MDEV-22854 Garbage returned with SELECT CASE..DEFAULT(timestamp_field_with_now_as_default)

Item_default_value did not override val_native(), so the inherited
Item_field::val_native() was called. As a result Item_default_value::calculate()
was not called and Item_field::val_native() was called on a Field
with a non-initialized ptr.

Implementing Item_default_value::val_native() properly.

mysql-test/main/type_timestamp.result

@@ -1308,5 +1308,17 @@ Warnings:
 Warning	1292	Incorrect datetime value: '2010-00-01 00:00:00'
 DROP TABLE t1;
 #
+# MDEV-22854 Garbage returned with SELECT CASE..DEFAULT(timestamp_field_with_now_as_default)
+#
+SET timestamp=UNIX_TIMESTAMP('2001-01-01 10:20:30.456789');
+CREATE TABLE t1 (a TIMESTAMP(3) DEFAULT CURRENT_TIMESTAMP);
+INSERT INTO t1 VALUES ('2019-02-23 11:31:04'),('2023-02-09 00:00:00');
+SELECT CASE WHEN a THEN DEFAULT(a) END FROM t1;
+CASE WHEN a THEN DEFAULT(a) END
+2001-01-01 10:20:30.456
+2001-01-01 10:20:30.456
+DROP TABLE t1;
+SET timestamp=DEFAULT;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_timestamp.test

@@ -863,6 +863,18 @@ SELECT * FROM t1 WHERE c='2010-00-01 00:00:00';
 DROP TABLE t1;
 
 
+--echo #
+--echo # MDEV-22854 Garbage returned with SELECT CASE..DEFAULT(timestamp_field_with_now_as_default)
+--echo #
+
+SET timestamp=UNIX_TIMESTAMP('2001-01-01 10:20:30.456789');
+CREATE TABLE t1 (a TIMESTAMP(3) DEFAULT CURRENT_TIMESTAMP);
+INSERT INTO t1 VALUES ('2019-02-23 11:31:04'),('2023-02-09 00:00:00');
+SELECT CASE WHEN a THEN DEFAULT(a) END FROM t1;
+DROP TABLE t1;
+SET timestamp=DEFAULT;
+
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/item.cc

@@ -9338,6 +9338,12 @@ void Item_default_value::calculate()
   DEBUG_SYNC(field->table->in_use, "after_Item_default_value_calculate");
 }
 
+bool Item_default_value::val_native(THD *thd, Native *to)
+{
+  calculate();
+  return Item_field::val_native(thd, to);
+}
+
 String *Item_default_value::val_str(String *str)
 {
   calculate();

sql/item.h

@@ -6320,6 +6320,7 @@ class Item_default_value : public Item_field
   longlong val_int();
   my_decimal *val_decimal(my_decimal *decimal_value);
   bool get_date(THD *thd, MYSQL_TIME *ltime,date_mode_t fuzzydate);
+  bool val_native(THD *thd, Native *to);
   bool send(Protocol *protocol, st_value *buffer);
   int save_in_field(Field *field_arg, bool no_conversions);
   bool save_in_param(THD *thd, Item_param *param)