A fix and test case for Bug#6297 "prepared statement, wrong handling

 of <parameter> IS NULL":
we must not only set Item::null_value in Item_param, but implement
Item_param::is_null() to work well with IS NULL/IS NOT NULL clauses.

mysql-test/r/ps.result

@@ -450,3 +450,24 @@ PREPARE stmt FROM 'UPDATE t1 AS P1 INNER JOIN (SELECT N FROM t1 GROUP BY N HAVIN
 EXECUTE stmt;
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
+prepare stmt from "select ? is null, ? is not null, ?";
+select @no_such_var is null, @no_such_var is not null, @no_such_var;
+@no_such_var is null	@no_such_var is not null	@no_such_var
+1	0	NULL
+execute stmt using @no_such_var, @no_such_var, @no_such_var;
+? is null	? is not null	?
+1	0	NULL
+set @var='abc';
+select @var is null, @var is not null, @var;
+@var is null	@var is not null	@var
+0	1	abc
+execute stmt using @var, @var, @var;
+? is null	? is not null	?
+0	1	abc
+set @var=null;
+select @var is null, @var is not null, @var;
+@var is null	@var is not null	@var
+1	0	NULL
+execute stmt using @var, @var, @var;
+? is null	? is not null	?
+1	0	NULL

mysql-test/t/ps.test

@@ -458,3 +458,17 @@ EXECUTE stmt;
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 
+# 
+# Bug#6297 "prepared statement, wrong handling of <parameter> IS NULL"
+# Test that placeholders work with IS NULL/IS NOT NULL clauses. 
+#
+prepare stmt from "select ? is null, ? is not null, ?";
+select @no_such_var is null, @no_such_var is not null, @no_such_var;
+execute stmt using @no_such_var, @no_such_var, @no_such_var;
+set @var='abc';
+select @var is null, @var is not null, @var;
+execute stmt using @var, @var, @var;
+set @var=null;
+select @var is null, @var is not null, @var;
+execute stmt using @var, @var, @var;
+

sql/item.h

@@ -266,6 +266,14 @@ class Item {
   virtual bool get_time(TIME *ltime);
   virtual bool get_date_result(TIME *ltime,uint fuzzydate)
   { return get_date(ltime,fuzzydate); }
+  /*
+    This function is used only in Item_func_isnull/Item_func_isnotnull
+    (implementations of IS NULL/IS NOT NULL clauses). Item_func_is{not}null
+    calls this method instead of one of val/result*() methods, which
+    normally will set null_value. This allows to determine nullness of
+    a complex expression without fully evaluating it.
+    Any new item which can be NULL must implement this call.
+  */
   virtual bool is_null() { return 0; }
   /*
     it is "top level" item of WHERE clause and we do not need correct NULL
@@ -573,6 +581,8 @@ class Item_param :public Item
   void print(String *str) { str->append('?'); }
   /* parameter never equal to other parameter of other item */
   bool eq(const Item *item, bool binary_cmp) const { return 0; }
+  bool is_null()
+  { DBUG_ASSERT(state != NO_VALUE); return state == NULL_VALUE; }
 };
 
 class Item_int :public Item_num