Normative: Disallow negative day lengths

[anba]

User-defined time zones can produce negative day lengths.

class TimeZone extends Temporal.TimeZone {
  #count = 0;
  #nanoseconds;

  constructor(nanoseconds) {
    super("UTC");
    this.#nanoseconds = nanoseconds;
  }
  getPossibleInstantsFor(dateTime) {
    if (++this.#count === 2) {
      return [new Temporal.Instant(this.#nanoseconds)];
    }
    return super.getPossibleInstantsFor(dateTime);
  }
}

function test(epochNanoseconds, tomorrowEpochNanoseconds, testCases) {
  for (let [roundingMode, expected] of Object.entries(testCases)) {
    let timeZone = new TimeZone(tomorrowEpochNanoseconds);
    let zoned = new Temporal.ZonedDateTime(epochNanoseconds, timeZone);
    let result = zoned.round({ smallestUnit: "days", roundingMode });
    console.log(result.epochNanoseconds, expected);
  }
}

const oneDay = 24n * 60n * 60n * 1000n * 1000n * 1000n;

test(3n, -10n, {
  ceil: 0n,
  floor: -oneDay,
  trunc: 0n,
  halfExpand: 0n,
});
test(-3n, -10n, {
  ceil: oneDay,
  floor: 0n,
  trunc: 0n,
  halfExpand: 0n,
});

[codecov]

Codecov Report

Merging #2261 (47a819a) into main (7fe29eb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2261   +/-   ##
=======================================
  Coverage   91.16%   91.16%           
=======================================
  Files          19       19           
  Lines       10524    10524           
  Branches     1688     1688           
=======================================
  Hits         9594     9594           
  Misses        918      918           
  Partials       12       12           

Flag	Coverage Δ
test262	83.19% <ø> (ø)
tests	82.10% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7fe29eb...47a819a. Read the comment docs.

[ptomato]

Thanks, good catch. I'll convert this to draft in advance of presenting it to TC39.

[ptomato]

This change reached consensus in the July 2022 TC39 plenary meeting. Tests are in tc39/test262#3610.

[ptomato]

Whoops, I merged the wrong PR in reference to the above test262 pull request. This change does not yet have tests.

[ptomato]

(To close the loop, now it has tests.)