Index.generate() raises NotImplementedError by default

CHANGES

@@ -8,6 +8,8 @@ Incompatible changes
   members by default. Thanks to Luc Saffre.
 * LaTeX ``\includegraphics`` command isn't overloaded: only ``\sphinxincludegraphics``
   has the custom code to fit image to available width if oversized.
+* The subclasses of ``sphinx.domains.Index`` should override ``generate()``
+  method.  The default implementation raises NotImplmentedError
 
 Features removed
 ----------------

sphinx/domains/__init__.py

@@ -107,7 +107,7 @@ class Index(object):
 
         Qualifier and description are not rendered e.g. in LaTeX output.
         """
-        return tuple()
+        raise NotImplementedError
 
 
 class Domain(object):