diff -r cf6e782a7f94 Lib/test/test_unicode.py
--- a/Lib/test/test_unicode.py	Sun May 31 20:21:00 2015 +0300
+++ b/Lib/test/test_unicode.py	Sun May 31 20:56:24 2015 +0300
@@ -1036,10 +1036,12 @@ class UnicodeTest(
         self.assertRaises(UnicodeError, unicode, 'Andr\202 x', 'ascii','strict')
         self.assertEqual(unicode('Andr\202 x','ascii','ignore'), u"Andr x")
         self.assertEqual(unicode('Andr\202 x','ascii','replace'), u'Andr\uFFFD x')
-        self.assertEqual(u'abcde'.decode('ascii', 'ignore'),
-                         u'abcde'.decode('ascii', errors='ignore'))
-        self.assertEqual(u'abcde'.decode('ascii', 'replace'),
-                         u'abcde'.decode(encoding='ascii', errors='replace'))
+        with test_support.check_py3k_warnings():
+            self.assertEqual(u'abcde'.decode('ascii', 'ignore'),
+                             u'abcde'.decode('ascii', errors='ignore'))
+        with test_support.check_py3k_warnings():
+            self.assertEqual(u'abcde'.decode('ascii', 'replace'),
+                             u'abcde'.decode(encoding='ascii', errors='replace'))
 
         # Error handling (unknown character names)
         self.assertEqual("\\N{foo}xx".decode("unicode-escape", "ignore"), u"xx")
diff -r cf6e782a7f94 Objects/unicodeobject.c
--- a/Objects/unicodeobject.c	Sun May 31 20:21:00 2015 +0300
+++ b/Objects/unicodeobject.c	Sun May 31 20:56:24 2015 +0300
@@ -1288,6 +1288,9 @@ PyObject *PyUnicode_AsDecodedObject(PyOb
         goto onError;
     }
 
+    if (PyErr_WarnPy3k("decoding Unicode is not supported in 3.x", 1) < 0)
+        goto onError;
+
     if (encoding == NULL)
         encoding = PyUnicode_GetDefaultEncoding();