This assumption in Item_cache_datetime::cache_value_int
was wrong:
-  /* Assume here that the underlying item will do correct conversion.*/
-  int_value= example->val_int_result();


mysql-test/r/subselect_innodb.result:
  New test case.
mysql-test/t/subselect_innodb.test:
  New test case.
sql/item.cc:
  In Item_cache_datetime::cache_value_int()
   - call get_time() or get_date() depending on desired type
   - convert the returned MYSQL_TIME value to longlong depending on desired type
sql/item.h:
  The cached int_value in Item_cache_datetime should not be unsigned:
   - it is used mostly in signed context
   - it can actually have negative value (for TIME data type)
sql/item_cmpfunc.cc:
  Add comment on Bug#59685
sql/item_subselect.cc:
  Add some DBUG_TRACE for easier bug-hunting.