Re: [PATCH v7 4/4] checkout: -m (--merge) uses autostash when switching branches

>  === 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 the stash application results in conflicts, they are not
> +resolved and the stash is saved to the stash list:

> ...
> +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.
>  ------------
> +
> +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`.

> diff --git a/Documentation/git-switch.adoc b/Documentation/git-switch.adoc
> index 87707e9265..3147023dc3 100644
> --- a/Documentation/git-switch.adoc
> +++ b/Documentation/git-switch.adoc
> @@ -126,15 +126,16 @@ variable.
>  	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.  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.

> @@ -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/t/t7201-co.sh b/t/t7201-co.sh
> index 9bcf7c0b40..857a5a6bd1 100755
> --- a/t/t7201-co.sh
> +++ b/t/t7201-co.sh
> @@ -210,6 +210,194 @@ 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 skips stash when no conflict' '
> +	git checkout -f main &&
> +	git clean -f &&
> +
> +	fill 0 x y z >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 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 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 checkout -m nonoverlap_base 2>stderr &&
> +	test_grep "Applied autostash" stderr &&
> +	test_grep ! "resulted in conflicts" stderr &&
> +
> +	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
> +'