Subject: [PATCH v11 4/4] checkout: -m (--merge) uses autostash when switching branches

From: "Harald Nordgren via GitGitGadget" <[email-masked]>
Date: Tue, 14 Apr 2026 10:50:19 +0000

From: Harald Nordgren <[email-masked]>

When switching branches with "git checkout -m", local modifications
can block the switch.  Teach the -m flow to create a temporary stash
before switching and reapply it after.  On success, only "Applied
autostash." is shown.  If reapplying causes conflicts, the stash is
kept and the user is told they can resolve and run "git stash drop",
or run "git reset --hard" and later "git stash pop" to recover their
changes.

Signed-off-by: Harald Nordgren <[email-masked]>
---
 Documentation/git-checkout.adoc |  58 ++++-----
 Documentation/git-switch.adoc   |  33 ++---
 builtin/checkout.c              | 147 ++++++++++------------
 sequencer.c                     |  62 +++++++---
 sequencer.h                     |   6 +-
 t/t3420-rebase-autostash.sh     |  24 +++-
 t/t7201-co.sh                   | 208 ++++++++++++++++++++++++++++++++
 t/t7600-merge.sh                |   2 +-
 xdiff-interface.c               |  12 ++
 xdiff-interface.h               |   1 +
 10 files changed, 403 insertions(+), 150 deletions(-)

diff --git a/Documentation/git-checkout.adoc b/Documentation/git-checkout.adoc
index 43ccf47cf6..70dd211ee3 100644
--- a/Documentation/git-checkout.adoc
+++ b/Documentation/git-checkout.adoc
@@ -251,20 +251,19 @@ working tree, by copying them from elsewhere, extracting a tarball, etc.
 	are different between the current branch and the branch to
 	which you are switching, the command refuses to switch
 	branches in order to preserve your modifications in context.
-	However, with this option, a three-way merge between the current
-	branch, your working tree contents, and the new branch
-	is done, and you will be on the new branch.
-+
-When a merge conflict happens, the index entries for conflicting
-paths are left unmerged, and you need to resolve the conflicts
-and mark the resolved paths with `git add` (or `git rm` if the merge
-should result in deletion of the path).
+	With this option, the conflicting local changes are
+	automatically stashed before the switch and reapplied
+	afterwards.  If the local changes do not overlap with the
+	differences between branches, the switch proceeds without
+	stashing.  If reapplying the stash results in conflicts, the
+	entry is saved to the stash list.  Resolve the conflicts
+	and run `git stash drop` when done, or clear the working
+	tree (e.g. with `git reset --hard`) before running `git stash
+	pop` later to re-apply your changes.
 +
 When checking out paths from the index, this option lets you recreate
 the conflicted merge in the specified paths.  This option cannot be
 used when checking out paths from a tree-ish.
-+
-When switching branches with `--merge`, staged changes may be lost.
 
 `--conflict=<style>`::
 	The same as `--merge` option above, but changes the way the
@@ -578,39 +577,44 @@ $ git checkout mytopic
 error: You have local changes to 'frotz'; not switching branches.
 ------------
 
-You can give the `-m` flag to the command, which would try a
-three-way merge:
+You can give the `-m` flag to the command, which would carry your local
+changes to the new branch:
 
 ------------
 $ git checkout -m mytopic
-Auto-merging frotz
+Switched to branch 'mytopic'
 ------------
 
-After this three-way merge, the local modifications are _not_
+After the switch, the local modifications are reapplied and are _not_
 registered in your index file, so `git diff` would show you what
 changes you made since the tip of the new branch.
 
 === 3. Merge conflict
 
-When a merge conflict happens during switching branches with
-the `-m` option, you would see something like this:
+When the `--merge` (`-m`) option is in effect and the locally
+modified files overlap with files that need to be updated by the
+branch switch, the changes are stashed and reapplied after the
+switch.  If this process results in conflicts, a stash entry is saved
+and made available in `git stash list`:
 
 ------------
 $ git checkout -m mytopic
-Auto-merging frotz
-ERROR: Merge conflict in frotz
-fatal: merge program failed
-------------
+Your local changes are stashed, however, applying it to carry
+forward your local changes resulted in conflicts:
 
-At this point, `git diff` shows the changes cleanly merged as in
-the previous example, as well as the changes in the conflicted
-files.  Edit and resolve the conflict and mark it resolved with
-`git add` as usual:
+ - You can try resolving them now.  If you resolved them
+   successfully, discard the stash entry with "git stash drop".
 
