merge: only apply autostash when appropriate

If a merge failed and we are leaving conflicts in the working directory
for the user to resolve, we should not attempt to apply any autostash.

Further, if we fail to apply the autostash (because either the merge
failed, or the user requested --no-commit), then we should instruct the
user how to apply it later.

Add a testcase verifying we have corrected this behavior.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

builtin/merge.c

@@ -492,7 +492,8 @@ static void finish(struct commit *head_commit,
 	/* Run a post-merge hook */
 	run_hooks_l("post-merge", squash ? "1" : "0", NULL);
 
-	apply_autostash(git_path_merge_autostash(the_repository));
+	if (new_head)
+		apply_autostash(git_path_merge_autostash(the_repository));
 	strbuf_release(&reflog_message);
 }
 
@@ -1754,6 +1755,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			"stopped before committing as requested\n"));
 	else
 		ret = suggest_conflicts();
+	if (autostash)
+		printf(_("When finished, apply stashed changes with `git stash pop`\n"));
 
 done:
 	if (!automerge_was_ok) {

t/t7600-merge.sh

@@ -255,6 +255,15 @@ test_expect_success 'merge --squash c3 with c7' '
 	test_cmp expect actual
 '
 
+test_expect_success 'merge --squash --autostash conflict does not attempt to apply autostash' '
+	git reset --hard c3 &&
+	>unrelated &&
+	git add unrelated &&
+	test_must_fail git merge --squash c7 --autostash >out 2>err &&
+	! grep "Applying autostash resulted in conflicts." err &&
+	grep "When finished, apply stashed changes with \`git stash pop\`" out
+'
+
 test_expect_success 'merge c3 with c7 with commit.cleanup = scissors' '
 	git config commit.cleanup scissors &&
 	git reset --hard c3 &&