--- a/mercurial/merge.py	Fri Dec 16 21:09:41 2011 +0900
+++ b/mercurial/merge.py	Fri Dec 16 21:21:27 2011 +0900
@@ -96,7 +96,7 @@
             raise util.Abort(_("untracked file in working directory differs"
                                " from file in requested revision: '%s'") % fn)
 
-def _checkcollision(mctx):
+def _checkcollision(mctx, wctx):
     "check for case folding collisions in the destination context"
     folded = {}
     for fn in mctx:
@@ -106,6 +106,14 @@
                              % (fn, folded[fold]))
         folded[fold] = fn
 
+    if wctx:
+        for fn in wctx:
+            fold = util.normcase(fn)
+            mfn = folded.get(fold, None)
+            if mfn and (mfn != fn):
+                raise util.Abort(_("case-folding collision between %s and %s")
+                                 % (mfn, fn))
+
 def _forgetremoved(wctx, mctx, branchmerge):
     """
     Forget removed files
@@ -549,7 +557,7 @@
         if not force:
             _checkunknown(wc, p2, folding)
         if folding:
-            _checkcollision(p2)
+            _checkcollision(p2, branchmerge and p1)
         action += _forgetremoved(wc, p2, branchmerge)
         action += manifestmerge(repo, wc, p2, pa, overwrite, partial)
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-casecollision-merge.t	Fri Dec 16 21:21:27 2011 +0900
@@ -0,0 +1,109 @@
+run only on case-insensitive filesystems
+
+  $ "$TESTDIR/hghave" icasefs || exit 80
+
+################################
+test for branch merging
+################################
+
+  $ hg init repo1
+  $ cd repo1
+
+create base revision
+
+  $ echo base > base.txt
+  $ hg add base.txt
+  $ hg commit -m 'base'
+
+add same file in different case on both heads
+
+  $ echo a > a.txt
+  $ hg add a.txt
+  $ hg commit -m 'add a.txt'
+
+  $ hg update 0
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+  $ echo A > A.TXT
+  $ hg add A.TXT
+  $ hg commit -m 'add A.TXT'
+  created new head
+
+merge another, and fail with case-folding collision
+
+  $ hg merge
+  abort: case-folding collision between a.txt and A.TXT
+  [255]
+
+check clean-ness of working directory
+
+  $ hg status
+  $ hg parents --template '{rev}\n'
+  2
+  $ cd ..
+
+################################
+test for linear updates
+################################
+
+  $ hg init repo2
+  $ cd repo2
+
+create base revision (rev:0)
+
+  $ hg import --bypass --exact - <<EOF
+  > # HG changeset patch
+  > # User null
+  > # Date 1 0
+  > # Node ID e1bdf414b0ea9c831fd3a14e94a0a18e1410f98b
+  > # Parent  0000000000000000000000000000000000000000
+  > add a
+  > 
+  > diff --git a/a b/a
+  > new file mode 100644
+  > --- /dev/null
+  > +++ b/a
+  > @@ -0,0 +1,3 @@
+  > +this is line 1
+  > +this is line 2
+  > +this is line 3
+  > EOF
+  applying patch from stdin
+
+create rename revision (rev:1)
+
+  $ hg import --bypass --exact - <<EOF
+  > # HG changeset patch
+  > # User null
+  > # Date 1 0
+  > # Node ID 9dca9f19bb91851bc693544b598b0740629edfad
+  > # Parent  e1bdf414b0ea9c831fd3a14e94a0a18e1410f98b
+  > rename a to A
+  > 
+  > diff --git a/a b/A
+  > rename from a
+  > rename to A
+  > EOF
+  applying patch from stdin
+
+update to base revision, and modify 'a'
+
+  $ hg update 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo 'this is added line' >> a
+
+update to current tip linearly
+
+  $ hg update 1
+  merging a and A to A
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+
+check status and contents of file
+
+  $ hg status -A
+  M A
+  $ cat A
+  this is line 1
+  this is line 2
+  this is line 3
+  this is added line