Fix for bug #5595: NULLIF() IS NULL returns false if NULLIF() returns NULL

mysql-test/r/func_if.result

@@ -64,3 +64,6 @@ select if(1>2,a,avg(a)) from t1;
 if(1>2,a,avg(a))
 1.5000
 drop table t1;
+SELECT NULLIF(5,5) IS NULL, NULLIF(5,5) IS NOT NULL;
+NULLIF(5,5) IS NULL	NULLIF(5,5) IS NOT NULL
+1	0

mysql-test/t/func_if.test

@@ -47,3 +47,7 @@ insert t1 values (1),(2);
 select if(1>2,a,avg(a)) from t1;
 drop table t1;
 
+#
+# Bug #5595  NULLIF() IS NULL returns false if NULLIF() returns NULL
+#
+SELECT NULLIF(5,5) IS NULL, NULLIF(5,5) IS NOT NULL;

sql/item_cmpfunc.cc

@@ -654,6 +654,15 @@ Item_func_nullif::val_str(String *str)
   return res;
 }
 
+
+bool
+Item_func_nullif::is_null()
+{
+  if (!(this->*cmp_func)())
+    return null_value=1;
+  return 0;
+}
+
 /*
   CASE expression 
   Return the matching ITEM or NULL if all compares (including else) failed

sql/item_cmpfunc.h

@@ -240,6 +240,7 @@ class Item_func_nullif :public Item_bool_func2
   void fix_length_and_dec();
   const char *func_name() const { return "nullif"; }
   table_map not_null_tables() const { return 0; }
+  bool is_null();
 };