Fix date-only parsing failure when local midnight is skipped by DST

gson/src/main/java/com/google/gson/internal/bind/util/ISO8601Utils.java

@@ -178,9 +178,26 @@ public static Date parse(String date, ParsePosition pos) throws ParseException {
       if (!hasT && (date.length() <= offset)) {
         Calendar calendar = new GregorianCalendar(year, month - 1, day);
         calendar.setLenient(false);
-
-        pos.setIndex(offset);
-        return calendar.getTime();
+        try {
+          pos.setIndex(offset);
+          return calendar.getTime();
+        } catch (IllegalArgumentException e) {
+          // The calendar date can be valid while local midnight does not exist in the default
+          // time zone, for example '1966-11-01' in America/Sao_Paulo where clocks were shifted
+          // forward from 0:00 to 1:00. In that case resolve the date like java.time's
+          // LocalDate.atStartOfDay(ZoneId) by using the first existing instant of that day.
+          // Truly invalid dates (such as '2021-02-30') also fail above, but lenient resolution
+          // would shift them to a different day, so they are re-thrown below.
+          Calendar lenientCalendar = new GregorianCalendar(year, month - 1, day);
+          Date resolved = lenientCalendar.getTime();
+          if (lenientCalendar.get(Calendar.YEAR) != year
+              || lenientCalendar.get(Calendar.MONTH) != month - 1
+              || lenientCalendar.get(Calendar.DAY_OF_MONTH) != day) {
+            throw e;
+          }
+          pos.setIndex(offset);
+          return resolved;
+        }
       }
 
       if (hasT) {

gson/src/test/java/com/google/gson/internal/bind/util/ISO8601UtilsTest.java

@@ -43,6 +43,50 @@ private static GregorianCalendar createUtcCalendar() {
     return calendar;
   }
 
+  @Test
+  public void testParseDateOnlyDSTGap() throws ParseException {
+    TimeZone defaultTimeZone = TimeZone.getDefault();
+    try {
+      // 1966-11-01 00:00 does not exist in America/Sao_Paulo because clocks were shifted
+      // forward from 0:00 to 1:00 at midnight; parsing must not fail for this valid date
+      TimeZone.setDefault(TimeZone.getTimeZone("America/Sao_Paulo"));
+      Date date = ISO8601Utils.parse("1966-11-01", new ParsePosition(0));
+
+      GregorianCalendar calendar =
+          new GregorianCalendar(TimeZone.getTimeZone("America/Sao_Paulo"), Locale.US);
+      // Calendar was created with current time, must clear it
+      calendar.clear();
+      calendar.setTime(date);
+      assertThat(calendar.get(Calendar.YEAR)).isEqualTo(1966);
+      assertThat(calendar.get(Calendar.MONTH)).isEqualTo(Calendar.NOVEMBER);
+      assertThat(calendar.get(Calendar.DAY_OF_MONTH)).isEqualTo(1);
+      // The resolved hour depends on the DST rules of the JDK's bundled timezone data
+      // (midnight itself, or the first hour after the 0:00 -> 1:00 transition), so only
+      // assert that the date fields are preserved while the hour stays within the day
+      assertThat(calendar.get(Calendar.HOUR_OF_DAY)).isAnyOf(0, 1);
+    } finally {
+      TimeZone.setDefault(defaultTimeZone);
+    }
+  }
+
+  @Test
+  public void testParseInvalidDateOnlyStillFails() {
+    TimeZone defaultTimeZone = TimeZone.getDefault();
+    try {
+      // Strict parsing introduced for date-only values must keep rejecting dates which do
+      // not exist, even in time zones where midnight is skipped by a DST transition
+      TimeZone.setDefault(TimeZone.getTimeZone("America/Sao_Paulo"));
+      assertThrows(
+          ParseException.class, () -> ISO8601Utils.parse("2021-02-30", new ParsePosition(0)));
+      assertThrows(
+          ParseException.class, () -> ISO8601Utils.parse("2021-13-01", new ParsePosition(0)));
+      assertThrows(
+          ParseException.class, () -> ISO8601Utils.parse("1966-00-01", new ParsePosition(0)));
+    } finally {
+      TimeZone.setDefault(defaultTimeZone);
+    }
+  }
+
   @Test
   public void testDateFormatString() {
     GregorianCalendar calendar = new GregorianCalendar(utcTimeZone(), Locale.US);