MDEV-29841 Partition by system_time can be converted into table but not back

Wrong error code was returned because of prematurely tested condition
for wrong partition type. Now the condition for CONVERT IN is tested
first.

mysql-test/suite/versioning/r/partition.result

@@ -2049,6 +2049,15 @@ t1	CREATE TABLE `t1` (
  PARTITION `pn` VALUES LESS THAN MAXVALUE ENGINE = X)
 drop tables t1, tp1;
 #
+# MDEV-29841 Partition by system_time can be converted into table but not back
+#
+create or replace table t (a int) with system versioning
+partition by system_time limit 10 partitions 3;
+alter table t convert partition p0 to table tp;
+alter table t convert table tp to partition p0;
+ERROR HY000: CONVERT TABLE TO PARTITION can only be used on RANGE/LIST partitions
+drop tables t, tp;
+#
 # End of 10.7 tests
 #
 set global innodb_stats_persistent= @save_persistent;

mysql-test/suite/versioning/t/partition.test

@@ -1677,6 +1677,16 @@ show create table t1;
 drop tables t1, tp1;
 }
 
+--echo #
+--echo # MDEV-29841 Partition by system_time can be converted into table but not back
+--echo #
+create or replace table t (a int) with system versioning
+partition by system_time limit 10 partitions 3;
+alter table t convert partition p0 to table tp;
+--error ER_ONLY_ON_RANGE_LIST_PARTITION
+alter table t convert table tp to partition p0;
+drop tables t, tp;
+
 --echo #
 --echo # End of 10.7 tests
 --echo #

sql/sql_partition.cc

@@ -5057,6 +5057,13 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
     if ((alter_info->partition_flags & ALTER_PARTITION_ADD) ||
         (alter_info->partition_flags & ALTER_PARTITION_REORGANIZE))
     {
+      if ((alter_info->partition_flags & ALTER_PARTITION_CONVERT_IN) &&
+          !(tab_part_info->part_type == RANGE_PARTITION ||
+            tab_part_info->part_type == LIST_PARTITION))
+      {
+        my_error(ER_ONLY_ON_RANGE_LIST_PARTITION, MYF(0), "CONVERT TABLE TO");
+        goto err;
+      }
       if (thd->work_part_info->part_type != tab_part_info->part_type)
       {
         if (thd->work_part_info->part_type == NOT_A_PARTITION)
@@ -5126,13 +5133,6 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
     }
     if (alter_info->partition_flags & ALTER_PARTITION_ADD)
     {
-      if ((alter_info->partition_flags & ALTER_PARTITION_CONVERT_IN) &&
-          !(tab_part_info->part_type == RANGE_PARTITION ||
-            tab_part_info->part_type == LIST_PARTITION))
-      {
-        my_error(ER_ONLY_ON_RANGE_LIST_PARTITION, MYF(0), "CONVERT TABLE TO");
-        goto err;
-      }
       if (*fast_alter_table && thd->locked_tables_mode)
       {
         MEM_ROOT *old_root= thd->mem_root;