--- a/hgext/rebase.py	Wed Apr 28 08:48:10 2021 -0700
+++ b/hgext/rebase.py	Tue Feb 23 10:28:42 2021 -0800
@@ -446,8 +446,15 @@
             rebaseset = set(destmap.keys())
             rebaseset -= set(self.obsolete_with_successor_in_destination)
             rebaseset -= self.obsolete_with_successor_in_rebase_set
+            # We have our own divergence-checking in the rebase extension
+            overrides = {}
+            if obsolete.isenabled(self.repo, obsolete.createmarkersopt):
+                overrides = {
+                    (b'experimental', b'evolution.allowdivergence'): b'true'
+                }
             try:
-                rewriteutil.precheck(self.repo, rebaseset, action=b'rebase')
+                with self.ui.configoverride(overrides):
+                    rewriteutil.precheck(self.repo, rebaseset, action=b'rebase')
             except error.Abort as e:
                 if e.hint is None:
                     e.hint = _(b'use --keep to keep original changesets')

--- a/mercurial/obsolete.py	Wed Apr 28 08:48:10 2021 -0700
+++ b/mercurial/obsolete.py	Tue Feb 23 10:28:42 2021 -0800
@@ -106,6 +106,7 @@
 # Options for obsolescence
 createmarkersopt = b'createmarkers'
 allowunstableopt = b'allowunstable'
+allowdivergenceopt = b'allowdivergence'
 exchangeopt = b'exchange'
 
 
@@ -144,10 +145,13 @@
 
     createmarkersvalue = _getoptionvalue(repo, createmarkersopt)
     unstablevalue = _getoptionvalue(repo, allowunstableopt)
+    divergencevalue = _getoptionvalue(repo, allowdivergenceopt)
     exchangevalue = _getoptionvalue(repo, exchangeopt)
 
     # createmarkers must be enabled if other options are enabled
-    if (unstablevalue or exchangevalue) and not createmarkersvalue:
+    if (
+        unstablevalue or divergencevalue or exchangevalue
+    ) and not createmarkersvalue:
         raise error.Abort(
             _(
                 b"'createmarkers' obsolete option must be enabled "
@@ -158,6 +162,7 @@
     return {
         createmarkersopt: createmarkersvalue,
         allowunstableopt: unstablevalue,
+        allowdivergenceopt: divergencevalue,
         exchangeopt: exchangevalue,
     }
 

--- a/mercurial/rewriteutil.py	Wed Apr 28 08:48:10 2021 -0700
+++ b/mercurial/rewriteutil.py	Tue Feb 23 10:28:42 2021 -0800
@@ -44,7 +44,9 @@
         revs = (r.rev() for r in revs)
 
     if len(repo[None].parents()) > 1:
-        raise error.StateError(_(b"cannot %s changesets while merging") % action)
+        raise error.StateError(
+            _(b"cannot %s changesets while merging") % action
+        )
 
     publicrevs = repo.revs(b'%ld and public()', revs)
     if publicrevs:
@@ -59,6 +61,38 @@
             _(b"cannot %s changeset with children") % action, hint=hint
         )
 
+    if not obsolete.isenabled(repo, obsolete.allowdivergenceopt):
+        new_divergence = _find_new_divergence(repo, revs)
+        if new_divergence:
+            local_ctx, other_ctx, base_ctx = new_divergence
+            msg = _(
+                b'cannot %s %s, as that creates content-divergence with %s'
+            ) % (
+                action,
+                local_ctx,
+                other_ctx,
+            )
+            if local_ctx.rev() != base_ctx.rev():
+                msg += _(b', from %s') % base_ctx
+            if repo.ui.verbose:
+                if local_ctx.rev() != base_ctx.rev():
+                    msg += _(
+                        b'\n    changeset %s is a successor of ' b'changeset %s'
+                    ) % (local_ctx, base_ctx)
+                msg += _(
+                    b'\n    changeset %s already has a successor in '
+                    b'changeset %s\n'
+                    b'    rewriting changeset %s would create '
+                    b'"content-divergence"\n'
+                    b'    set experimental.evolution.allowdivergence=True to '
+                    b'skip this check'
+                ) % (base_ctx, other_ctx, local_ctx)
+                raise error.InputError(msg)
+            else:
+                raise error.InputError(
+                    msg, hint=_(b"add --verbose for details")
+                )
+
 
 def disallowednewunstable(repo, revs):
     """Checks whether editing the revs will create new unstable changesets and
@@ -73,6 +107,40 @@
     return repo.revs(b"(%ld::) - %ld", revs, revs)
 
