Fix for bug#8656: Crash with group_concat on alias in outer table

fixed result_field support of Item_ref

mysql-test/r/func_gconcat.result

@@ -462,3 +462,10 @@ SELECT GROUP_CONCAT(id) AS gc FROM t1 HAVING gc IS NULL;
 gc
 NULL
 DROP TABLE t1;
+create table r2 (a int, b int);
+insert into r2 values (1,1), (2,2);
+select  b x, (select group_concat(x) from r2) from  r2;
+x	(select group_concat(x) from r2)
+1	1,1
+2	2,2
+drop table r2;

mysql-test/t/func_gconcat.test

@@ -284,3 +284,11 @@ drop table t1;
 CREATE TABLE t1 (id int);
 SELECT GROUP_CONCAT(id) AS gc FROM t1 HAVING gc IS NULL;
 DROP TABLE t1;
+
+#
+# Bug #8656: Crash with group_concat on alias in outer table
+#
+create table r2 (a int, b int);
+insert into r2 values (1,1), (2,2);
+select  b x, (select group_concat(x) from r2) from  r2;
+drop table r2;

sql/item.h

@@ -902,6 +902,7 @@ class Item_ref :public Item_ident
   void save_org_in_field(Field *field)	{ (*ref)->save_org_in_field(field); }
   enum Item_result result_type () const { return (*ref)->result_type(); }
   enum_field_types field_type() const   { return (*ref)->field_type(); }
+  Field *get_tmp_table_field() { return result_field; }
   table_map used_tables() const		
   { 
     return depended_from ? OUTER_REF_TABLE_BIT : (*ref)->used_tables();