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

> 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 */
>   
>   			/*
>