type of the result.

There are several functions that accept parameters of different types.
The result field type of such functions was determined based on
the aggregated result type of its arguments. As the DATE and the DATETIME
types are represented by the STRING type, the result field type
of the affected functions was always STRING for DATE/DATETIME arguments.
The affected functions are COALESCE, IF, IFNULL, CASE, LEAST/GREATEST, CASE.

Now the affected functions aggregate the field types of their arguments rather
than their result types and return the result of aggregation as their result
field type.
The cached_field_type member variable is added to the number of classes to
hold the aggregated result field type.
The str_to_date() function's result field type now defaults to the
MYSQL_TYPE_DATETIME.
The agg_field_type() function is added. It aggregates field types with help
of the Field::field_type_merge() function.
The create_table_from_items() function now uses the 
item->tmp_table_field_from_field_type() function to get the proper field
when the item is a function with a STRING result type.


libmysql/libmysql.c:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The client library now accepts the NEWDATE type as a string.
sql/item_cmpfunc.cc:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  Now the fix_length_and_dec functions of the
  Item_func_ifnull,Item_func_if, Item_func_case, Item_func_coalesce
  classes are use agg_field_type function to find out the type of their 
  result field.
sql/item_cmpfunc.h:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The cached_field type is added to the Item_func_case, Item_func_if and
  Item_func_coalesce classes.
  The field_type function is added to the Item_func_ifnull, Item_func_if,
  Item_func_coalesce, Item_func_case classes.
sql/item_func.cc:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The Item_func_min_max::fix_length_and_dec function now uses the agg_field_type
  function to find out the correct type of the result field.
sql/item_func.h:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The cached_field_type variable is added to the Item_func_min_max class.
sql/item_timefunc.cc:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The result of the str_to_date function now defaults to the
  DATETIME type.
sql/mysql_priv.h:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The prototype of the agg_field_type function is added.
sql/protocol.cc:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The Protocol_simple::store function is now used to store fields of NEWDATE type.
sql/sql_insert.cc:
  Bug#27216: functions with parameters of different date types may return wrong
  type of the result.
  The create_table_from_items function now uses the
  tmp_table_field_from_field_type function to get field for items with the
  STRING result type.
mysql-test/r/date_formats.result:
  A test case result corrected after fixing bug#27216.
mysql-test/r/type_datetime.result:
  Added a test case for the bug#27216: functions with parameters of different
  date types may return wrong type of the result.
mysql-test/t/type_datetime.test:
  Added a test case for the bug#27216: functions with parameters of different
  date types may return wrong type of the result.