Bug#28208: Wrong result of a non-const STRING function with a const DATETIME

function.

A wrong  condition was used to check that the
Arg_comparator::can_compare_as_dates() function calculated the value of the
string constant. When comparing a non-const STRING function with a constant
DATETIME function it leads to saving an arbitrary value as a cached value of
the DATETIME function.

Now the Arg_comparator::set_cmp_func() function initializes the const_value
variable to the impossible DATETIME value (-1) and this const_value is
cached only if it was changed by the Arg_comparator::can_compare_as_dates()
function.

mysql-test/r/type_datetime.result

@@ -346,3 +346,9 @@ call test27759();
 a	b	a_then_b	b_then_a	c_then_a
 2007-04-10	2007-04-11	2007-04-10	2007-04-10	2004-04-09 00:00:00
 drop procedure test27759;
+create table t1 (f1 date);
+insert into t1 values (curdate());
+select left(f1,10) = curdate() from t1;
+left(f1,10) = curdate()
+1
+drop table t1;

mysql-test/t/type_datetime.test

@@ -225,3 +225,12 @@ END;|
 DELIMITER ;|
 call test27759();
 drop procedure test27759;
+
+#
+# Bug#28208: Wrong result of a non-const STRING function with a const
+#            DATETIME function.
+#
+create table t1 (f1 date);
+insert into t1 values (curdate());
+select left(f1,10) = curdate() from t1;
+drop table t1;

sql/item_cmpfunc.cc

@@ -661,7 +661,7 @@ int Arg_comparator::set_cmp_func(Item_bool_func2 *owner_arg,
                                         Item_result type)
 {
   enum enum_date_cmp_type cmp_type;
-  ulonglong const_value;
+  ulonglong const_value= (ulonglong)-1;
   a= a1;
   b= a2;
 
@@ -674,8 +674,7 @@ int Arg_comparator::set_cmp_func(Item_bool_func2 *owner_arg,
     a_cache= 0;
     b_cache= 0;
 
-    if (cmp_type != CMP_DATE_WITH_DATE &&
-        ((*b)->const_item() || (*a)->const_item()))
+    if (const_value != (ulonglong)-1)
     {
       Item_cache_int *cache= new Item_cache_int();
       /* Mark the cache as non-const to prevent re-caching. */