+ - Alternatively you can "git reset --hard" if you do not want
+   to deal with them right now, and later "git stash pop" to
+   recover your local changes.
 ------------
-$ edit frotz
-$ git add frotz
-------------
+
+You can try resolving the conflicts now.  Edit the conflicting files
+and mark them resolved with `git add` as usual, then run `git stash
+drop` to discard the stash entry.  Alternatively, you can clear the
+working tree with `git reset --hard` and recover your local changes
+later with `git stash pop`.
 
 CONFIGURATION
 -------------
diff --git a/Documentation/git-switch.adoc b/Documentation/git-switch.adoc
index 87707e9265..ee58a4d0fd 100644
--- a/Documentation/git-switch.adoc
+++ b/Documentation/git-switch.adoc
@@ -123,18 +123,19 @@ variable.
 
 `-m`::
 `--merge`::
-	If you have local modifications to one or more files that are
-	different between the current branch and the branch to which
-	you are switching, the command refuses to switch branches in
-	order to preserve your modifications in context.  However,
-	with this option, a three-way merge between the current
-	branch, your working tree contents, and the new branch is
-	done, and you will be on the new branch.
-+
-When a merge conflict happens, the index entries for conflicting
-paths are left unmerged, and you need to resolve the conflicts
-and mark the resolved paths with `git add` (or `git rm` if the merge
-should result in deletion of the path).
+	If you have local modifications to one or more files that
+	are different between the current branch and the branch to
+	which you are switching, the command normally refuses to
+	switch branches in order to preserve your modifications in
+	context.  However, with this option, the conflicting local
+	changes are automatically stashed before the switch and
+	reapplied afterwards.  If the local changes do not overlap
+	with the differences between branches, the switch proceeds
+	without stashing.  If reapplying the stash results in
+	conflicts, the entry is saved to the stash list.  Resolve
+	the conflicts and run `git stash drop` when done, or clear
+	the working tree (e.g. with `git reset --hard`) before
+	running `git stash pop` later to re-apply your changes.
 
 `--conflict=<style>`::
 	The same as `--merge` option above, but changes the way the
@@ -217,15 +218,15 @@ $ git switch mytopic
 error: You have local changes to 'frotz'; not switching branches.
 ------------
 
-You can give the `-m` flag to the command, which would try a three-way
-merge:
+You can give the `-m` flag to the command, which would carry your local
+changes to the new branch:
 
 ------------
 $ git switch -m mytopic
-Auto-merging frotz
+Switched to branch 'mytopic'
 ------------
 
-After this three-way merge, the local modifications are _not_
+After the switch, the local modifications are reapplied and are _not_
 registered in your index file, so `git diff` would show you what
 changes you made since the tip of the new branch.
 
diff --git a/builtin/checkout.c b/builtin/checkout.c
index e031e61886..2adfefa085 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -17,7 +17,6 @@
 #include "merge-ll.h"
 #include "lockfile.h"
 #include "mem-pool.h"
-#include "merge-ort-wrappers.h"
 #include "object-file.h"
 #include "object-name.h"
 #include "odb.h"
@@ -30,6 +29,7 @@
 #include "repo-settings.h"
 #include "resolve-undo.h"
 #include "revision.h"
+#include "sequencer.h"
 #include "setup.h"
 #include "submodule.h"
 #include "symlinks.h"
@@ -783,8 +783,10 @@ static int merge_working_tree(const struct checkout_opts *opts,
 	struct tree *new_tree;
 
