Disallow u"..." + b"..." and b"..." + u"...".

Lib/test/test_bytes.py

@@ -374,7 +374,7 @@ class BytesTest(unittest.TestCase):
         self.assertEqual(b1 + "def", bytes("abcdef"))
         self.assertEqual("def" + b1, bytes("defabc"))
         self.assertRaises(TypeError, lambda: b1 + u"def")
-        ##self.assertRaises(TypeError, lambda: u"abc" + b2)  # XXX FIXME
+        self.assertRaises(TypeError, lambda: u"abc" + b2)
 
     def test_repeat(self):
         b = bytes("abc")

Objects/unicodeobject.c

@@ -5535,6 +5535,9 @@ PyObject *PyUnicode_Concat(PyObject *left,
 {
     PyUnicodeObject *u = NULL, *v = NULL, *w;
 
+    if (PyBytes_Check(left) || PyBytes_Check(right))
+        return PyBytes_Concat(left, right);
+
     /* Coerce the two arguments */
     u = (PyUnicodeObject *)PyUnicode_FromObject(left);
     if (u == NULL)