[PATCH] checkout.txt: note about losing staged changes with --merge

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

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.wood123@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 This is my "fix" for Phillip's second problem. I chose to reply here
 because this is where an actual fix was discussed. The test script to
 demonstate it is here

 https://public-inbox.org/git/7d3742d6-73e4-2750-6ecb-9edf761d96dd@gmail.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 f179b43732..877e5f503a 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 0e6037b296..f95e7975f7 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -726,6 +726,8 @@ 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;
 
@@ -736,6 +738,13 @@ 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 */
 
 			/*
-- 
2.21.0.548.gd3c7d92dc2