZonedDateTime/prototype/round/div-zero.js
new file mode 100644
index 0000000000..e56300836b
--- /dev/null
+++ b/polyfill/test/ZonedDateTime/prototype/round/div-zero.js
@@ -0,0 +1,23 @@
+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.round
+features: [Temporal]
+---*/
+
+class Calendar extends Temporal.Calendar {
+  constructor() {
+    super("iso8601")
+  }
+  dateAdd(d) {
+    return d;
+  }
+};
+
+const zdt = new Temporal.ZonedDateTime(0n, "UTC", new Calendar());
+
+const units = ["day", "hour", "minute", "second", "millisecond", "microsecond", "nanosecond"];
+for (const smallestUnit of units) {
+  assert.throws(RangeError, () => zdt.round({ smallestUnit }));
+}
diff --git a/spec/zoneddatetime.html b/spec/zoneddatetime.html
index 4e669de321..c227694020 100644
--- a/spec/zoneddatetime.html
+++ b/spec/zoneddatetime.html
@@ -789,6 +789,8 @@ <h1>Temporal.ZonedDateTime.prototype.round ( _options_ )</h1>
         1. Let _startNs_ be _instantStart_.[[Nanoseconds]].
         1. Let _endNs_ be ? AddZonedDateTime(_startNs_, _timeZone_, _zonedDateTime_.[[Calendar]], 0, 0, 0, 1, 0, 0, 0, 0, 0, 0).
         1. Let _dayLengthNs_ be _endNs_ − _startNs_.
+        1. If _dayLengthNs_ is 0, then
+          1. Throw a *RangeError* exception.
         1. Let _roundResult_ be ? RoundISODateTime(_temporalDateTime_.[[ISOYear]], _temporalDateTime_.[[ISOMonth]], _temporalDateTime_.[[ISODay]], _temporalDateTime_.[[ISOHour]], _temporalDateTime_.[[ISOMinute]], _temporalDateTime_.[[ISOSecond]], _temporalDateTime_.[[ISOMillisecond]], _temporalDateTime_.[[ISOMicrosecond]], _temporalDateTime_.[[ISONanosecond]], _roundingIncrement_, _smallestUnit_, _roundingMode_, _dayLengthNs_).
         1. Let _offsetNanoseconds_ be ? GetOffsetNanosecondsFor(_timeZone_, _instant_).
         1. Let _epochNanoseconds_ be ? InterpretISODateTimeOffset(_roundResult_.[[Year]], _roundResult_.[[Month]], _roundResult_.[[Day]], _roundResult_.[[Hour]], _roundResult_.[[Minute]], _roundResult_.[[Second]], _roundResult_.[[Millisecond]], _roundResult_.[[Microsecond]], _roundResult_.[[Nanosecond]], _offsetNanoseconds_, _timeZone_, *"compatible"*, *"prefer"*).