MDEV-13240 Wrong warning with MAX(datetime_field) OVER (...)

The problem resided in Item_window_func implementation,
and it was revealed by bb-10.2-ext specific changes:

Item_window_func::save_in_field() works differently in bb-10.2-ext vs 10.2:

- 10.2 goes through val_str()
- bb-10.2-ext goes through get_date(), due to Type_handler related changes.
  get_date() tries to convert empty string to DATETIME, hence the warning.

During a discussion with Vicentiu, it was decided to fix
Item_window_func::val_xxx() to return NULL
(instead of an "empty" value, such as 0 for numbers and '' for strings)
when force_return_blank is set.

1 files changed, 13 insertions, 0 deletions

diff --git a/mysql-test/t/win.test b/mysql-test/t/win.test
index 95d32c5bd14..dfcf00cabf9 100644
--- a/mysql-test/t/win.test
+++ b/mysql-test/t/win.test
@@ -1877,3 +1877,16 @@ select count(max(id)) over (order by max(id)) from t1 where id < 3;
 select max(id), rank() over (order by max(id)) from t1 where id < 3;
 
 drop table t1;
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-13240 Wrong warning with MAX(datetime_field) OVER (...)
+--echo #
+
+CREATE TABLE t1 (dt DATETIME);
+INSERT INTO t1 VALUES ('2017-05-17');
+SELECT MAX(dt) OVER (ORDER BY dt ROWS BETWEEN 1 FOLLOWING AND 1 FOLLOWING) FROM t1;
+DROP TABLE t1;