MDEV-19272: Assertion unireg_check..Field::NEXT_NUMBER failed

ha_innobase::check_if_supported_inplace_alter(): Relax a too strict
debug assertion when changing a column from NULL to NOT NULL.
InnoDB actually allows instant removal of an AUTO_INCREMENT attribute
since commit 8dffaaef (MDEV-12836).

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

@@ -107,5 +107,15 @@ alter table t1 engine=innodb;
 alter table t1 add column b int;
 drop table t1,t2;
 #
+# MDEV-19272 Assertion unireg_check...Field::NEXT_NUMBER failed
+#
+CREATE TABLE t1 (c INT AUTO_INCREMENT NULL UNIQUE) ENGINE=InnoDB;
+ALTER TABLE t1 MODIFY c INT NOT NULL, ALGORITHM=INPLACE;
+DROP TABLE t1;
+CREATE TABLE t1 (c TIMESTAMP AUTO_INCREMENT UNIQUE) ENGINE=InnoDB;
+ERROR 42000: Incorrect column specifier for column 'c'
+CREATE TABLE t1 (c DATETIME AUTO_INCREMENT UNIQUE) ENGINE=InnoDB;
+ERROR 42000: Incorrect column specifier for column 'c'
+#
 # End of 10.4 tests
 #

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

@@ -109,6 +109,17 @@ alter table t1 engine=innodb;
 alter table t1 add column b int;
 drop table t1,t2;
 
+--echo #
+--echo # MDEV-19272 Assertion unireg_check...Field::NEXT_NUMBER failed
+--echo #
+CREATE TABLE t1 (c INT AUTO_INCREMENT NULL UNIQUE) ENGINE=InnoDB;
+ALTER TABLE t1 MODIFY c INT NOT NULL, ALGORITHM=INPLACE;
+DROP TABLE t1;
+--error ER_WRONG_FIELD_SPEC
+CREATE TABLE t1 (c TIMESTAMP AUTO_INCREMENT UNIQUE) ENGINE=InnoDB;
+--error ER_WRONG_FIELD_SPEC
+CREATE TABLE t1 (c DATETIME AUTO_INCREMENT UNIQUE) ENGINE=InnoDB;
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

storage/innobase/handler/handler0alter.cc

@@ -2408,6 +2408,13 @@ ha_innobase::check_if_supported_inplace_alter(
 				& ALTER_ADD_COLUMN));
 
 		if (const Field* f = cf.field) {
+			/* An AUTO_INCREMENT attribute can only
+			be added to an existing column by ALGORITHM=COPY,
+			but we can remove the attribute. */
+			ut_ad((MTYP_TYPENR((*af)->unireg_check)
+			       != Field::NEXT_NUMBER)
+			      || (MTYP_TYPENR(f->unireg_check)
+				  == Field::NEXT_NUMBER));
 			if (!f->real_maybe_null() || (*af)->real_maybe_null())
 				goto next_column;
 			/* We are changing an existing column
@@ -2416,7 +2423,6 @@ ha_innobase::check_if_supported_inplace_alter(
 				    & ALTER_COLUMN_NOT_NULLABLE);
 			/* Virtual columns are never NOT NULL. */
 			DBUG_ASSERT(f->stored_in_db());
-
 			switch ((*af)->type()) {
 			case MYSQL_TYPE_TIMESTAMP:
 			case MYSQL_TYPE_TIMESTAMP2:
@@ -2436,14 +2442,7 @@ ha_innobase::check_if_supported_inplace_alter(
 				break;
 			default:
 				/* For any other data type, NULL
-				values are not converted.
-				(An AUTO_INCREMENT attribute cannot
-				be introduced to a column with
-				ALGORITHM=INPLACE.) */
-				ut_ad((MTYP_TYPENR((*af)->unireg_check)
-				       == Field::NEXT_NUMBER)
-				      == (MTYP_TYPENR(f->unireg_check)
-					  == Field::NEXT_NUMBER));
+				values are not converted. */
 				goto next_column;
 			}