From a7256debd4b6ba9f6ccb94c0958ef48cdd8cb664 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nguy=E1=BB=85n=20Th=C3=A1i=20Ng=E1=BB=8Dc=20Duy?=
 <pclouds@gmail.com>
Date: Tue, 19 Mar 2019 16:39:10 +0700
Subject: [PATCH] checkout.txt: note about losing staged changes with --merge
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If you have staged changes in path A and perform 'checkout
--merge' (which could result in conflicts in a totally unrelated path
B), changes in A will be gone. Which is unexpected. We are supposed
to keep all changes, or kick and scream otherwise.

This is the result of how --merge is implemented, from the very first
day in 1be0659efc (checkout: merge local modifications while switching
branches., 2006-01-12):

1. a merge is done, unmerged entries are collected
2. a hard switch to a new branch is done, then unmerged entries added
   back

There is no trivial fix for this. Going with 3-way merge one file at a
time loses rename detection. Going with 3-way merge by trees requires
teaching the algorithm to pick up staged changes. And even if we detect
staged changes with --merge and abort for safety, an option to continue
--merge is very weird. Such an option would keep worktree changes, but
drop staged changes.

Because the problem has been with us since the introduction of --merge
and everybody has been pretty happy (except Phillip, who found this
problem), I'll just take a note here to acknowledge it and wait for
merge wizards to come in and work their magic. There may be a way
forward [1].

[1] CABPp-BFoL_U=bzON4SEMaQSKU2TKwnOgNqjt5MUaOejTKGUJxw@mail.gmail.com

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git-checkout.txt | 2 ++
 builtin/checkout.c             | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 8c3d4128c2..3ea4579736 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -242,6 +242,8 @@ should result in deletion of the path).
 +
 When checking out paths from the index, this option lets you recreate
 the conflicted merge in the specified paths.
++
+When switching branches with `--merge`, staged changes may be lost.
 
 --conflict=<style>::
 	The same as --merge option above, but changes the way the
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 24b8593b93..a56fa534f1 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -676,6 +676,8 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			struct tree *result;
 			struct tree *work;
 			struct merge_options o;
+			struct strbuf sb = STRBUF_INIT;
+
 			if (!opts->merge)
 				return 1;
 
@@ -686,6 +688,13 @@ static int merge_working_tree(const struct checkout_opts *opts,
 			if (!old_branch_info->commit)
 				return 1;
 
+			if (repo_index_has_changes(the_repository,
+						   get_commit_tree(old_branch_info->commit),
+						   &sb))
+				warning(_("staged changes in the following files may be lost: %s"),
+					sb.buf);
+			strbuf_release(&sb);
+
 			/* Do more real merge */
 
 			/*