MDEV-9656 Assertion `0' failed in Item_sum_field::get_tmp_table_field().

Removing a wrong ASSERT. Item_sum_field now uses the inherited
Item::get_tmp_table_field().

mysql-test/r/func_group.result

@@ -2298,6 +2298,15 @@ id	avg	cast_avg
 2	9223372036854775807.0000	9223372036854775807
 DROP TABLE t1;
 #
+# MDEV-9656 Assertion `0' failed in Item_sum_field::get_tmp_table_field()
+#
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT DISTINCT VAR_POP(1) FROM t1 GROUP BY @a := 's';
+VAR_POP(1)
+0.0000
+DROP TABLE t1;
+#
 # MDEV-8921 Wrong result for CAST(AVG(double_column) AS SIGNED)
 #
 CREATE TABLE t1 (id INT, a DOUBLE);

mysql-test/t/func_group.test

@@ -1581,6 +1581,14 @@ SELECT id, AVG(a) AS avg, CAST(MIN(a) AS SIGNED) AS cast_min FROM t1 GROUP BY id
 SELECT id, AVG(a) AS avg, CAST(AVG(a) AS SIGNED) AS cast_avg FROM t1 GROUP BY id HAVING avg!=123 ORDER BY id;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-9656 Assertion `0' failed in Item_sum_field::get_tmp_table_field()
+--echo #
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT DISTINCT VAR_POP(1) FROM t1 GROUP BY @a := 's';
+DROP TABLE t1;
+
 --echo #
 --echo # MDEV-8921 Wrong result for CAST(AVG(double_column) AS SIGNED)
 --echo #

sql/item_sum.h

@@ -1084,7 +1084,6 @@ public:
     fixed= true;
   }
   table_map used_tables() const { return (table_map) 1L; }
-  Field *get_tmp_table_field() { DBUG_ASSERT(0); return NULL; }
   Field *tmp_table_field(TABLE *) { DBUG_ASSERT(0); return NULL; }
   void set_result_field(Field *) { DBUG_ASSERT(0); }
   void save_in_result_field(bool no_conversions) { DBUG_ASSERT(0); }