MDEV-31250 ROW variables do not get assigned from subselects

ROW variables did not get assigned from subselects in these contexts:

BEGIN
  DECLARE r ROW TYPE OF t1;
  SET r=(SELECT * FROM t1 WHERE a=1);
END;

BEGIN
  DECLARE r ROW TYPE OF t1 DEFAULT (SELECT * FROM t1 WHERE a=1);
END;

All fields of the ROW variable remained NULL.

mysql-test/main/sp-anchor-row-type-table.result

@@ -814,3 +814,47 @@ t2	CREATE TABLE `t2` (
   `text0` text DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
 DROP TABLE t1;
+#
+# End of 10.3 tests
+#
+#
+# Start of 10.4 tests
+#
+#
+# MDEV-31250 ROW variables do not get assigned from subselects
+#
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+BEGIN NOT ATOMIC
+DECLARE r ROW TYPE OF t1;
+SELECT * INTO r FROM t1 WHERE a=1;
+SELECT r.a, r.b;
+END;
+$$
+r.a	r.b
+1	b1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+BEGIN NOT ATOMIC
+DECLARE r ROW TYPE OF t1;
+SET r=(SELECT * FROM t1 WHERE a=1);
+SELECT r.a, r.b;
+END;
+$$
+r.a	r.b
+1	b1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+BEGIN NOT ATOMIC
+DECLARE r ROW TYPE OF t1 DEFAULT (SELECT * FROM t1 WHERE a=1);
+SELECT r.a, r.b;
+END;
+$$
+r.a	r.b
+1	b1
+DROP TABLE t1;
+#
+# End of 10.4 tests
+#

mysql-test/main/sp-anchor-row-type-table.test

@@ -881,3 +881,58 @@ END;
 $$
 DELIMITER ;$$
 DROP TABLE t1;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #
+
+
+--echo #
+--echo # Start of 10.4 tests
+--echo #
+
+--echo #
+--echo # MDEV-31250 ROW variables do not get assigned from subselects
+--echo #
+
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+DELIMITER $$;
+BEGIN NOT ATOMIC
+  DECLARE r ROW TYPE OF t1;
+  SELECT * INTO r FROM t1 WHERE a=1;
+  SELECT r.a, r.b;
+END;
+$$
+DELIMITER ;$$
+DROP TABLE t1;
+
+
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+DELIMITER $$;
+BEGIN NOT ATOMIC
+  DECLARE r ROW TYPE OF t1;
+  SET r=(SELECT * FROM t1 WHERE a=1);
+  SELECT r.a, r.b;
+END;
+$$
+DELIMITER ;$$
+DROP TABLE t1;
+
+
+CREATE TABLE t1 (a INT, b TEXT);
+INSERT INTO t1 VALUES (1,'b1');
+DELIMITER $$;
+BEGIN NOT ATOMIC
+  DECLARE r ROW TYPE OF t1 DEFAULT (SELECT * FROM t1 WHERE a=1);
+  SELECT r.a, r.b;
+END;
+$$
+DELIMITER ;$$
+DROP TABLE t1;
+
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

sql/field.cc

@@ -2543,6 +2543,8 @@ bool Field_row::sp_prepare_and_store_item(THD *thd, Item **value)
       fixed underlying Item_field pointing to Field_row.
     - In case if we're assigning from a ROW() value, src and value[0] will
       point to the same Item_row.
+    - In case if we're assigning from a subselect, src and value[0] also
+      point to the same Item_singlerow_subselect.
   */
   Item *src;
   if (!(src= thd->sp_fix_func_item(value)) ||
@@ -2554,6 +2556,7 @@ bool Field_row::sp_prepare_and_store_item(THD *thd, Item **value)
     DBUG_RETURN(true);
   }
 
+  src->bring_value();
   DBUG_RETURN(m_table->sp_set_all_fields_from_item(thd, src));
 }