MDEV-33739 Check field type of the first field in check_sequence_fields()

This avoids non-integral types breaking the call of
sequence_structure().

mysql-test/suite/sql_sequence/alter.result

@@ -739,5 +739,14 @@ next value for s
 1001
 drop sequence s;
 #
+# MDEV-33739 Assertion `0' failed in Type_handler_typelib::max_display_length_for_field
+#
+CREATE SEQUENCE s1 ;
+ALTER table s1 CHANGE `next_not_cached_value` next_not_cached_value SET('1','2','3','4','5','6','7','8','9','10');
+ERROR HY000: Sequence 'test.s1' table structure is invalid (next_not_cached_value)
+ALTER table s1 CHANGE `next_not_cached_value` next_not_cached_value bool;
+ERROR HY000: Sequence 'test.s1' table structure is invalid (next_not_cached_value)
+drop sequence s1;
+#
 # End of 11.5 test
 #

mysql-test/suite/sql_sequence/alter.test

@@ -410,6 +410,18 @@ alter sequence s maxvalue 9432738420582397432;
 show create sequence s;
 select next value for s;
 drop sequence s;
+
+--echo #
+--echo # MDEV-33739 Assertion `0' failed in Type_handler_typelib::max_display_length_for_field
+--echo #
+
+CREATE SEQUENCE s1 ;
+--error ER_SEQUENCE_INVALID_TABLE_STRUCTURE
+ALTER table s1 CHANGE `next_not_cached_value` next_not_cached_value SET('1','2','3','4','5','6','7','8','9','10');
+--error ER_SEQUENCE_INVALID_TABLE_STRUCTURE
+ALTER table s1 CHANGE `next_not_cached_value` next_not_cached_value bool;
+drop sequence s1;
+
 --enable_ps2_protocol
 
 --echo #

sql/sql_sequence.cc

@@ -349,6 +349,12 @@ bool check_sequence_fields(LEX *lex, List<Create_field> *fields,
     reason= my_get_err_msg(ER_SEQUENCE_TABLE_HAS_WRONG_NUMBER_OF_COLUMNS);
     goto err;
   }
+  if (!sequence_definition::is_allowed_value_type(
+        fields->head()->type_handler()->field_type()))
+  {
+    reason= fields->head()->field_name.str;
+    goto err;
+  }
   row_structure= sequence_structure(fields->head()->type_handler());
   if (field_count != array_elements(row_structure.fields)-1)
   {