 	repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
-	if (repo_read_index_preload(the_repository, NULL, 0) < 0)
+	if (repo_read_index_preload(the_repository, NULL, 0) < 0) {
+		rollback_lock_file(&lock_file);
 		return error(_("index file corrupt"));
+	}
 
 	resolve_undo_clear_index(the_repository->index);
 	if (opts->new_orphan_branch && opts->orphan_from_empty_tree) {
@@ -797,14 +799,18 @@ static int merge_working_tree(const struct checkout_opts *opts,
 	} else {
 		new_tree = repo_get_commit_tree(the_repository,
 						new_branch_info->commit);
-		if (!new_tree)
+		if (!new_tree) {
+			rollback_lock_file(&lock_file);
 			return error(_("unable to read tree (%s)"),
 				     oid_to_hex(&new_branch_info->commit->object.oid));
+		}
 	}
 	if (opts->discard_changes) {
 		ret = reset_tree(new_tree, opts, 1, writeout_error, new_branch_info);
-		if (ret)
+		if (ret) {
+			rollback_lock_file(&lock_file);
 			return ret;
+		}
 	} else {
 		struct tree_desc trees[2];
 		struct tree *tree;
@@ -814,6 +820,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		refresh_index(the_repository->index, REFRESH_QUIET, NULL, NULL, NULL);
 
 		if (unmerged_index(the_repository->index)) {
+			rollback_lock_file(&lock_file);
 			error(_("you need to resolve your current index first"));
 			return 1;
 		}
@@ -846,82 +853,8 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		ret = unpack_trees(2, trees, &topts);
 		clear_unpack_trees_porcelain(&topts);
 		if (ret == -1) {
-			/*
-			 * Unpack couldn't do a trivial merge; either
-			 * give up or do a real merge, depending on
-			 * whether the merge flag was used.
-			 */
-			struct tree *work;
-			struct tree *old_tree;
-			struct merge_options o;
-			struct strbuf sb = STRBUF_INIT;
-			struct strbuf old_commit_shortname = STRBUF_INIT;
-
-			if (!opts->merge)
-				return 1;
-
-			/*
-			 * Without old_branch_info->commit, the below is the same as
-			 * the two-tree unpack we already tried and failed.
-			 */
-			if (!old_branch_info->commit)
-				return 1;
-			old_tree = repo_get_commit_tree(the_repository,
-							old_branch_info->commit);
-
-			if (repo_index_has_changes(the_repository, old_tree, &sb))
-				die(_("cannot continue with staged changes in "
-				      "the following files:\n%s"), sb.buf);
-			strbuf_release(&sb);
-
-			/* Do more real merge */
-
-			/*
-			 * We update the index fully, then write the
-			 * tree from the index, then merge the new
-			 * branch with the current tree, with the old
-			 * branch as the base. Then we reset the index
-			 * (but not the working tree) to the new
-			 * branch, leaving the working tree as the
-			 * merged version, but skipping unmerged
-			 * entries in the index.
-			 */
-
-			add_files_to_cache(the_repository, NULL, NULL, NULL, 0,
-					0, 0);
-			init_ui_merge_options(&o, the_repository);
-			o.verbosity = 0;
-			work = write_in_core_index_as_tree(the_repository,
-							   the_repository->index);
-
-			ret = reset_tree(new_tree,
-					 opts, 1,
-					 writeout_error, new_branch_info);
-			if (ret)
-				return ret;
-			o.ancestor = old_branch_info->name;
-			if (!old_branch_info->name) {
-				strbuf_add_unique_abbrev(&old_commit_shortname,
-							 &old_branch_info->commit->object.oid,
-							 DEFAULT_ABBREV);
-				o.ancestor = old_commit_shortname.buf;
-			}
-			o.branch1 = new_branch_info->name;
-			o.branch2 = "local";
-			o.conflict_style = opts->conflict_style;
-			ret = merge_ort_nonrecursive(&o,
-						     new_tree,
-						     work,
-						     old_tree);
-			if (ret < 0)
-				die(NULL);
-			ret = reset_tree(new_tree,
-					 opts, 0,
-					 writeout_error, new_branch_info);
-			strbuf_release(&o.obuf);
-			strbuf_release(&old_commit_shortname);
-			if (ret)
-				return ret;
+			rollback_lock_file(&lock_file);
+			return 1;
 		}
 	}
 
