(Merge into trunk.)

Fix for SF bug #492345.  (I could've sworn I checked this in, but
apparently I didn't!)

This code:

    class Classic:
        pass

    class New(Classic):
        __metaclass__ = type

attempts to create a new-style class with only classic bases -- but it
doesn't work right.  Attempts to fix it so it works caused problems
elsewhere, so I'm now raising a TypeError in this case.

Lib/test/test_descr.py

@@ -922,6 +922,16 @@ def multi():
     vereq(m.m3method(), "M3 a")
     vereq(m.all_method(), "M3 b")
 
+    class Classic:
+        pass
+    try:
+        class New(Classic):
+            __metaclass__ = type
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "new class with only classic bases - shouldn't be"
+
 def diamond():
     if verbose: print "Testing multiple inheritance special cases..."
     class A(object):

Objects/typeobject.c

@@ -757,7 +757,9 @@ best_base(PyObject *bases)
 			return NULL;
 		}
 	}
-	assert(base != NULL);
+	if (base == NULL)
+		PyErr_SetString(PyExc_TypeError,
+			"a new-style class can't have only classic bases");
 	return base;
 }