diff --git a/mercurial/revsetlang.py b/mercurial/revsetlang.py
--- a/mercurial/revsetlang.py
+++ b/mercurial/revsetlang.py
@@ -434,9 +434,9 @@ def _optimize(x, small):
         flushss()
         if len(ts) == 1:
             return ws[0], ts[0] # 'or' operation is fully optimized out
-        # we can't reorder trees by weight because it would change the order.
-        # ("sort(a + b)" == "sort(b + a)", but "a + b" != "b + a")
-        #   ts = tuple(t for w, t in sorted(zip(ws, ts), key=lambda wt: wt[0]))
+        if order != defineorder:
+            # reorder by weight only when f(a + b) == f(b + a)
+            ts = [wt[1] for wt in sorted(zip(ws, ts), key=lambda wt: wt[0])]
         return max(ws), (op, ('list',) + tuple(ts), order)
     elif op == 'not':
         # Optimize not public() to _notpublic() because we have a fast version
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -1420,19 +1420,19 @@ ordering defined by it.
       define)
     (or
       (list
+        ('symbol', '2')
         (range
           ('symbol', '0')
           ('symbol', '1')
-          follow)
-        ('symbol', '2'))
+          follow))
       follow)
     define)
   * set:
   <filteredset
     <spanset- 0:2>,
     <addset
-      <spanset+ 0:1>,
-      <baseset [2]>>>
+      <baseset [2]>,
+      <spanset+ 0:1>>>
   2
   1
   0
@@ -1917,6 +1917,69 @@ ordering defined by it.
   1
   0
 
+ 'A + B' can be rewritten to 'B + A' by weight only when the order doesn't
+ matter (e.g. 'X & (A + B)' can be 'X & (B + A)', but '(A + B) & X' can't):
+
+  $ try -p optimized '0:2 & (reverse(contains("a")) + 2)'
+  * optimized:
+  (and
+    (range
+      ('symbol', '0')
+      ('symbol', '2')
+      define)
+    (or
+      (list
+        ('symbol', '2')
+        (func
+          ('symbol', 'reverse')
+          (func
+            ('symbol', 'contains')
+            ('string', 'a')
+            define)
+          follow))
+      follow)
+    define)
+  * set:
+  <filteredset
+    <spanset+ 0:2>,
+    <addset
+      <baseset [2]>,
+      <filteredset
+        <fullreposet+ 0:9>,
+        <contains 'a'>>>>
+  0
+  1
+  2
+
+  $ try -p optimized '(reverse(contains("a")) + 2) & 0:2'
+  * optimized:
+  (and
+    (range
+      ('symbol', '0')
+      ('symbol', '2')
+      follow)
+    (or
+      (list
+        (func
+          ('symbol', 'reverse')
+          (func
+            ('symbol', 'contains')
+            ('string', 'a')
+            define)
+          define)
+        ('symbol', '2'))
+      define)
+    define)
+  * set:
+  <addset
+    <filteredset
+      <spanset- 0:2>,
+      <contains 'a'>>,
+    <baseset [2]>>
+  1
+  0
+  2
+
 test sort revset
 --------------------------------------------