@@ -1166,6 +1099,10 @@ static int switch_branches(const struct checkout_opts *opts,
 	struct object_id rev;
 	int flag, writeout_error = 0;
 	int do_merge = 1;
+	int created_autostash = 0;
+	struct strbuf old_commit_shortname = STRBUF_INIT;
+	struct strbuf autostash_msg = STRBUF_INIT;
+	const char *stash_label_base = NULL;
 
 	trace2_cmd_mode("branch");
 
@@ -1203,10 +1140,37 @@ static int switch_branches(const struct checkout_opts *opts,
 			do_merge = 0;
 	}
 
+	if (old_branch_info.name)
+		stash_label_base = old_branch_info.name;
+	else if (old_branch_info.commit) {
+		strbuf_add_unique_abbrev(&old_commit_shortname,
+					 &old_branch_info.commit->object.oid,
+					 DEFAULT_ABBREV);
+		stash_label_base = old_commit_shortname.buf;
+	}
+
 	if (do_merge) {
 		ret = merge_working_tree(opts, &old_branch_info, new_branch_info, &writeout_error);
+		if (ret && opts->merge) {
+			strbuf_addf(&autostash_msg,
+				    "autostash while switching to '%s'",
+				    new_branch_info->name);
+			create_autostash_ref_silent_with_msg(the_repository,
+							    "CHECKOUT_AUTOSTASH_HEAD",
+							    autostash_msg.buf);
+			created_autostash = 1;
+			ret = merge_working_tree(opts, &old_branch_info, new_branch_info, &writeout_error);
+		}
 		if (ret) {
+			apply_autostash_ref_with_labels(the_repository,
+						       "CHECKOUT_AUTOSTASH_HEAD",
+						       new_branch_info->name,
+						       "local",
+						       stash_label_base,
+						       autostash_msg.len ? autostash_msg.buf : NULL);
 			branch_info_release(&old_branch_info);
+			strbuf_release(&old_commit_shortname);
+			strbuf_release(&autostash_msg);
 			return ret;
 		}
 	}
@@ -1216,8 +1180,31 @@ static int switch_branches(const struct checkout_opts *opts,
 
 	update_refs_for_switch(opts, &old_branch_info, new_branch_info);
 
+	if (opts->conflict_style >= 0) {
+		struct strbuf cfg = STRBUF_INIT;
+		strbuf_addf(&cfg, "merge.conflictStyle=%s",
+			    conflict_style_name(opts->conflict_style));
+		git_config_push_parameter(cfg.buf);
+		strbuf_release(&cfg);
+	}
+	apply_autostash_ref_with_labels(the_repository, "CHECKOUT_AUTOSTASH_HEAD",
+				       new_branch_info->name, "local",
+				       stash_label_base,
+				       autostash_msg.len ? autostash_msg.buf : NULL);
+
+	discard_index(the_repository->index);
+	if (repo_read_index(the_repository) < 0)
+		die(_("index file corrupt"));
+
+	if (created_autostash && !opts->discard_changes && !opts->quiet &&
+	    new_branch_info->commit)
+		show_local_changes(&new_branch_info->commit->object,
+				   &opts->diff_options);
+
 	ret = post_checkout_hook(old_branch_info.commit, new_branch_info->commit, 1);
 	branch_info_release(&old_branch_info);
+	strbuf_release(&old_commit_shortname);
+	strbuf_release(&autostash_msg);
 
 	return ret || writeout_error;
 }
diff --git a/sequencer.c b/sequencer.c
index 913be115f2..febdb5b20a 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4658,6 +4658,7 @@ static enum todo_command peek_command(struct todo_list *todo_list, int offset)
 static void create_autostash_internal(struct repository *r,
 				      const char *path,
 				      const char *refname,
+				      const char *message,
 				      bool silent)
 {
 	struct strbuf buf = STRBUF_INIT;
@@ -4680,7 +4681,8 @@ static void create_autostash_internal(struct repository *r,
 		struct object_id oid;
 
