MDEV-25403 ALTER TABLE wrongly checks for field's default value if AFTER is used

When a column is added to an non-empty table, existing rows will have
a column's default value for existing rows. Or a "zero value" if the
column has no default.

But this check should be skipped when an existing column is altered.

mysql-test/r/alter_table.result

@@ -2517,5 +2517,22 @@ ALTER TABLE t1 ALTER COLUMN k1 SET DEFAULT (SELECT 1 FROM t2 limit 1);
 ERROR HY000: Function or expression 'select ...' cannot be used in the DEFAULT clause of `k1`
 DROP TABLE t1,t2;
 #
+# MDEV-25403 ALTER TABLE wrongly checks for field's default value if AFTER is used
+#
+create table t1(t int, d date not null);
+insert into t1 values (1,'2001-1-1');
+set sql_mode = "no_zero_date";
+alter table t1 change d d date not null after t, add i int;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `t` int(11) DEFAULT NULL,
+  `d` date NOT NULL,
+  `i` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table t1 add x date not null;
+ERROR 22007: Incorrect date value: '0000-00-00' for column `test`.`t1`.`x` at row 1
+drop table t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/alter_table.test

@@ -2044,6 +2044,18 @@ CREATE TABLE t2 (i1 int);
 ALTER TABLE t1 ALTER COLUMN k1 SET DEFAULT (SELECT 1 FROM t2 limit 1);
 DROP TABLE t1,t2;
 
+--echo #
+--echo # MDEV-25403 ALTER TABLE wrongly checks for field's default value if AFTER is used
+--echo #
+create table t1(t int, d date not null);
+insert into t1 values (1,'2001-1-1');
+set sql_mode = "no_zero_date";
+alter table t1 change d d date not null after t, add i int;
+show create table t1;
+--error ER_TRUNCATED_WRONG_VALUE
+alter table t1 add x date not null;
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_table.cc

@@ -7906,7 +7906,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
          def->sql_type == MYSQL_TYPE_NEWDATE ||
          def->sql_type == MYSQL_TYPE_DATETIME ||
          def->sql_type == MYSQL_TYPE_DATETIME2) &&
-         !alter_ctx->datetime_field &&
+         !alter_ctx->datetime_field && !def->field &&
          !(~def->flags & (NO_DEFAULT_VALUE_FLAG | NOT_NULL_FLAG)) &&
          thd->variables.sql_mode & MODE_NO_ZERO_DATE)
     {