Polyfill: clean up ISO parser & align to spec

To make it easier to find actual Test262 gaps, this PR removes
unreachable and unused code in ISO parsing.

It also removes offset normalization code from ParseISODateTime,
to better mach the spec that only normalizes downstream from ISO
string parsing. Doing this exposed a test bug that's fixed in the second
commit of tc39/test262#3877.

polyfill/lib/ecmascript.mjs

@@ -437,23 +437,11 @@ export function ParseISODateTime(isoString) {
   if (match[13]) {
     offset = undefined;
     z = true;
-  } else if (match[14] && match[15]) {
-    const offsetSign = match[14] === '-' || match[14] === '\u2212' ? '-' : '+';
-    const offsetHours = match[15] || '00';
-    const offsetMinutes = match[16] || '00';
-    const offsetSeconds = match[17] || '00';
-    let offsetFraction = match[18] || '0';
-    offset = `${offsetSign}${offsetHours}:${offsetMinutes}`;
-    if (+offsetFraction) {
-      while (offsetFraction.endsWith('0')) offsetFraction = offsetFraction.slice(0, -1);
-      offset += `:${offsetSeconds}.${offsetFraction}`;
-    } else if (+offsetSeconds) {
-      offset += `:${offsetSeconds}`;
-    }
-    if (offset === '-00:00') offset = '+00:00';
+  } else if (match[14]) {
+    offset = match[14];
   }
-  const tzName = match[19];
-  const calendar = processAnnotations(match[20]);
+  const tzName = match[15];
+  const calendar = processAnnotations(match[16]);
   RejectDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
   return {
     year,
@@ -505,7 +493,7 @@ export function ParseTemporalTimeString(isoString) {
     millisecond = ToIntegerOrInfinity(fraction.slice(0, 3));
     microsecond = ToIntegerOrInfinity(fraction.slice(3, 6));
     nanosecond = ToIntegerOrInfinity(fraction.slice(6, 9));
-    processAnnotations(match[14]); // ignore found calendar
+    processAnnotations(match[10]); // ignore found calendar
     if (match[8]) throw new RangeError('Z designator not supported for PlainTime');
   } else {
     let z, hasTime;
@@ -2608,7 +2596,7 @@ export function IsOffsetTimeZoneIdentifier(string) {
 }
 
 export function ParseDateTimeUTCOffset(string) {
-  const match = OFFSET.exec(string);
+  const match = OFFSET_WITH_PARTS.exec(string);
   if (!match) {
     throw new RangeError(`invalid time zone offset: ${string}`);
   }
@@ -5616,6 +5604,7 @@ export function ASCIILowercase(str) {
 }
 
 const OFFSET = new RegExp(`^${PARSE.offset.source}$`);
+const OFFSET_WITH_PARTS = new RegExp(`^${PARSE.offsetWithParts.source}$`);
 
 function bisect(getState, left, right, lstate = getState(left), rstate = getState(right)) {
   left = bigInt(left);

polyfill/lib/regex.mjs

@@ -22,7 +22,9 @@ export const datesplit = new RegExp(
   `(${yearpart.source})(?:-(${monthpart.source})-(${daypart.source})|(${monthpart.source})(${daypart.source}))`
 );
 const timesplit = /(\d{2})(?::(\d{2})(?::(\d{2})(?:[.,](\d{1,9}))?)?|(\d{2})(?:(\d{2})(?:[.,](\d{1,9}))?)?)?/;
-export const offset = /([+\u2212-])([01][0-9]|2[0-3])(?::?([0-5][0-9])(?::?([0-5][0-9])(?:[.,](\d{1,9}))?)?)?/;
+export const offsetWithParts = /([+\u2212-])([01][0-9]|2[0-3])(?::?([0-5][0-9])(?::?([0-5][0-9])(?:[.,](\d{1,9}))?)?)?/;
+export const offset =
+  /((?:[+\u2212-])(?:[01][0-9]|2[0-3])(?::?(?:[0-5][0-9])(?::?(?:[0-5][0-9])(?:[.,](?:\d{1,9}))?)?)?)/;
 const offsetpart = new RegExp(`([zZ])|${offset.source}?`);
 export const offsetIdentifier = /([+\u2212-])([01][0-9]|2[0-3])(?::?([0-5][0-9])?)?/;
 export const annotation = /\[(!)?([a-z_][a-z0-9_-]*)=([A-Za-z0-9]+(?:-[A-Za-z0-9]+)*)\]/g;

polyfill/test/validStrings.mjs

@@ -6,7 +6,6 @@
 
 import assert from 'assert';
 import * as ES from '../lib/ecmascript.mjs';
-import { Instant } from '../lib/instant.mjs';
 
 const timezoneNames = Intl.supportedValuesOf('timeZone');
 const calendarNames = Intl.supportedValuesOf('calendar');
@@ -249,12 +248,7 @@ const temporalSign = withCode(
 );
 const temporalDecimalFraction = fraction;
 function saveOffset(data, result) {
-  // To canonicalize an offset string that may include nanoseconds, we use GetOffsetStringFor
-  const instant = new Instant(0n);
-  const fakeTimeZone = {
-    getOffsetNanosecondsFor: () => ES.ParseDateTimeUTCOffset(result).offsetNanoseconds
-  };
-  data.offset = ES.GetOffsetStringFor(fakeTimeZone, instant);
+  data.offset = ES.ParseDateTimeUTCOffset(result);
 }
 const utcOffsetSubMinutePrecision = withCode(
   seq(
@@ -468,7 +462,13 @@ function fuzzMode(mode) {
       for (let prop of comparisonItems[mode]) {
         let expected = generatedData[prop];
         if (prop !== 'tzName' && prop !== 'offset' && prop !== 'calendar') expected = expected || 0;
-        assert.equal(parsed[prop], expected, prop);
+        if (prop === 'offset' && expected) {
+          const parsedResult = ES.ParseDateTimeUTCOffset(parsed[prop]);
+          assert.equal(parsedResult.offsetNanoseconds, expected.offsetNanoseconds);
+          assert.equal(parsedResult.hasSubMinutePrecision, expected.hasSubMinutePrecision);
+        } else {
+          assert.equal(parsed[prop], expected, prop);
+        }
       }
       console.log(`${fuzzed} => ok`);
     } catch (e) {