 		strvec_pushl(&stash.args,
-			     "stash", "create", "autostash", NULL);
+			     "stash", "create",
+			     message ? message : "autostash", NULL);
 		stash.git_cmd = 1;
 		stash.no_stdin = 1;
 		strbuf_reset(&buf);
@@ -4716,22 +4718,30 @@ static void create_autostash_internal(struct repository *r,
 
 void create_autostash(struct repository *r, const char *path)
 {
-	create_autostash_internal(r, path, NULL, false);
+	create_autostash_internal(r, path, NULL, NULL, false);
 }
 
 void create_autostash_ref(struct repository *r, const char *refname)
 {
-	create_autostash_internal(r, NULL, refname, false);
+	create_autostash_internal(r, NULL, refname, NULL, false);
 }
 
 void create_autostash_ref_silent(struct repository *r, const char *refname)
 {
-	create_autostash_internal(r, NULL, refname, true);
+	create_autostash_internal(r, NULL, refname, NULL, true);
+}
+
+void create_autostash_ref_silent_with_msg(struct repository *r,
+					  const char *refname,
+					  const char *message)
+{
+	create_autostash_internal(r, NULL, refname, message, true);
 }
 
 static int apply_save_autostash_oid(const char *stash_oid, int attempt_apply,
 				    const char *label_ours, const char *label_theirs,
-				    const char *label_base)
+				    const char *label_base,
+				    const char *stash_msg)
 {
 	struct child_process child = CHILD_PROCESS_INIT;
 	int ret = 0;
@@ -4761,20 +4771,28 @@ static int apply_save_autostash_oid(const char *stash_oid, int attempt_apply,
 		strvec_push(&store.args, "stash");
 		strvec_push(&store.args, "store");
 		strvec_push(&store.args, "-m");
-		strvec_push(&store.args, "autostash");
+		strvec_push(&store.args, stash_msg ? stash_msg : "autostash");
 		strvec_push(&store.args, "-q");
 		strvec_push(&store.args, stash_oid);
 		if (run_command(&store))
 			ret = error(_("cannot store %s"), stash_oid);
+		else if (attempt_apply)
+			fprintf(stderr,
+				_("Your local changes are stashed, however, applying it to carry\n"
+				  "forward your local changes resulted in conflicts:\n"
+				  "\n"
+				  " - You can try resolving them now.  If you resolved them\n"
+				  "   successfully, discard the stash entry with \"git stash drop\".\n"
+				  "\n"
+				  " - Alternatively you can \"git reset --hard\" if you do not want\n"
+				  "   to deal with them right now, and later \"git stash pop\" to\n"
+				  "   recover your local changes.\n"));
 		else
 			fprintf(stderr,
-				_("%s\n"
+				_("Autostash exists; creating a new stash entry.\n"
 				  "Your changes are safe in the stash.\n"
 				  "You can run \"git stash pop\" or"
-				  " \"git stash drop\" at any time.\n"),
-				attempt_apply ?
-				_("Applying autostash resulted in conflicts.") :
-				_("Autostash exists; creating a new stash entry."));
+				  " \"git stash drop\" at any time.\n"));
 	}
 
 	return ret;
@@ -4793,7 +4811,7 @@ static int apply_save_autostash(const char *path, int attempt_apply)
 	strbuf_trim(&stash_oid);
 
 	ret = apply_save_autostash_oid(stash_oid.buf, attempt_apply,
-				      NULL, NULL, NULL);
+				      NULL, NULL, NULL, NULL);
 
 	unlink(path);
 	strbuf_release(&stash_oid);
@@ -4812,13 +4830,14 @@ int apply_autostash(const char *path)
 
 int apply_autostash_oid(const char *stash_oid)
 {
-	return apply_save_autostash_oid(stash_oid, 1, NULL, NULL, NULL);
+	return apply_save_autostash_oid(stash_oid, 1, NULL, NULL, NULL, NULL);
 }
 
