--- a/mercurial/commands.py	Sun Nov 13 21:37:14 2011 +0100
+++ b/mercurial/commands.py	Wed Nov 16 12:53:10 2011 +0100
@@ -3510,6 +3510,12 @@
                 try:
                     p1 = repo[p1]
                     p2 = repo[p2]
+                    # Without any options, consider p2 only if the
+                    # patch is being applied on top of the recorded
+                    # first parent.
+                    if p1 != parents[0]:
+                        p1 = parents[0]
+                        p2 = repo[nullid]
                 except error.RepoError:
                     p1, p2 = parents
             else:
@@ -3517,9 +3523,9 @@
 
             n = None
             if update:
-                if opts.get('exact') and p1 != parents[0]:
+                if p1 != parents[0]:
                     hg.clean(repo, p1.node())
-                if p1 != parents[0] and p2 != parents[1]:
+                if p2 != parents[1]:
                     repo.dirstate.setparents(p1.node(), p2.node())
 
                 if opts.get('exact') or opts.get('import_branch'):
@@ -3533,7 +3539,10 @@
                     if message:
                         msgs.append(message)
                 else:
-                    if opts.get('exact'):
+                    if opts.get('exact') or p2:
+                        # If you got here, you either use --force and know what
+                        # you are doing or used --exact or a merge patch while
+                        # being updated to its first parent.
                         m = None
                     else:
                         m = scmutil.matchfiles(repo, files or [])

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-import-merge.t	Wed Nov 16 12:53:10 2011 +0100
@@ -0,0 +1,114 @@
+  $ echo "[extensions]" >> $HGRCPATH
+  $ echo "mq=" >> $HGRCPATH
+
+  $ tipparents() {
+  > hg parents --template "{rev}:{node|short} {desc|firstline}\n" -r tip
+  > }
+
+Test import and merge diffs
+
+  $ hg init repo
+  $ cd repo
+  $ echo a > a
+  $ hg ci -Am adda
+  adding a
+  $ echo a >> a
+  $ hg ci -m changea
+  $ echo c > c
+  $ hg ci -Am addc
+  adding c
+  $ hg up 0
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo b > b
+  $ hg ci -Am addb
+  adding b
+  created new head
+  $ hg up 1
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg merge 3
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg ci -m merge
+  $ hg export . > ../merge.diff
+  $ cd ..
+  $ hg clone -r2 repo repo2
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 2 files
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd repo2
+  $ hg pull -r3 ../repo
+  pulling from ../repo
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  (run 'hg heads' to see heads, 'hg merge' to merge)
+
+Test without --exact and diff.p1 == workingdir.p1
+
+  $ hg up 1
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg import ../merge.diff
+  applying ../merge.diff
+  $ tipparents
+  1:540395c44225 changea
+  3:102a90ea7b4a addb
+  $ hg strip --no-backup tip
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+Test without --exact and diff.p1 != workingdir.p1
+
+  $ hg up 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg import ../merge.diff
+  applying ../merge.diff
+  $ tipparents
+  2:890ecaa90481 addc
+  $ hg strip --no-backup tip
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+Test with --exact
+
+  $ hg import --exact ../merge.diff
+  applying ../merge.diff
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ tipparents
+  1:540395c44225 changea
+  3:102a90ea7b4a addb
+  $ hg strip --no-backup tip
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
+Test with --bypass and diff.p1 == workingdir.p1
+
+  $ hg up 1
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg import --bypass ../merge.diff
+  applying ../merge.diff
+  $ tipparents
+  1:540395c44225 changea
+  3:102a90ea7b4a addb
+  $ hg strip --no-backup tip
+
+Test with --bypass and diff.p1 != workingdir.p1
+
+  $ hg up 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg import --bypass ../merge.diff
+  applying ../merge.diff
+  $ tipparents
+  2:890ecaa90481 addc
+  $ hg strip --no-backup tip
+
+Test with --bypass and --exact
+
+  $ hg import --bypass --exact ../merge.diff
+  applying ../merge.diff
+  $ tipparents
+  1:540395c44225 changea
+  3:102a90ea7b4a addb
+  $ hg strip --no-backup tip
+