Bug#29850: Wrong charset of GROUP_CONCAT result when the select employs

a temporary table.

The result string of the Item_func_group_concat wasn't initialized in the 
copying constructor of the Item_func_group_concat class. This led to a
wrong charset of GROUP_CONCAT result when the select employs a temporary
table.

The copying constructor of the Item_func_group_concat class now correctly
initializes the charset of the result string.

mysql-test/r/func_gconcat.result

@@ -810,4 +810,13 @@ LENGTH( GROUP_CONCAT( a ) )
 65535
 SET group_concat_max_len= DEFAULT;
 DROP TABLE t1, t2, t3;
+set names latin1;
+create table t1 (id int, name varchar(20)) DEFAULT CHARSET=utf8;
+insert into t1 (id, name) values (1, "ra");
+insert into t1 (id, name) values (2, "ra");
+select b.id, group_concat(b.name) from t1 a, t1 b group by b.id;
+id	group_concat(b.name)
+1	ra,ra
+2	ra,ra
+drop table t1;
 End of 5.0 tests

mysql-test/t/func_gconcat.test

@@ -551,4 +551,15 @@ SELECT LENGTH( GROUP_CONCAT( a ) ) FROM t3 WHERE b = 3;
 SET group_concat_max_len= DEFAULT;
 DROP TABLE t1, t2, t3;
 
+#
+# Bug#29850: Wrong charset of the GROUP_CONCAT result when the select employs
+#            a temporary table.
+#
+set names latin1;
+create table t1 (id int, name varchar(20)) DEFAULT CHARSET=utf8;
+insert into t1 (id, name) values (1, "ra");
+insert into t1 (id, name) values (2, "ra");
+select b.id, group_concat(b.name) from t1 a, t1 b group by b.id;
+drop table t1;
+
 --echo End of 5.0 tests

sql/item_sum.cc

@@ -3068,6 +3068,7 @@ Item_func_group_concat::Item_func_group_concat(THD *thd,
   original(item)
 {
   quick_group= item->quick_group;
+  result.set_charset(collation.collation);
 }