 static int apply_save_autostash_ref(struct repository *r, const char *refname,
 				    int attempt_apply,
 				    const char *label_ours, const char *label_theirs,
-				    const char *label_base)
+				    const char *label_base,
+				    const char *stash_msg)
 {
 	struct object_id stash_oid;
 	char stash_oid_hex[GIT_MAX_HEXSZ + 1];
@@ -4835,7 +4854,8 @@ static int apply_save_autostash_ref(struct repository *r, const char *refname,
 
 	oid_to_hex_r(stash_oid_hex, &stash_oid);
 	ret = apply_save_autostash_oid(stash_oid_hex, attempt_apply,
-				       label_ours, label_theirs, label_base);
+				       label_ours, label_theirs, label_base,
+				       stash_msg);
 
 	refs_delete_ref(get_main_ref_store(r), "", refname,
 			&stash_oid, REF_NO_DEREF);
@@ -4845,20 +4865,24 @@ static int apply_save_autostash_ref(struct repository *r, const char *refname,
 
 int save_autostash_ref(struct repository *r, const char *refname)
 {
-	return apply_save_autostash_ref(r, refname, 0, NULL, NULL, NULL);
+	return apply_save_autostash_ref(r, refname, 0,
+					NULL, NULL, NULL, NULL);
 }
 
 int apply_autostash_ref(struct repository *r, const char *refname)
 {
-	return apply_save_autostash_ref(r, refname, 1, NULL, NULL, NULL);
+	return apply_save_autostash_ref(r, refname, 1,
+					NULL, NULL, NULL, NULL);
 }
 
 int apply_autostash_ref_with_labels(struct repository *r, const char *refname,
 				    const char *label_ours, const char *label_theirs,
-				    const char *label_base)
+				    const char *label_base,
+				    const char *stash_msg)
 {
 	return apply_save_autostash_ref(r, refname, 1,
-					label_ours, label_theirs, label_base);
+					label_ours, label_theirs, label_base,
+					stash_msg);
 }
 
 static int checkout_onto(struct repository *r, struct replay_opts *opts,
diff --git a/sequencer.h b/sequencer.h
index 2c4ff17c4e..67de755053 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -231,6 +231,9 @@ void commit_post_rewrite(struct repository *r,
 void create_autostash(struct repository *r, const char *path);
 void create_autostash_ref(struct repository *r, const char *refname);
 void create_autostash_ref_silent(struct repository *r, const char *refname);
+void create_autostash_ref_silent_with_msg(struct repository *r,
+					  const char *refname,
+					  const char *message);
 int save_autostash(const char *path);
 int save_autostash_ref(struct repository *r, const char *refname);
 int apply_autostash(const char *path);
@@ -238,7 +241,8 @@ int apply_autostash_oid(const char *stash_oid);
 int apply_autostash_ref(struct repository *r, const char *refname);
 int apply_autostash_ref_with_labels(struct repository *r, const char *refname,
 				    const char *label_ours, const char *label_theirs,
-				    const char *label_base);
+				    const char *label_base,
+				    const char *stash_msg);
 
 #define SUMMARY_INITIAL_COMMIT   (1 << 0)
 #define SUMMARY_SHOW_AUTHOR_DATE (1 << 1)
diff --git a/t/t3420-rebase-autostash.sh b/t/t3420-rebase-autostash.sh
index ad3ba6a984..e4e2cb19ce 100755
--- a/t/t3420-rebase-autostash.sh
+++ b/t/t3420-rebase-autostash.sh
@@ -61,18 +61,30 @@ create_expected_failure_apply () {
 	First, rewinding head to replay your work on top of it...
 	Applying: second commit
 	Applying: third commit
-	Applying autostash resulted in conflicts.
-	Your changes are safe in the stash.
-	You can run "git stash pop" or "git stash drop" at any time.
+	Your local changes are stashed, however, applying it to carry
+	forward your local changes resulted in conflicts:
+
+	 - You can try resolving them now.  If you resolved them
+	   successfully, discard the stash entry with "git stash drop".
+
+	 - Alternatively you can "git reset --hard" if you do not want
+	   to deal with them right now, and later "git stash pop" to
+	   recover your local changes.
 	EOF
 }
 
