diff -r a271925699d6 -r 9903261dcc81 mercurial/revset.py
--- a/mercurial/revset.py	Thu Oct 15 01:35:44 2015 +0100
+++ b/mercurial/revset.py	Thu Oct 15 01:11:00 2015 +0100
@@ -477,58 +477,7 @@
     # # XXX: - taking rev as arguments,
     # # XXX: - bailing out in case of ambiguity vs returning all data.
     getargs(x, 0, 0, _("_mergedefaultdest takes no arguments"))
-    if repo._activebookmark:
-        bmheads = repo.bookmarkheads(repo._activebookmark)
-        curhead = repo[repo._activebookmark].node()
-        if len(bmheads) == 2:
-            if curhead == bmheads[0]:
-                node = bmheads[1]
-            else:
-                node = bmheads[0]
-        elif len(bmheads) > 2:
-            raise error.Abort(_("multiple matching bookmarks to merge - "
-                "please merge with an explicit rev or bookmark"),
-                hint=_("run 'hg heads' to see all heads"))
-        elif len(bmheads) <= 1:
-            raise error.Abort(_("no matching bookmark to merge - "
-                "please merge with an explicit rev or bookmark"),
-                hint=_("run 'hg heads' to see all heads"))
-    else:
-        branch = repo[None].branch()
-        bheads = repo.branchheads(branch)
-        nbhs = [bh for bh in bheads if not repo[bh].bookmarks()]
-
-        if len(nbhs) > 2:
-            raise error.Abort(_("branch '%s' has %d heads - "
-                               "please merge with an explicit rev")
-                             % (branch, len(bheads)),
-                             hint=_("run 'hg heads .' to see heads"))
-
-        parent = repo.dirstate.p1()
-        if len(nbhs) <= 1:
-            if len(bheads) > 1:
-                raise error.Abort(_("heads are bookmarked - "
-                                   "please merge with an explicit rev"),
-                                 hint=_("run 'hg heads' to see all heads"))
-            if len(repo.heads()) > 1:
-                raise error.Abort(_("branch '%s' has one head - "
-                                   "please merge with an explicit rev")
-                                 % branch,
-                                 hint=_("run 'hg heads' to see all heads"))
-            msg, hint = _('nothing to merge'), None
-            if parent != repo.lookup(branch):
-                hint = _("use 'hg update' instead")
-            raise error.Abort(msg, hint=hint)
-
-        if parent not in bheads:
-            raise error.Abort(_('working directory not at a head revision'),
-                             hint=_("use 'hg update' or merge with an "
-                                    "explicit revision"))
-        if parent == nbhs[0]:
-            node = nbhs[-1]
-        else:
-            node = nbhs[0]
-    return subset & baseset([repo[node].rev()])
+    return subset & baseset([destutil.destmerge(repo)])
 
 def adds(repo, subset, x):
     """``adds(pattern)``