MDEV-25672 table alias from previous statement interferes later commands

only perform the "correct table name" check for *new* generated columns,
but not for already existing ones - they're guaranteed to be valid

mysql-test/suite/vcol/r/vcol_syntax.result

-drop table if exists t1;
 set @OLD_SQL_MODE=@@SESSION.SQL_MODE;
 create table t1 (a int, b int generated always  as (a+1));
 show create table t1;
@@ -88,3 +87,13 @@ create table t1 (x int, y int default test2.t1.x);
 ERROR 42S22: Unknown column '`test2`.`t1`.`x`' in 'DEFAULT'
 create table t1 (x int, check (test2.t1.x > 0));
 ERROR 42S22: Unknown column '`test2`.`t1`.`x`' in 'CHECK'
+#
+# MDEV-25672 table alias from previous statement interferes later commands
+#
+create table t1 (a int, v_a int generated always as (a));
+update t1 as x set a = 1;
+alter table t1 force;
+drop table t1;
+#
+# End of 10.2 tests
+#

mysql-test/suite/vcol/t/vcol_syntax.test

 #
 # test syntax
 #
---disable_warnings
-drop table if exists t1;
---enable_warnings
-
 set @OLD_SQL_MODE=@@SESSION.SQL_MODE;
 create table t1 (a int, b int generated always  as (a+1));
 show create table t1;
@@ -72,3 +68,16 @@ create table t1 (x int, y int check (y > test2.t1.x));
 create table t1 (x int, y int default test2.t1.x);
 --error ER_BAD_FIELD_ERROR
 create table t1 (x int, check (test2.t1.x > 0));
+
+--echo #
+--echo # MDEV-25672 table alias from previous statement interferes later commands
+--echo #
+create table t1 (a int, v_a int generated always as (a));
+update t1 as x set a = 1;
+alter table t1 force;
+drop table t1;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/item.h

@@ -2841,7 +2841,7 @@ class Item_field :public Item_ident,
   bool check_table_name_processor(void *arg)
   {
     Check_table_name_prm &p= *(Check_table_name_prm *) arg;
-    if (p.table_name.length && table_name)
+    if (!field && p.table_name.length && table_name)
     {
       DBUG_ASSERT(p.db.length);
       if ((db_name &&