--- a/hgext3rd/topic/__init__.py	Tue Mar 15 17:26:57 2016 +0000
+++ b/hgext3rd/topic/__init__.py	Mon Mar 14 18:39:19 2016 +0000
@@ -12,6 +12,7 @@
 """
 import functools
 import contextlib
+import re
 
 from mercurial.i18n import _
 from mercurial import branchmap
@@ -51,7 +52,22 @@
     return self.extra().get(constants.extrakey, '')
 context.basectx.topic = _contexttopic
 
+topicrev = re.compile(r'^t\d+$')
+
+
 def _namemap(repo, name):
+    if topicrev.match(name):
+        idx = int(name[1:])
+        topic = repo.currenttopic
+        if not topic:
+            raise error.Abort(_('cannot resolve "%s": no active topic') % name)
+        revs = list(stack.getstack(repo, topic))
+        try:
+            r = revs[idx]
+        except IndexError:
+            msg = _('cannot resolve "%s": topic "%s" has only %d changesets')
+            raise error.Abort(msg % (name, topic, len(revs)))
+        return [repo[r].node()]
     return [ctx.node() for ctx in
             repo.set('not public() and extra(topic, %s)', name)]
 

--- a/hgext3rd/topic/discovery.py	Tue Mar 15 17:26:57 2016 +0000
+++ b/hgext3rd/topic/discovery.py	Mon Mar 14 18:39:19 2016 +0000
@@ -107,5 +107,3 @@
     if repo.peer().capable('topics'):
         caps.append('topics')
     return caps
-
-

--- a/hgext3rd/topic/stack.py	Tue Mar 15 17:26:57 2016 +0000
+++ b/hgext3rd/topic/stack.py	Mon Mar 14 18:39:19 2016 +0000
@@ -8,7 +8,7 @@
 from mercurial import extensions
 from mercurial import obsolete
 
-def _getstack(repo, topic):
+def getstack(repo, topic):
     # XXX need sorting
     trevs = repo.revs("topic(%s) - obsolete()", topic)
     return _orderrevs(repo, trevs)
@@ -18,7 +18,7 @@
         topic = repo.currenttopic
     if not topic:
         raise error.Abort(_('no active topic to list'))
-    for idx, r in enumerate(_getstack(repo, topic)):
+    for idx, r in enumerate(getstack(repo, topic)):
         # super crude initial version
         l = "%d: %s\n" % (idx, repo[r].description().splitlines()[0])
         ui.write(l)

--- a/tests/test-topic-stack.t	Tue Mar 15 17:26:57 2016 +0000
+++ b/tests/test-topic-stack.t	Mon Mar 14 18:39:19 2016 +0000
@@ -69,12 +69,23 @@
   abort: no active topic to list
   [255]
 
+Test "t#" reference
+-------------------
+
+
+  $ hg up t1
+  abort: cannot resolve "t1": no active topic
+  [255]
+  $ hg topic foo
+  $ hg up t42
+  abort: cannot resolve "t42": topic "foo" has only 4 changesets
+  [255]
+  $ hg up t1
+  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
+
 Case with some of the topic unstable
 ------------------------------------
 
-  $ hg up 'desc(c_d)'
-  switching to topic foo
-  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ echo bbb > ddd
   $ hg commit --amend
   $ hg log -G