Temporal: Add a test for rounding negative instants

test/built-ins/Temporal/Instant/prototype/round/negative-instant.js

@@ -0,0 +1,67 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.round
+description: RoundNumberToIncrementAsIfPositive should multiply the remainder by an extra sign
+             before comparing it
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Instant(-1000000000000000000n);  // 1938-04-24T22:13:20Z
+const roundedDown = new Temporal.Instant(-1000000800000000000n); // 1938-04-24T22:00:00Z
+const roundedUp = new Temporal.Instant(-999997200000000000n); // 1938-04-24T23:00:00Z
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "halfCeil" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with halfCeil rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "halfFloor" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with halfFloor rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "halfExpand" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with halfExpand rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "halfTrunc" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with halfTrunc rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "halfEven" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with halfEven rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "ceil" }).epochNanoseconds,
+  roundedUp.epochNanoseconds,
+  "Rounding with ceil rounds to the next hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "floor" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with floor rounds to the closest hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "expand" }).epochNanoseconds,
+  roundedUp.epochNanoseconds,
+  "Rounding with expand rounds to the next hour"
+);
+
+assert.sameValue(
+  instance.round({ smallestUnit: "hour", roundingIncrement: 1, roundingMode: "trunc" }).epochNanoseconds,
+  roundedDown.epochNanoseconds,
+  "Rounding with trunc rounds to the closest hour"
+);