can lead to a wrong result.

All date/time functions has the STRING result type thus their results are
compared as strings. The string date representation allows a user to skip 
some of leading zeros. This can lead to wrong comparison result if a date/time 
function result is compared to such a string constant.

The idea behind this bug fix is to compare results of date/time functions
and data/time constants as ints, because that date/time representation is 
more exact. To achieve this the agg_cmp_type() is changed to take in the
account that a date/time field or an date/time item should be compared 
as ints.

This bug fix is partially back ported from 5.0.

The agg_cmp_type() function now accepts THD as one of parameters. 
In addition, it now checks if a date/time field/function is present in the
list. If so, it tries to coerce all constants to INT to make date/time
comparison return correct result. The field for the constant coercion is
taken from the Item_field or constructed from the Item_func. In latter case
the constructed field will be freed after conversion of all constant items.
Otherwise the result is same as before - aggregated with help of the
item_cmp_type() function.

From the Item_func_between::fix_length_and_dec() function removed the part
which was converting date/time constants to int if possible. Now this is 
done by the agg_cmp_type() function.

The new function result_as_longlong() is added to the Item class. 
It indicates that the item is a date/time item and result of it can be
compared as int. Such items are date/time fields/functions.

Correct val_int() methods are implemented for classes Item_date_typecast, 
Item_func_makedate, Item_time_typecast, Item_datetime_typecast. All these
classes are derived from Item_str_func and Item_str_func::val_int() converts
its string value to int without regard to the date/time type of these items.

Arg_comparator::set_compare_func() and Arg_comparator::set_cmp_func()
functions are changed to substitute result type of an item with the INT_RESULT
if the item is a date/time item and another item is a constant. This is done
to get a correct result of comparisons like date_time_function() = string_constant.


mysql-test/r/cast.result:
  Fixed wrong test case result after bug fix#16377.
sql/item_timefunc.h:
  Fixed bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.
  The result_as_longlong() function is set to return TRUE for these classes:
  Item_date, Item_date_func, Item_func_curtime, Item_func_sec_to_time,
  Item_date_typecast, Item_time_typecast, Item_datetime_typecast,
  Item_func_makedate.
sql/item_timefunc.cc:
  Fixed bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.Correct val_int() methods are implemented for classes Item_date_typecast, 
  Item_func_makedate, Item_time_typecast, Item_datetime_typecast.
sql/item_cmpfunc.h:
  Fixed bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.
  Arg_comparator::set_compare_func() and Arg_comparator::set_cmp_func()
  functions are changed to substitute result type of an item with the INT_RESULT
  if the item is a date/time item and another item is a constant.
sql/field.cc:
  Fixed bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.
  Field::set_warning(), Field::set_datetime_warning() now use current_thd to get thd if table isn't set.
sql/item_cmpfunc.cc:
  Fixed bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.
  The agg_cmp_type() function now accepts THD as one of parameters. 
  In addition, it now checks if a date/time field/function is present in the
  list. If so, it tries to coerce all constants to INT to make date/time
  comparison return correct result. The field for the constant coercion is
  taken from the Item_field or constructed from the Item_func. In latter case
  the constructed field will be freed after conversion of all constant items.
  Otherwise the result is same as before - aggregated with help of the
  item_cmp_type() function.
sql/item.h:
  The new function result_as_longlong() is added to the Item class. 
  It indicates that the item is a date/time item and result of it can be
  compared as int. Such items are date/time fields/functions.
mysql-test/t/func_time.test:
  Added test case fot bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.
mysql-test/r/func_time.result:
  Added test case fot bug#16377: result of DATE/TIME functions were compared as strings which
  can lead to a wrong result.