LibJS: Add Add IsValidISODate assertions

This is an editorial change in the Temporal spec.

See: https://github.com/tc39/proposal-temporal/commit/46f97ea

Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTime.cpp

@@ -76,21 +76,24 @@ auto const DATETIME_NANOSECONDS_MAX = "8640000086400000000000"_sbigint;
 // 5.5.2 ISODateTimeWithinLimits ( year, month, day, hour, minute, second, millisecond, microsecond, nanosecond ), https://tc39.es/proposal-temporal/#sec-temporal-isodatetimewithinlimits
 bool iso_date_time_within_limits(GlobalObject& global_object, i32 year, u8 month, u8 day, u8 hour, u8 minute, u8 second, u16 millisecond, u16 microsecond, u16 nanosecond)
 {
-    // 1. Let ns be ℝ(GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond)).
+    // 1. Assert: IsValidISODate(year, month, day) is true.
+    VERIFY(is_valid_iso_date(year, month, day));
+
+    // 2. Let ns be ℝ(GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond)).
     auto ns = get_epoch_from_iso_parts(global_object, year, month, day, hour, minute, second, millisecond, microsecond, nanosecond)->big_integer();
 
-    // 2. If ns ≤ nsMinInstant - nsPerDay, then
+    // 3. If ns ≤ nsMinInstant - nsPerDay, then
     if (ns <= DATETIME_NANOSECONDS_MIN) {
         // a. Return false.
         return false;
     }
 
-    // 3. If ns ≥ nsMaxInstant + nsPerDay, then
+    // 4. If ns ≥ nsMaxInstant + nsPerDay, then
     if (ns >= DATETIME_NANOSECONDS_MAX) {
         // a. Return false.
         return false;
     }
-    // 4. Return true.
+    // 5. Return true.
     return true;
 }
 

Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp

@@ -155,6 +155,7 @@ MarkedVector<BigInt*> get_iana_time_zone_epoch_value(GlobalObject& global_object
     // The implementation-defined abstract operation GetIANATimeZoneEpochValue takes arguments timeZoneIdentifier (a String), year (an integer), month (an integer between 1 and 12 inclusive), day (an integer between 1 and 31 inclusive), hour (an integer between 0 and 23 inclusive), minute (an integer between 0 and 59 inclusive), second (an integer between 0 and 59 inclusive), millisecond (an integer between 0 and 999 inclusive), microsecond (an integer between 0 and 999 inclusive), and nanosecond (an integer between 0 and 999 inclusive) and returns a List of BigInts.
     // Each value in the returned List represents a number of nanoseconds since the Unix epoch in UTC that corresponds to the given ISO 8601 calendar date and wall-clock time in the IANA time zone identified by timeZoneIdentifier.
     // When the input represents a local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change), the returned List will have more than one element. The elements are sorted in numerical order. When the input represents a skipped local time at a positive time zone transition (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change), the returned List will be empty. Otherwise, the returned List will have one element.
+    // It is an error to call GetIANATimeZoneEpochValue with arguments such that IsValidISODate(year, month, day) is false.
 
     // FIXME: Implement this properly for non-UTC timezones.
     auto& vm = global_object.vm();