 
+def _find_new_divergence(repo, revs):
+    obsrevs = repo.revs(b'%ld and obsolete()', revs)
+    for r in obsrevs:
+        div = find_new_divergence_from(repo, repo[r])
+        if div:
+            return (repo[r], repo[div[0]], repo[div[1]])
+    return None
+
+
+def find_new_divergence_from(repo, ctx):
+    """return divergent revision if rewriting an obsolete cset (ctx) will
+    create divergence
+
+    Returns (<other node>, <common ancestor node>) or None
+    """
+    if not ctx.obsolete():
+        return None
+    # We need to check two cases that can cause divergence:
+    # case 1: the rev being rewritten has a non-obsolete successor (easily
+    #     detected by successorssets)
+    sset = obsutil.successorssets(repo, ctx.node())
+    if sset:
+        return (sset[0][0], ctx.node())
+    else:
+        # case 2: one of the precursors of the rev being revived has a
+        #     non-obsolete successor (we need divergentsets for this)
+        divsets = obsutil.divergentsets(repo, ctx)
+        if divsets:
+            nsuccset = divsets[0][b'divergentnodes']
+            prec = divsets[0][b'commonpredecessor']
+            return (nsuccset[0], prec)
+        return None
+
+
 def skip_empty_successor(ui, command):
     empty_successor = ui.config(b'rewrite', b'empty-successor')
     if empty_successor == b'skip':

--- a/tests/test-amend.t	Wed Apr 28 08:48:10 2021 -0700
+++ b/tests/test-amend.t	Tue Feb 23 10:28:42 2021 -0800
@@ -232,6 +232,17 @@
   $ hg debugobsolete -r .
   112478962961147124edd43549aedd1a335e44bf be169c7e8dbe21cd10b3d79691cbe7f241e3c21c 0 (Thu Jan 01 00:00:00 1970 +0000) {'ef1': '8', 'operation': 'amend', 'user': 'test'}
   be169c7e8dbe21cd10b3d79691cbe7f241e3c21c 16084da537dd8f84cfdb3055c633772269d62e1b 0 (Thu Jan 01 00:00:00 1970 +0000) {'ef1': '8', 'note': 'adding bar', 'operation': 'amend', 'user': 'test'}
+
+Cannot cause divergence by default
+
+  $ hg co --hidden 1
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg amend -m divergent
+  abort: cannot amend 112478962961, as that creates content-divergence with 16084da537dd
+  (add --verbose for details)
+  [10]
+  $ hg amend -m divergent --config experimental.evolution.allowdivergence=true
+  2 new content-divergent changesets
 #endif
 
 Cannot amend public changeset

--- a/tests/test-branch-change.t	Wed Apr 28 08:48:10 2021 -0700
+++ b/tests/test-branch-change.t	Tue Feb 23 10:28:42 2021 -0800
@@ -150,7 +150,8 @@
   [255]
 
   $ hg branch -r 4 --hidden foobar
-  abort: cannot change branch of a obsolete changeset
+  abort: cannot change branch of 3938acfb5c0f, as that creates content-divergence with 7c1991464886
+  (add --verbose for details)
   [10]
 
 Make sure bookmark movement is correct

--- a/tests/test-obshistory.t	Wed Apr 28 08:48:10 2021 -0700
+++ b/tests/test-obshistory.t	Tue Feb 23 10:28:42 2021 -0800
@@ -13,6 +13,7 @@
   > [experimental]
   > evolution.createmarkers = yes
   > evolution.effect-flags = yes
+  > evolution.allowdivergence=true
   > EOF
 
 Test output on amended commit

--- a/tests/test-obsmarker-template.t	Wed Apr 28 08:48:10 2021 -0700
+++ b/tests/test-obsmarker-template.t	Tue Feb 23 10:28:42 2021 -0800
@@ -11,6 +11,7 @@
   > publish=False
   > [experimental]
   > evolution=true
+  > evolution.allowdivergence=true
   > [templates]
   > obsfatesuccessors = "{if(successors, " as ")}{join(successors, ", ")}"
   > obsfateverb = "{obsfateverb(successors, markers)}"

--- a/tests/test-unamend.t	Wed Apr 28 08:48:10 2021 -0700
+++ b/tests/test-unamend.t	Tue Feb 23 10:28:42 2021 -0800
@@ -6,6 +6,7 @@
   > glog = log -G -T '{rev}:{node|short}  {desc}'
   > [experimental]
   > evolution = createmarkers, allowunstable
+  > evolution.allowdivergence = true
   > [extensions]
   > rebase =
   > amend =