diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -2283,12 +2283,7 @@ class abstractsmartset(object):
         """Returns a new object with the union of the two collections.
 
         This is part of the mandatory API for smartset."""
-        kwargs = {}
-        if self.isascending() and other.isascending():
-            kwargs['ascending'] = True
-        if self.isdescending() and other.isdescending():
-            kwargs['ascending'] = False
-        return addset(self, other, **kwargs)
+        return addset(self, other)
 
     def __sub__(self, other):
         """Returns a new object with the substraction of the two collections.
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -572,6 +572,29 @@ test intersecting something with an adds
   5
   8
 
+test that `or` operation combines elements in the right order:
+
+  $ log '3:4 or 2:5'
+  3
+  4
+  2
+  5
+  $ log '3:4 or 5:2'
+  3
+  4
+  5
+  2
+  $ log 'sort(3:4 or 2:5)'
+  2
+  3
+  4
+  5
+  $ log 'sort(3:4 or 5:2)'
+  2
+  3
+  4
+  5
+
 check that conversion to only works
   $ try --optimize '::3 - ::1'
   (minus