[PATCH] merge: give clearer error messages when there is nothing to merge

[Greg Ward]

# HG changeset patch
# User Greg Ward <greg-hg at gerg.ca>
# Date 1276536677 14400
# Node ID c6971a790206f824df9b25607fa4d030baad92fd
# Parent  39d4534094c12f1b679be1e203e6b5cc21a4f859
merge: give clearer error messages when there is nothing to merge

diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -462,18 +462,28 @@
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)
         fastforward = False
 
+        errprefix = _('nothing to merge')
+        errself = _('attempt to merge current changeset with itself')
+        errancestor = _('attempt to merge with an ancestor of the working dir')
+        errdesc = _('attempt to merge with a descendant of the working dir')
+
         ### check phase
         if not overwrite and len(pl) > 1:
             raise util.Abort(_("outstanding uncommitted merges"))
         if branchmerge:
-            if pa == p2:
-                raise util.Abort(_("can't merge with ancestor"))
+            if p1 == p2:
+                raise util.Abort('%s\n(%s)' % (errprefix, errself))
+            elif pa == p2:
+                raise util.Abort('%s\n(%s)' % (errprefix, errancestor))
             elif pa == p1:
                 if p1.branch() != p2.branch():
                     fastforward = True
                 else:
-                    raise util.Abort(_("nothing to merge (use 'hg update'"
-                                       " or check 'hg heads')"))
+                    # we could advise the user to update to the latest branch
+                    # head, but we should check that there is a later branch
+                    # head first
+                    raise util.Abort('%s\n(%s)' % (errprefix, errdesc))
+
             if not force and (wc.files() or wc.deleted()):
                 raise util.Abort(_("outstanding uncommitted changes "
                                    "(use 'hg status' to list changes)"))
diff --git a/tests/test-issue619.out b/tests/test-issue619.out
--- a/tests/test-issue619.out
+++ b/tests/test-issue619.out
@@ -6,5 +6,6 @@
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 (branch merge, don't forget to commit)
 bogus fast-forward should fail
-abort: can't merge with ancestor
+abort: nothing to merge
+(attempt to merge with an ancestor of the working dir)
 done