The problem happens when::

  import compiler

  class Visitor(object):
      def visitReturn(self, node):
          print node.value

  source = """
  def f():
      return
  """
  compiler.walk(compiler.parse(source), Visitor())

I think the value of `node.value` should have been `None` instead of
`Const(None)` as it is for most other nodes when an optional part is
missing.  The same problem (getting `Const(None)` instead of `None`)
exists for `Sliceobj.nodes[i]` and `Discard.expr`.

"return" is syntactic shorthand for "return None", so Const(None) is the right value; same thing for Sliceobj. This isn't a bug.