MDEV-32337 Assertion `pos < table->n_def' failed in dict_table_get_nth_col

While checking for altered column in foreign key constraints,
InnoDB fails to ignore virtual columns. This issue caused
by commit 5f09b53(MDEV-31086).

Author: Thirunarayanan

mysql-test/suite/innodb/r/fk_col_alter.result

@@ -94,4 +94,15 @@ ALTER TABLE t2 MODIFY b VARCHAR(16), ADD KEY(b);
 ERROR 42S02: Table 'test.t2' doesn't exist
 DROP TABLE t2, t1;
 ERROR 42S02: Unknown table 'test.t2'
+#
+#  MDEV-32337  Assertion `pos < table->n_def' failed
+#		in dict_table_get_nth_col
+#
+CREATE TABLE t (a INT, va INT AS (a), b INT, vb INT AS (b),
+c INT, vc INT AS (c), vf VARCHAR(16) AS (f),
+f VARCHAR(4)) ENGINE=InnoDB;
+ALTER TABLE t MODIFY f VARCHAR(8);
+ALTER TABLE t MODIFY vf VARCHAR(18);
+ERROR HY000: This is not yet supported for generated columns
+DROP TABLE t;
 # End of 10.4 tests

mysql-test/suite/innodb/t/fk_col_alter.test

@@ -126,4 +126,17 @@ SET SESSION FOREIGN_KEY_CHECKS = ON;
 ALTER TABLE t2 MODIFY b VARCHAR(16), ADD KEY(b);
 --error ER_BAD_TABLE_ERROR
 DROP TABLE t2, t1;
+
+--echo #
+--echo #  MDEV-32337  Assertion `pos < table->n_def' failed
+--echo #		in dict_table_get_nth_col
+--echo #
+CREATE TABLE t (a INT, va INT AS (a), b INT, vb INT AS (b),
+		c INT, vc INT AS (c), vf VARCHAR(16) AS (f),
+		f VARCHAR(4)) ENGINE=InnoDB;
+ALTER TABLE t MODIFY f VARCHAR(8);
+# Altering the virtual column is not supported
+--error ER_UNSUPPORTED_ACTION_ON_GENERATED_COLUMN
+ALTER TABLE t MODIFY vf VARCHAR(18);
+DROP TABLE t;
 --echo # End of 10.4 tests

storage/innobase/handler/handler0alter.cc

@@ -8333,8 +8333,17 @@ ha_innobase::prepare_inplace_alter_table(
 	if (ha_alter_info->handler_flags
 			& ALTER_COLUMN_TYPE_CHANGE_BY_ENGINE) {
 
-		for (uint i= 0; i < table->s->fields; i++) {
+		for (uint i= 0, n_v_col= 0; i < table->s->fields;
+		     i++) {
 			Field* field = table->field[i];
+
+			/* Altering the virtual column is not
+			supported for inplace alter algorithm */
+			if (field->vcol_info) {
+				n_v_col++;
+				continue;
+			}
+
 			for (const Create_field& new_field :
 				ha_alter_info->alter_info->create_list) {
 				if (new_field.field == field) {
@@ -8349,7 +8358,7 @@ ha_innobase::prepare_inplace_alter_table(
 field_changed:
 			const char* col_name= field->field_name.str;
 			dict_col_t *col= dict_table_get_nth_col(
-				m_prebuilt->table, i);
+				m_prebuilt->table, i - n_v_col);
 			if (check_col_is_in_fk_indexes(
 				m_prebuilt->table, col, col_name,
 				span<const dict_foreign_t*>(