bug #16546 (DATETIME + 0 not always coerced in the same way)

  fix for cast( AS DATETIME) + 0 operation.
  I just implemented Item_datetime_typecast::val() method
  as it is usually done in other classes.
  Should be fixed more radically in 5.0


mysql-test/r/type_datetime.result:
  result added
mysql-test/t/type_datetime.test:
  testcase
sql/item_timefunc.h:
  added double conversion to Item_datetime_typecast

mysql-test/r/type_datetime.result

@@ -166,3 +166,6 @@ dt
 0000-00-00 00:00:00
 0000-00-00 00:00:00
 drop table t1;
+select cast('2006-12-05 22:10:10' as datetime) + 0;
+cast('2006-12-05 22:10:10' as datetime) + 0
+20061205221010.000000

mysql-test/t/type_datetime.test

@@ -113,4 +113,10 @@ insert into t1 values ("00-00-00"), ("00-00-00 00:00:00");
 select * from t1;
 drop table t1;
 
+#
+# Bug #16546 DATETIME+0 not always coerced the same way 
+#
+select cast('2006-12-05 22:10:10' as datetime) + 0;
+
+
 # End of 4.1 tests

sql/item_timefunc.h

@@ -751,12 +751,19 @@ public:
   String *val_str(String *str);
   const char *cast_type() const { return "datetime"; }
   enum_field_types field_type() const { return MYSQL_TYPE_DATETIME; }
+  void fix_length_and_dec()
+  {
+    Item_typecast_maybe_null::fix_length_and_dec();
+    decimals= DATETIME_DEC;
+  }
+
   Field *tmp_table_field(TABLE *t_arg)
   {
     return (new Field_datetime(maybe_null, name, t_arg, &my_charset_bin));
   }
   bool result_as_longlong() { return TRUE; }
   longlong val_int();
+  double val() { return (double) val_int(); }
 };
 
 class Item_func_makedate :public Item_str_func