Bug #20868: Client connection is broken on SQL query error

 An aggregate function reference was resolved incorrectly and
caused a crash in count_field_types.
 Must use real_item() to get to the real Item instance through
the reference

mysql-test/r/func_group.result

@@ -988,3 +988,14 @@ SUM(a)
 6
 DROP TABLE t1;
 set div_precision_increment= @sav_dpi;
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+INSERT INTO t1 VALUES (1,1), (2,2);
+CREATE TABLE t2 (a INT PRIMARY KEY, b INT);
+INSERT INTO t2 VALUES (1,1), (3,3);
+SELECT SQL_NO_CACHE 
+(SELECT SUM(c.a) FROM t1 ttt, t2 ccc 
+WHERE ttt.a = ccc.b AND ttt.a = t.a GROUP BY ttt.a) AS minid   
+FROM t1 t, t2 c WHERE t.a = c.b;
+minid
+NULL
+DROP TABLE t1,t2;

mysql-test/t/func_group.test

@@ -660,3 +660,18 @@ SELECT SUM(a) FROM t1 GROUP BY b/c;
 DROP TABLE t1;
 set div_precision_increment= @sav_dpi;
 
+#
+# Bug #20868: Client connection is broken on SQL query error
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+INSERT INTO t1 VALUES (1,1), (2,2);
+
+CREATE TABLE t2 (a INT PRIMARY KEY, b INT);
+INSERT INTO t2 VALUES (1,1), (3,3);
+
+SELECT SQL_NO_CACHE 
+  (SELECT SUM(c.a) FROM t1 ttt, t2 ccc 
+   WHERE ttt.a = ccc.b AND ttt.a = t.a GROUP BY ttt.a) AS minid   
+FROM t1 t, t2 c WHERE t.a = c.b;
+
+DROP TABLE t1,t2;

sql/sql_select.cc

@@ -12806,7 +12806,7 @@ count_field_types(TMP_TABLE_PARAM *param, List<Item> &fields,
     {
       if (! field->const_item())
       {
-	Item_sum *sum_item=(Item_sum*) field;
+	Item_sum *sum_item=(Item_sum*) field->real_item();
 	if (!sum_item->quick_group)
 	  param->quick_group=0;			// UDF SUM function
 	param->sum_func_count++;