Bug#23656: Wrong conversion result of a DATETIME to integer using CAST function.

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/r/cast.result

@@ -281,4 +281,10 @@ DROP TABLE t1;
 select isnull(date(NULL)), isnull(cast(NULL as DATE));
 isnull(date(NULL))	isnull(cast(NULL as DATE))
 1	1
+SELECT CAST(cast('01-01-01' as date) AS UNSIGNED);
+CAST(cast('01-01-01' as date) AS UNSIGNED)
+20010101
+SELECT CAST(cast('01-01-01' as date) AS SIGNED);
+CAST(cast('01-01-01' as date) AS SIGNED)
+20010101
 End of 4.1 tests

mysql-test/t/cast.test

@@ -173,4 +173,10 @@ DROP TABLE t1;
 
 select isnull(date(NULL)), isnull(cast(NULL as DATE));
 
+#
+# Bug#23656: Wrong result of CAST from DATE to int
+#
+SELECT CAST(cast('01-01-01' as date) AS UNSIGNED);
+SELECT CAST(cast('01-01-01' as date) AS SIGNED);
+
 --echo End of 4.1 tests

sql/item_func.cc

@@ -477,7 +477,8 @@ longlong Item_func_signed::val_int()
   longlong value;
   int error;
 
-  if (args[0]->cast_to_int_type() != STRING_RESULT)
+  if (args[0]->cast_to_int_type() != STRING_RESULT ||
+      args[0]->result_as_longlong())
   {
     value= args[0]->val_int();
     null_value= args[0]->null_value; 
@@ -529,7 +530,8 @@ longlong Item_func_unsigned::val_int()
     return (longlong)  (dvalue + (dvalue > 0 ? 0.5 : -0.5));
   }
 
-  if (args[0]->cast_to_int_type() != STRING_RESULT)
+  if (args[0]->cast_to_int_type() != STRING_RESULT ||
+      args[0]->result_as_longlong())
   {
     value= args[0]->val_int();
     null_value= args[0]->null_value;