diff -r ca8170b5d370 -r 08b068240a1a mercurial/merge.py
--- a/mercurial/merge.py	Wed Oct 14 20:35:06 2015 -0700
+++ b/mercurial/merge.py	Mon Oct 05 21:42:09 2015 -0700
@@ -1080,6 +1080,9 @@
             pas = [repo[ancestor]]
 
         if node is None:
+            if (repo.ui.configbool('devel', 'all-warnings')
+                    or repo.ui.configbool('devel', 'oldapi')):
+                repo.ui.develwarn('update with no target')
             rev, _mark, _act = destutil.destupdate(repo)
             node = repo[rev].node()