Fix date_trunct for December dates that are in the next year, e.g.:

bmomjian · bmomjian · commit b9fe8ee225f9 · 2005-07-04T14:38:31.000Z
SELECT date_trunc('week', '2002-12-31'::date);

Backpatch to 8.0.X.

Per report from Nick Johnson.
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.128 2005/06/30 03:48:58 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.129 2005/07/04 14:38:31 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2793,9 +2793,12 @@ timestamp_trunc(PG_FUNCTION_ARGS)
 				/*
 				 *	If it is week 52/53 and the month is January,
 				 *	then the week must belong to the previous year.
+				 *	Also, some December dates belong to the next year.
 				 */
 				if (woy >= 52 && tm->tm_mon == 1)
 					--tm->tm_year;
+				if (woy <= 1 && tm->tm_mon == 12)
+					++tm->tm_year;
 				isoweek2date(woy, &(tm->tm_year), &(tm->tm_mon), &(tm->tm_mday));
 				tm->tm_hour = 0;
 				tm->tm_min = 0;
@@ -2924,9 +2927,12 @@ timestamptz_trunc(PG_FUNCTION_ARGS)
 				/*
 				 *	If it is week 52/53 and the month is January,
 				 *	then the week must belong to the previous year.
+				 *	Also, some December dates belong to the next year.
 				 */
 				if (woy >= 52 && tm->tm_mon == 1)
 					--tm->tm_year;
+				if (woy <= 1 && tm->tm_mon == 12)
+					++tm->tm_year;
 				isoweek2date(woy, &(tm->tm_year), &(tm->tm_mon), &(tm->tm_mday));
 				tm->tm_hour = 0;
 				tm->tm_min = 0;
