--- a/mercurial/commands.py	Thu Jun 18 23:08:33 2009 -0500
+++ b/mercurial/commands.py	Sat Jun 20 08:29:41 2009 -0500
@@ -2975,7 +2975,7 @@
 
     return postincoming(ui, repo, modheads, opts.get('update'), None)
 
-def update(ui, repo, node=None, rev=None, clean=False, date=None):
+def update(ui, repo, node=None, rev=None, clean=False, date=None, check=False):
     """update working directory
 
     Update the repository's working directory to the specified
@@ -2991,7 +2991,8 @@
 
     When there are uncommitted changes, use option -C/--clean to
     discard them, forcibly replacing the state of the working
-    directory with the requested revision.
+    directory with the requested revision. Alternately, use -c/--check
+    to abort.
 
     When there are uncommitted changes and option -C/--clean is not
     used, and the parent revision and requested revision are on the
@@ -3011,6 +3012,12 @@
     if not rev:
         rev = node
 
+    if not clean and check:
+        # we could use dirty() but we can ignore merge and branch trivia
+        c = repo[None]
+        if c.modified() or c.added() or c.removed():
+            raise util.Abort(_("uncommitted local changes"))
+
     if date:
         if rev:
             raise util.Abort(_("you can't specify a revision and a date"))
@@ -3502,6 +3509,7 @@
     "^update|up|checkout|co":
         (update,
          [('C', 'clean', None, _('overwrite locally modified files (no backup)')),
+          ('c', 'check', None, _('check for uncommitted changes')),
           ('d', 'date', '', _('tipmost revision matching date')),
           ('r', 'rev', '', _('revision'))],
          _('[-C] [-d DATE] [[-r] REV]')),

--- a/tests/test-debugcomplete.out	Thu Jun 18 23:08:33 2009 -0500
+++ b/tests/test-debugcomplete.out	Sat Jun 20 08:29:41 2009 -0500
@@ -176,7 +176,7 @@
 remove: after, force, include, exclude
 serve: accesslog, daemon, daemon-pipefds, errorlog, port, address, prefix, name, webdir-conf, pid-file, stdio, templates, style, ipv6, certificate
 status: all, modified, added, removed, deleted, clean, unknown, ignored, no-status, copies, print0, rev, include, exclude
-update: clean, date, rev
+update: clean, check, date, rev
 addremove: similarity, include, exclude, dry-run
 archive: no-decode, prefix, rev, type, include, exclude
 backout: merge, parent, rev, include, exclude, message, logfile, date, user

--- a/tests/test-up-issue1456	Thu Jun 18 23:08:33 2009 -0500
+++ b/tests/test-up-issue1456	Sat Jun 20 08:29:41 2009 -0500
@@ -10,7 +10,7 @@
 hg ci -m1
 hg co -q 0
 echo dirty > foo
-sleep 1
+hg up -c
 hg up -q
 cat foo
 hg st -A

--- a/tests/test-up-issue1456.out	Thu Jun 18 23:08:33 2009 -0500
+++ b/tests/test-up-issue1456.out	Sat Jun 20 08:29:41 2009 -0500
@@ -1,3 +1,4 @@
+abort: uncommitted local changes
 dirty
 M foo
 % validate update of standalone execute bit change