Temporal: Tests for disallowed corner case in rounding durations

This test covers a normative change to disallow an ambiguous corner case
in the options bag for Temporal.Duration.prototype.round().

See tc39/proposal-temporal#2916

test/built-ins/Temporal/Duration/prototype/round/round-and-balance-calendar-units-with-increment-disallowed.js

@@ -0,0 +1,81 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.round
+description: >
+  Disallow rounding to an increment of calendar units >1 if also balancing
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const relativeTo = new Temporal.PlainDate(2024, 1, 1);
+
+const months = Temporal.Duration.from({ months: 9 });
+
+TemporalHelpers.assertDuration(months.round({
+  relativeTo,
+  smallestUnit: "months",
+  roundingIncrement: 8,
+  roundingMode: "ceil",
+}), 0, 16, 0, 0, 0, 0, 0, 0, 0, 0, "OK to round to an increment of months");
+
+TemporalHelpers.assertDuration(months.round({
+  relativeTo,
+  largestUnit: "years",
+  smallestUnit: "months",
+}), 0, 9, 0, 0, 0, 0, 0, 0, 0, 0, "OK to balance to years while rounding to 1 month");
+
+assert.throws(RangeError, () => months.round({
+  relativeTo,
+  largestUnit: "years",
+  smallestUnit: "months",
+  roundingIncrement: 8,
+  roundingMode: "ceil",
+}), "Cannot round to an increment of months while also balancing to years");
+
+const weeks = Temporal.Duration.from({ weeks: 7 });
+
+TemporalHelpers.assertDuration(weeks.round({
+  relativeTo,
+  smallestUnit: "weeks",
+  roundingIncrement: 6,
+  roundingMode: "ceil",
+}), 0, 0, 12, 0, 0, 0, 0, 0, 0, 0, "OK to round to an increment of weeks");
+
+TemporalHelpers.assertDuration(weeks.round({
+  relativeTo,
+  largestUnit: "months",
+  smallestUnit: "weeks",
+}), 0, 1, 3, 0, 0, 0, 0, 0, 0, 0, "OK to balance to months while rounding to 1 week");
+
+assert.throws(RangeError, () => weeks.round({
+  relativeTo,
+  largestUnit: "months",
+  smallestUnit: "weeks",
+  roundingIncrement: 6,
+  roundingMode: "ceil",
+}), "Cannot round to an increment of weeks while also balancing to months");
+
+const days = Temporal.Duration.from({ days: 31 });
+
+TemporalHelpers.assertDuration(days.round({
+  relativeTo,
+  smallestUnit: "days",
+  roundingIncrement: 30,
+  roundingMode: "ceil",
+}), 0, 0, 0, 60, 0, 0, 0, 0, 0, 0, "OK to round to an increment of days");
+
+TemporalHelpers.assertDuration(days.round({
+  relativeTo,
+  largestUnit: "weeks",
+  smallestUnit: "days",
+}), 0, 0, 4, 3, 0, 0, 0, 0, 0, 0, "OK to balance to weeks while rounding to 1 day");
+
+assert.throws(RangeError, () => days.round({
+  relativeTo,
+  largestUnit: "weeks",
+  smallestUnit: "days",
+  roundingIncrement: 30,
+  roundingMode: "ceil",
+}), "Cannot round to an increment of days while also balancing to weeks");