Add Temporal half-rounding boundary tests for PlainDate, PlainDateTime, PlainYearMonth, and ZonedDateTime#4996
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…and dayOfWeek The existing rounding mode tests for PlainDate.prototype.since() and PlainDate.prototype.until() use dates that produce ~31.97 months of difference, well above the 0.5 boundary. All half-* rounding modes produce identical results, making it impossible to distinguish halfExpand from halfTrunc, or halfEven from halfCeil. These new tests use dates that produce exactly 0.5 fractional progress (183/366 days in a leap year), causing all nine rounding modes to produce distinct result patterns. The 2.5-year case specifically distinguishes halfEven (rounds to nearest even integer 2) from halfExpand (rounds away from zero to 3). Also adds: - inLeapYear century-year tests (1700, 1800, 1900, 2100, 2200) exercising the 100/400 rule that the basic test does not cover - dayOfWeek tests across all 12 months of a year, since the basic test only checks 7 consecutive days within a single month Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Remove unrelated dayOfWeek and inLeapYear tests. Add RoundRelativeDuration spec references to info blocks. Extend half-boundary coverage to PlainDateTime, PlainYearMonth, and ZonedDateTime (until + since). PlainYearMonth uses June-starting dates because RoundRelativeDuration converts month remainders to days: Jun-Nov = 183 days in a 366-day year span (Jun 2019 - Jun 2020 crossing Feb 29), giving exactly 183/366 = 0.5. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Note
This PR was drafted with the help of Claude Code. Apologies if that's not welcome here — happy to revise anything by hand.
Context
We're building temporal-php, a PHP 8.4 port of the TC39 Temporal API. We run the test262 suite as part of our CI (transpiled to PHP) and also use Infection for mutation testing. Infection systematically modifies source code and checks whether the test suite catches each mutation. Out of 11,983 mutations, several hundred escaped — and many of those escaped because the upstream test262 data doesn't exercise certain code paths. This PR adds tests to close the most impactful gaps we found.
Summary
Eight new test files across four Temporal types, all exercising the exact 0.5 fractional boundary in
RoundRelativeDuration:PlainDate,PlainDateTime,ZonedDateTime(since/anduntil/): Test all 9 rounding modes at exact 0.5 fractional progress (183 days out of a 366-day leap year). The existingroundingmode-*.jstests use dates producing ~2.663 fractional years, where all half-* modes round identically. These new tests include both 1.5-year (odd integer) and 2.5-year (even integer) cases to distinguishhalfEvenfromhalfExpand.PlainYearMonth(since/anduntil/): Same boundary test, but uses June-starting dates becauseRoundRelativeDurationconverts month remainders to days: Jun–Nov = 183 days in the 366-day year span from Jun 2019 to Jun 2020 (crossing Feb 29), giving exactly 183/366 = 0.5.The
untiltests cover the positive direction; thesincetests cover the negative direction (wherehalfExpandandhalfCeildiverge).How the gaps were found
Infection rewrites code like swapping
halfExpandmatch arms, etc. If no test fails, the mutant "escapes." We traced ~80 escaped mutants back to the fact that all half-* rounding modes produce the same output with the current test data (no value near the 0.5 boundary), so entirematcharms can be deleted or swapped without detection.All expected values were verified against V8's
Temporalimplementation viatest262-harnesswithesvu-installed V8 (d8).