# HG changeset patch
# User Yuya Nishihara <yuya@tcha.org>
# Date 1531404449 -32400
# Node ID e4b270a32ba84b9287235c87df5de0e61c889ca8
# Parent  607e2a2501e69d946b2b2848e52bf7aa8c338d0e
revset: special case commonancestors(none()) to be empty set

This matches the behavior of ancestor(none()).

From an implementation perspective, ancestor() and commonancestors() are
intersection, and ancestors() is union, so it would make some sense that
commonancestors(none()) returned all revisions. However, ancestor(none())
isn't implemented as such, which breaks ancestor(x) == max(commonancestors(x)).

From a user perspective, ancestors of nothing is nothing whichever type
of operation the ancestor predicate does.

diff -r 607e2a2501e6 -r e4b270a32ba8 mercurial/revset.py
--- a/mercurial/revset.py	Tue Jul 10 23:01:53 2018 +0900
+++ b/mercurial/revset.py	Thu Jul 12 23:07:29 2018 +0900
@@ -632,7 +632,10 @@
 
     """
     # only wants the heads of the set passed in
-    for r in heads(repo, fullreposet(repo), x, anyorder):
+    h = heads(repo, fullreposet(repo), x, anyorder)
+    if not h:
+        return baseset()
+    for r in h:
         subset &= dagop.revancestors(repo, baseset([r]))
 
     return subset
diff -r 607e2a2501e6 -r e4b270a32ba8 tests/test-revset.t
--- a/tests/test-revset.t	Tue Jul 10 23:01:53 2018 +0900
+++ b/tests/test-revset.t	Thu Jul 12 23:07:29 2018 +0900
@@ -1063,6 +1063,12 @@
   8
   9
 
+test ancestor variants of empty revision
+
+  $ log 'ancestor(none())'
+  $ log 'ancestors(none())'
+  $ log 'commonancestors(none())'
+
 test ancestors with depth limit
 
  (depth=0 selects the node itself)