--- a/hgext3rd/topic/revset.py	Sat Dec 22 01:29:59 2018 -0500
+++ b/hgext3rd/topic/revset.py	Sun Dec 16 11:22:04 2018 +0800
@@ -106,3 +106,37 @@
     else:
         branch = repo[None].branch()
     return revset.baseset(stack.stack(repo, branch=branch, topic=topic)[1:]) & subset
+
+if util.safehasattr(revset, 'subscriptrelations'):
+    def stackrel(repo, subset, x, rel, n, order):
+        """This is a revset-flavored implementation of stack aliases.
+
+        The syntax is: rev#stack[n] or rev#s[n]. Plenty of logic is borrowed
+        from topic._namemap, but unlike that function, which prefers to abort
+        (e.g. when stack index is too high), this returns empty set to be more
+        revset-friendly.
+        """
+        if n < 0:
+            return revset.baseset()
+        s = revset.getset(repo, revset.fullreposet(repo), x)
+        if not s:
+            return revset.baseset()
+        revs = []
+        for r in s:
+            topic = repo[r].topic()
+            if topic:
+                st = stack.stack(repo, topic=topic)
+            else:
+                st = stack.stack(repo, branch=repo[r].branch())
+            try:
+                rev = list(st)[n]
+            except IndexError:
+                continue
+            if rev == -1 and n == 0:
+                continue
+            if rev not in revs:
+                revs.append(rev)
+        return subset & revset.baseset(revs)
+
+    revset.subscriptrelations['stack'] = stackrel
+    revset.subscriptrelations['s'] = stackrel

--- a/tests/test-topic-stack.t	Sat Dec 22 01:29:59 2018 -0500
+++ b/tests/test-topic-stack.t	Sun Dec 16 11:22:04 2018 +0800
@@ -373,6 +373,17 @@
   hg: parse error: stack takes no arguments, it works on current topic
   [255]
 
+Stack relation subscript:
+
+  $ hg log -r 'foo#stack[0]'
+  1 default {} public c_b
+  $ hg log -r 's0 and foo#stack[0]'
+  1 default {} public c_b
+  $ hg log -r 'foo#stack[4]'
+  5 default {foo} draft c_f
+  $ hg log -r 's4 and foo#stack[4]'
+  5 default {foo} draft c_f
+
 Case with multiple heads on the topic
 -------------------------------------
 

--- a/tests/test-topic.t	Sat Dec 22 01:29:59 2018 -0500
+++ b/tests/test-topic.t	Sun Dec 16 11:22:04 2018 +0800
@@ -823,6 +823,86 @@
 
   $ cd ..
 
+Stack relation subscript in revsets
+===================================
+
+  $ hg init more-than-one-commit-per-topic
+  $ cd more-than-one-commit-per-topic
+  $ cat > .hg/hgrc << EOF
+  > [phases]
+  > publish=false
+  > EOF
+
+  $ echo 0 > foo
+  $ hg ci -qAm 0
+  $ hg topic featureA
+  marked working directory as topic: featureA
+  $ echo 1 > foo
+  $ hg ci -qm 1
+  $ echo 2 > foo
+  $ hg ci -qm 2
+  $ echo 3 > foo
+  $ hg ci -qm 3
+  $ hg topic --clear
+  $ echo 4 > foo
+  $ hg ci -qm 4
+
+  $ tlog 'all()'
+  0: 
+  1: featureA
+  2: featureA
+  3: featureA
+  4: 
+
+  $ hg stack
+  ### target: default (branch)
+  s2@ 4 (current)
+    ^ 3
+  s1: 0
+
+  $ tlog 'tip#stack[0]'
+  $ tlog 'tip#stack[1]'
+  0: 
+  $ tlog 'tip#stack[2]'
+  4: 
+
+  $ hg stack featureA
+  ### topic: featureA
+  ### target: default (branch), 3 behind
+  s3: 3
+  s2: 2
+  s1: 1
+  s0^ 0 (base)
+
+  $ tlog 'featureA#s[0]'
+  0: 
+  $ tlog 'featureA#s[1]'
+  1: featureA
+  $ tlog 'featureA#s[2]'
+  2: featureA
+  $ tlog 'featureA#s[3]'
+  3: featureA
+
+  $ tlog 'all()#s[-1]'
+  $ tlog 'all()#s[0]'
+  0: 
+  $ tlog 'all()#s[1]'
+  0: 
+  1: featureA
+  $ tlog 'all()#s[9999]'
+
+  $ hg topic featureB
+  marked working directory as topic: featureB
+  $ hg stack
+  ### topic: featureB
+  ### target: default (branch)
+  (stack is empty)
+  s0^ 4 (base current)
+  $ tlog 'wdir()#s[0]'
+  4: 
+
+  $ cd ..
+
 Testing the new config knob to forbid untopiced commit
 ======================================================