Editorial: Adjust RangeError condition in ToTemporalInstant

We must avoid calling GetUTCEpochNanoseconds with dates that are too large
because it is ill-defined in ECMA-262 what is supposed to happen. (See
tc39/ecma262#1087). We had a condition in
ToTemporalInstant that was supposed to prevent this, but was incorrect
because subtracting the UTC offset could push the date out of range.

Note that this is editorial, because IsValidEpochNanoseconds would throw
anyway in this case even if GetUTCEpochNanoseconds was fully defined for
large inputs.

See: #2985

polyfill/lib/ecmascript.mjs

@@ -1363,7 +1363,7 @@ export function ToTemporalInstant(item) {
 
   // ParseTemporalInstantString ensures that either `z` is true or or `offset` is non-undefined
   const offsetNanoseconds = z ? 0 : ParseDateTimeUTCOffset(offset);
-  const epochNanoseconds = GetUTCEpochNanoseconds(
+  const balanced = BalanceISODateTime(
     year,
     month,
     day,
@@ -1372,8 +1372,22 @@ export function ToTemporalInstant(item) {
     second,
     millisecond,
     microsecond,
-    nanosecond
-  ).subtract(offsetNanoseconds);
+    nanosecond - offsetNanoseconds
+  );
+  if (MathAbs(ISODateToEpochDays(balanced.year, balanced.month - 1, balanced.day)) > 1e8) {
+    throw new RangeErrorCtor('date/time value is outside the supported range');
+  }
+  const epochNanoseconds = GetUTCEpochNanoseconds(
+    balanced.year,
+    balanced.month,
+    balanced.day,
+    balanced.hour,
+    balanced.minute,
+    balanced.second,
+    balanced.millisecond,
+    balanced.microsecond,
+    balanced.nanosecond
+  );
   ValidateEpochNanoseconds(epochNanoseconds);
   return new TemporalInstant(epochNanoseconds);
 }

spec/instant.html

@@ -412,12 +412,13 @@ <h1>
         1. Let _parsed_ be ? ParseISODateTime(_item_, « |TemporalInstantString| »).
         1. Assert: Either _parsed_.[[TimeZone]].[[OffsetString]] is not ~empty~ or _parsed_.[[TimeZone]].[[Z]] is *true*, but not both.
         1. If _parsed_.[[TimeZone]].[[Z]] is *true*, let _offsetNanoseconds_ be 0; otherwise, let _offsetNanoseconds_ be ! ParseDateTimeUTCOffset(_parsed_.[[TimeZone]].[[OffsetString]]).
-        1. If abs(ISODateToEpochDays(_parsed_.[[Year]], _parsed_.[[Month]] - 1, _parsed_.[[Day]])) > 10<sup>8</sup>, throw a *RangeError* exception.
         1. If _parsed_.[[Time]] is ~start-of-day~, then
           1. Let _time_ be Time Record { [[Days]]: 0, [[Hour]]: 0, [[Minute]]: 0, [[Second]]: 0, [[Millisecond]]: 0, [[Microsecond]]: 0, [[Nanosecond]]: 0 }.
         1. Else,
           1. Let _time_ be _parsed_.[[Time]].
-        1. Let _epochNanoseconds_ be GetUTCEpochNanoseconds(_parsed_.[[Year]], _parsed_.[[Month]], _parsed_.[[Day]], _time_.[[Hour]], _time_.[[Minute]], _time_.[[Second]], _time_.[[Millisecond]], _time_.[[Microsecond]], _time_.[[Nanosecond]]) - ℤ(_offsetNanoseconds_).
+        1. Let _balanced_ be BalanceISODateTime(_parsed_.[[Year]], _parsed_.[[Month]], _parsed_.[[Day]], _time_.[[Hour]], _time_.[[Minute]], _time_.[[Second]], _time_.[[Millisecond]], _time_.[[Microsecond]], _time_.[[Nanosecond]] - _offsetNanoseconds_).
+        1. If abs(ISODateToEpochDays(_balanced_.[[Year]], _balanced_.[[Month]] - 1, _balanced_.[[Day]])) > 10<sup>8</sup>, throw a *RangeError* exception.
+        1. Let _epochNanoseconds_ be GetUTCEpochNanoseconds(_balanced_.[[Year]], _balanced_.[[Month]], _balanced_.[[Day]], _balanced_.[[Hour]], _balanced_.[[Minute]], _balanced_.[[Second]], _balanced_.[[Millisecond]], _balanced_.[[Microsecond]], _balanced_.[[Nanosecond]]).
         1. If IsValidEpochNanoseconds(_epochNanoseconds_) is *false*, throw a *RangeError* exception.
         1. Return ! CreateTemporalInstant(_epochNanoseconds_).
       </emu-alg>