[PATCH v3 11/14] switch-branch: only allow explicit detached HEAD

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

"git checkout <commit>" will checkout the commit in question and
detach HEAD from the current branch. It is naturally a right thing to
do once you get git references. But detached HEAD is a scary concept
to new users because we show a lot of warnings and stuff, and it could
be hard to get out of (until you know better).

To keep switch-branch a bit more friendly to new users, we only allow
entering detached HEAD mode when --detach is given. "git
switch-branch" must take a branch (unless you create a new branch,
then of course switch-branch can take any commit-ish)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 builtin/checkout.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index c7ae068d2c..fbfebba2d9 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -49,6 +49,7 @@ struct checkout_opts {
 	int merge;
 	int force;
 	int force_detach;
+	int implicit_detach;
 	int writeout_stage;
 	int overwrite_ignore;
 	int ignore_skipworktree;
@@ -1241,6 +1242,13 @@ static int checkout_branch(struct checkout_opts *opts,
 	    !opts->force_detach)
 		die(_("nothing to do"));
 
+	if (!opts->implicit_detach &&
+	    !opts->new_branch &&
+	    !opts->new_branch_force &&
+	    new_branch_info->name &&
+	    !new_branch_info->path)
+		die(_("a branch is expected, got %s"), new_branch_info->name);
+
 	if (new_branch_info->path && !opts->force_detach && !opts->new_branch &&
 	    !opts->ignore_other_worktrees) {
 		int flag;
@@ -1485,6 +1493,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.dwim_new_local_branch = 1;
 	opts.switch_branch_doing_nothing_not_ok = 0;
 	opts.accept_pathspec = 1;
+	opts.implicit_detach = 1;
 
 	options = parse_options_dup(checkout_options);
 	options = add_common_options(&opts, options);
@@ -1513,6 +1522,7 @@ int cmd_switch_branch(int argc, const char **argv, const char *prefix)
 	opts.dwim_new_local_branch = 1;
 	opts.accept_pathspec = 0;
 	opts.switch_branch_doing_nothing_not_ok = 1;
+	opts.implicit_detach = 0;
 
 	options = parse_options_dup(switch_options);
 	options = add_common_options(&opts, options);
-- 
2.20.0.rc1.380.g3eb999425c.dirty