 create_expected_failure_merge () {
 	cat >expected <<-EOF
 	$(grep "^Created autostash: [0-9a-f][0-9a-f]*\$" actual)
-	Applying autostash resulted in conflicts.
-	Your changes are safe in the stash.
-	You can run "git stash pop" or "git stash drop" at any time.
+	Your local changes are stashed, however, applying it to carry
+	forward your local changes resulted in conflicts:
+
+	 - You can try resolving them now.  If you resolved them
+	   successfully, discard the stash entry with "git stash drop".
+
+	 - Alternatively you can "git reset --hard" if you do not want
+	   to deal with them right now, and later "git stash pop" to
+	   recover your local changes.
 	Successfully rebased and updated refs/heads/rebased-feature-branch.
 	EOF
 }
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 9bcf7c0b40..c474c6759f 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -210,6 +210,214 @@ test_expect_success 'checkout --merge --conflict=diff3 <branch>' '
 	test_cmp expect two
 '
 
+test_expect_success 'checkout --merge --conflict=zdiff3 <branch>' '
+	git checkout -f main &&
+	git reset --hard &&
+	git clean -f &&
+
+	fill a b X d e >two &&
+	git checkout --merge --conflict=zdiff3 simple &&
+
+	cat <<-EOF >expect &&
+	a
+	<<<<<<< simple
+	c
+	||||||| main
+	b
+	c
+	d
+	=======
+	b
+	X
+	d
+	>>>>>>> local
+	e
+	EOF
+	test_cmp expect two
+'
+
+test_expect_success 'checkout -m respects merge.conflictStyle config' '
+	git checkout -f main &&
+	git reset --hard &&
+	git clean -f &&
+
+	test_config merge.conflictStyle diff3 &&
+	fill b d >two &&
+	git checkout -m simple &&
+
+	cat <<-EOF >expect &&
+	<<<<<<< simple
+	a
+	c
+	e
+	||||||| main
+	a
+	b
+	c
+	d
+	e
+	=======
+	b
+	d
+	>>>>>>> local
+	EOF
+	test_cmp expect two
+'
+
+test_expect_success 'checkout -m skips stash when no conflict' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 0 x y z >same &&
+	git stash list >stash-before &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	git stash list >stash-after &&
+	test_cmp stash-before stash-after &&
+	fill 0 x y z >expect &&
+	test_cmp expect same
+'
+
+test_expect_success 'checkout -m skips stash with non-conflicting dirty index' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 0 x y z >same &&
+	git add same &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	fill 0 x y z >expect &&
+	test_cmp expect same
+'
+
+test_expect_success 'checkout -m stashes and applies on conflicting changes' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 1 2 3 4 5 6 7 >one &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	test_grep "Applied autostash" actual &&
+	fill 1 2 3 4 5 6 7 >expect &&
+	test_cmp expect one
+'
+
+test_expect_success 'checkout -m with mixed staged and unstaged changes' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 0 x y z >same &&
+	git add same &&
+	fill 1 2 3 4 5 6 7 >one &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	test_grep "Applied autostash" actual &&
+	fill 0 x y z >expect &&
+	test_cmp expect same &&
+	fill 1 2 3 4 5 6 7 >expect &&
+	test_cmp expect one
+'
+
+test_expect_success 'checkout -m stashes on truly conflicting changes' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 1 2 3 4 5 >one &&
+	test_must_fail git checkout side 2>stderr &&
+	test_grep "Your local changes" stderr &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	test_grep "resulted in conflicts" actual &&
+	test_grep "git stash drop" actual &&
+	git stash drop &&
+	git reset --hard
+'
+
+test_expect_success 'checkout -m produces usable stash on conflict' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 1 2 3 4 5 >one &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep "recover your local changes" actual &&
+	git checkout -f main &&
