The generic string to int conversion was used by the Item_func_signed and
the Item_func_unsigned classes to convert DATE/DATETIME values to the
SIGNED/UNSIGNED type. But this conversion produces wrong results for such
values.

Now if the item which result has to be converted can return its result as
longlong then the item->val_int() method is used to allow the item to carry
out the conversion itself and return the correct result.
This condition is checked in the Item_func_signed::val_int() and the
Item_func_unsigned::val_int() functions.


mysql-test/t/cast.test:
  Added a test case for the bug#23656: Wrong conversion result of a DATETIME to integer using CAST function.
mysql-test/r/cast.result:
  Added a test case for the bug#23656: Wrong conversion result of a DATETIME to integer using CAST function.
sql/item_func.cc:
  Bug#23656: Wrong conversion result of a DATETIME to integer using CAST function.
  Now if the item which result has to be converted can return its result as
  longlong then the item->val_int() method is used to allow the item to carry
  out the conversion itself and return the correct result.
  This condition is checked in the Item_func_signed::val_int() and the
  Item_func_unsigned::val_int() functions.