MDEV-24072 Assertion 'ib_table.n_v_cols' failed in instant_alter_column_possible()

instant_alter_column_possible(): Relax a too strict debug assertion.
The existence of an index stub or a corrupted index on virtual columns
does not imply that virtual columns exist.

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

@@ -436,4 +436,16 @@ check table t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 drop table t1;
+#
+# MDEV-24072 Assertion 'ib_table.n_v_cols' failed
+# in instant_alter_column_possible()
+#
+CREATE TABLE t (a BLOB) ENGINE=InnoDB;
+INSERT INTO t VALUES ('a');
+ALTER TABLE t ADD c INT GENERATED ALWAYS AS (a+1) VIRTUAL, ADD KEY(c);
+ERROR 22007: Truncated incorrect DOUBLE value: 'a'
+ALTER TABLE t ADD d INT;
+affected rows: 0
+info: Records: 0  Duplicates: 0  Warnings: 0
+DROP TABLE t;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

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

@@ -453,4 +453,17 @@ select * from t1;
 check table t1;
 drop table t1;
 
+--echo #
+--echo # MDEV-24072 Assertion 'ib_table.n_v_cols' failed
+--echo # in instant_alter_column_possible()
+--echo #
+CREATE TABLE t (a BLOB) ENGINE=InnoDB;
+INSERT INTO t VALUES ('a');
+--error ER_TRUNCATED_WRONG_VALUE
+ALTER TABLE t ADD c INT GENERATED ALWAYS AS (a+1) VIRTUAL, ADD KEY(c);
+--enable_info
+ALTER TABLE t ADD d INT;
+--disable_info
+DROP TABLE t;
+
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

storage/innobase/handler/handler0alter.cc

@@ -1505,7 +1505,8 @@ instant_alter_column_possible(
 		for (const dict_index_t* index = ib_table.indexes.start;
 		     index; index = index->indexes.next) {
 			if (index->has_virtual()) {
-				ut_ad(ib_table.n_v_cols);
+				ut_ad(ib_table.n_v_cols
+				      || index->is_corrupted());
 				return false;
 			}
 		}