+	git stash pop &&
+	fill 1 2 3 4 5 >expect &&
+	test_cmp expect one
+'
+
+test_expect_success 'checkout -m autostash message includes target branch' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 1 2 3 4 5 >one &&
+	git checkout -m side >actual 2>&1 &&
+	git stash list >stash-list &&
+	test_grep "autostash while switching to .side." stash-list &&
+	git stash drop &&
+	git checkout -f main &&
+	git reset --hard
+'
+
+test_expect_success 'checkout -m stashes on staged conflicting changes' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 1 2 3 4 5 >one &&
+	git add one &&
+	git checkout -m side >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	test_grep "resulted in conflicts" actual &&
+	test_grep "git stash drop" actual &&
+	git stash drop &&
+	git reset --hard
+'
+
+test_expect_success 'checkout -m applies stash cleanly with non-overlapping changes in same file' '
+	git checkout -f main &&
+	git reset --hard &&
+	git clean -f &&
+
+	git checkout -b nonoverlap_base &&
+	fill a b c d >file &&
+	git add file &&
+	git commit -m "add file" &&
+
+	git checkout -b nonoverlap_child &&
+	fill a b c INSERTED d >file &&
+	git commit -a -m "insert line near end of file" &&
+
+	fill DIRTY a b c INSERTED d >file &&
+
+	git stash list >stash-before &&
+	git checkout -m nonoverlap_base 2>stderr &&
+	test_grep "Applied autostash" stderr &&
+	test_grep ! "resulted in conflicts" stderr &&
+
+	git stash list >stash-after &&
+	test_cmp stash-before stash-after &&
+
+	fill DIRTY a b c d >expect &&
+	test_cmp expect file &&
+
+	git checkout -f main &&
+	git branch -D nonoverlap_base &&
+	git branch -D nonoverlap_child
+'
+
+test_expect_success 'checkout -m -b skips stash with dirty tree' '
+	git checkout -f main &&
+	git clean -f &&
+
+	fill 0 x y z >same &&
+	git checkout -m -b newbranch >actual 2>&1 &&
+	test_grep ! "Created autostash" actual &&
+	fill 0 x y z >expect &&
+	test_cmp expect same &&
+	git checkout main &&
+	git branch -D newbranch
+'
+
 test_expect_success 'switch to another branch while carrying a deletion' '
 	git checkout -f main &&
 	git reset --hard &&
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 9838094b66..cbef8a534e 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -914,7 +914,7 @@ test_expect_success 'merge with conflicted --autostash changes' '
 	git diff >expect &&
 	test_when_finished "test_might_fail git stash drop" &&
 	git merge --autostash c3 2>err &&
-	test_grep "Applying autostash resulted in conflicts." err &&
+	test_grep "your local changes resulted in conflicts" err &&
 	git show HEAD:file >merge-result &&
 	test_cmp result.1-9 merge-result &&
 	git stash show -p >actual &&
diff --git a/xdiff-interface.c b/xdiff-interface.c
index f043330f2a..5ee2b96d0a 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -325,6 +325,18 @@ int parse_conflict_style_name(const char *value)
 		return -1;
 }
 
+const char *conflict_style_name(int style)
+{
+	switch (style) {
+	case XDL_MERGE_DIFF3:
+		return "diff3";
+	case XDL_MERGE_ZEALOUS_DIFF3:
+		return "zdiff3";
+	default:
+		return "merge";
+	}
+}
+
 int git_xmerge_style = -1;
 
 int git_xmerge_config(const char *var, const char *value,
diff --git a/xdiff-interface.h b/xdiff-interface.h
index fbc4ceec40..ce54e1c0e0 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -55,6 +55,7 @@ void xdiff_set_find_func(xdemitconf_t *xecfg, const char *line, int cflags);
 void xdiff_clear_find_func(xdemitconf_t *xecfg);
 struct config_context;
 int parse_conflict_style_name(const char *value);
+const char *conflict_style_name(int style);
 int git_xmerge_config(const char *var, const char *value,
 		      const struct config_context *ctx, void *cb);
 extern int git_xmerge_style;
-- 
gitgitgadget