--- a/hgext3rd/topic/__init__.py	Sat Sep 30 23:00:21 2017 +0100
+++ b/hgext3rd/topic/__init__.py	Sat Sep 30 23:18:29 2017 +0100
@@ -59,7 +59,7 @@
     # behavior when commit is made without an active topic
     topic-mode = ignore # do nothing special (default)
     topic-mode = warning # print a warning
-    topic-mode = enforce # abort the commit
+    topic-mode = enforce # abort the commit (except for merge)
 """
 
 from __future__ import absolute_import
@@ -911,16 +911,23 @@
 def commitwrap(orig, ui, repo, *args, **opts):
     with repo.wlock():
         topicmode = _configtopicmode(ui)
+        ismergecommit = len(repo[None].parents()) == 2
+
+        notopic = not repo.currenttopic
+        mayabort = (topicmode == "enforce" and not ismergecommit)
+        maywarn = (topicmode == "warning"
+                   or (topicmode == "enforce" and ismergecommit))
+
         if opts.get('topic'):
             t = opts['topic']
             with repo.vfs.open('topic', 'w') as f:
                 f.write(t)
-        elif not repo.currenttopic and topicmode == "enforce":
+        elif notopic and mayabort:
             msg = _("no active topic")
             hint = _("set a current topic or use '--config " +
                      "experimental.topic-mode=off' to commit without a topic")
             raise error.Abort(msg, hint=hint)
-        elif not repo.currenttopic and topicmode == 'warning':
+        elif notopic and maywarn:
             ui.warn(_("warning: new draft commit without topic\n"))
         return orig(ui, repo, *args, **opts)
 

--- a/tests/test-topicmode.t	Sat Sep 30 23:00:21 2017 +0100
+++ b/tests/test-topicmode.t	Sat Sep 30 23:18:29 2017 +0100
@@ -66,3 +66,65 @@
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     added a'
   
+
+Testing the new config knob to warn about untopiced merge commit
+================================================================
+
+  $ hg init $TESTTMP/test-untopic-merge-commit
+  $ cd $TESTTMP/test-untopic-merge-commit
+  $ cat <<EOF >> .hg/hgrc
+  > [phases]
+  > publish=false
+  > EOF
+  $ cat <<EOF >> $HGRCPATH
+  > [experimental]
+  > topic-mode = enforce
+  > EOF
+  $ touch ROOT
+  $ hg commit -A -m "ROOT" --config experimental.topic-mode=off
+  adding ROOT
+  $ touch a
+  $ hg add a
+  $ hg topic mytopic
+  marked working directory as topic: mytopic
+  $ hg ci -m "Added a"
+  active topic 'mytopic' grew its first changeset
+
+  $ hg up -r "desc('ROOT')"
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ touch default
+  $ hg add default
+  $ hg commit -m "default" --config experimental.topic-mode=off
+
+  $ hg merge mytopic
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg commit -m "merge mytopic"
+  warning: new draft commit without topic
+
+  $ hg log -G
+  @    changeset:   3:676a445d1c09
+  |\   tag:         tip
+  | |  parent:      2:a4da109ee59f
+  | |  parent:      1:e5b6c632bd8e
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     merge mytopic
+  | |
+  | o  changeset:   2:a4da109ee59f
+  | |  parent:      0:ec1d2790416d
+  | |  user:        test
+  | |  date:        Thu Jan 01 00:00:00 1970 +0000
+  | |  summary:     default
+  | |
+  o |  changeset:   1:e5b6c632bd8e
+  |/   topic:       mytopic
+  |    user:        test
+  |    date:        Thu Jan 01 00:00:00 1970 +0000
+  |    summary:     Added a
+  |
+  o  changeset:   0:ec1d2790416d
+     user:        test
+     date:        Thu Jan 01 00:00:00 1970 +0000
+     summary:     ROOT
+