Bug: `git switch --detach` gives inapplicable advice

>
> By definition, a detached HEAD state is tentative and there is no
> configured "upstream" that it always wants to integrate with.  But
> if you detach from a branch that is behind its upstream, e.g.,
>
>     $ git checkout -t -b main origin/main
>     $ git checkout main
>     $ git reset --hard HEAD^
>     $ git checkout --detach main
>
> you'd see "you are behind your upstream origin/main".  This does not
> happen when you replace the last step in the above with any of these
>
>     $ git checkout HEAD^0
>     $ git checkout --detach HEAD
>     $ git checkout --detach origin/main
>
> Before 32669671 (checkout: introduce --detach synonym for "git
> checkout foo^{commit}", 2011-02-08) introduced the "--detach"
> option, the rule to decide if we show the tracking information
> used to be:
>
>     If --quiet is not given, and if the given branch name is a real
>     local branch (i.e. the one we can compute the file path under
>     .git/, like 'refs/heads/master' or "HEAD" which stand for the
>     name of the current branch", then give the tracking information.
>
> to exclude things like "git checkout master^0" (which was the
> official way to detach HEAD at the commit before that commit) and
> "git checkout origin/master^0" from showing tracking information,
> but still do show the tracking information for the current branch
> for "git checkout HEAD".  The introduction of an explicit option
> "--detach" broke this subtley.  The new rule should have been
>
>     If --quiet is given, do not bother with tracking info.
>     If --detach is given, do not bother with tracking info.
>
>     Otherwise, if we know that the branch name given is a real local
>     branch, or if we were given "HEAD" and "HEAD" is not detached,
>     then attempt to show the tracking info.
>
> but it allowed "git checkout --detach master" to also show the
> tracking info by mistake.  Let's tighten the rule to fix this.
>
> Reported-by: mirth hickford <mirth.hickford@gmail.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>
>     mirth hickford <mirth.hickford@gmail.com> writes:
>
>     >> git switch --detach main
>     > HEAD is now at 5a07c3bde Refactor...
>     > Your branch is behind 'origin/main' by 1 commit, and can be fast-forwarded.
>     >   (use "git pull" to update your local branch)
>     >
>     >> git pull
>     > You are not currently on a branch.
>
>     I think the instruction meant to say "use 'git pull' with
>     appropriate options to update" but had to ellide the details due
>     to limited screen real estate.  And it is quite expected that a
>     "git pull" from a detached HEAD would not do anything.
>
>     Because my local branches do not track any upstream branches, I
>     have little experience with the message in question.  Given
>     that, I doubt that I am the best person to comment on this
>     issue, but having said that, I observe that this is shared
>     between "checkout" and "switch".
>
>     Having said that, I think that the instruction is what is wrong.
>     What is wrong is the fact that the command assumed that you want
>     to keep up with the upstream of the branch you detached from.
>
>  builtin/checkout.c         | 3 ++-
>  t/t2020-checkout-detach.sh | 5 ++++-
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/builtin/checkout.c b/builtin/checkout.c
> index 2e8b0d18f4..26e1a64569 100644
> --- a/builtin/checkout.c
> +++ b/builtin/checkout.c
> @@ -1030,7 +1030,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
>         remove_branch_state(the_repository, !opts->quiet);
>         strbuf_release(&msg);
>         if (!opts->quiet &&
> -           (new_branch_info->path || (!opts->force_detach && !strcmp(new_branch_info->name, "HEAD"))))
> +           !opts->force_detach &&
> +           (new_branch_info->path || !strcmp(new_branch_info->name, "HEAD")))
>                 report_tracking(new_branch_info);
>  }
>
> diff --git a/t/t2020-checkout-detach.sh b/t/t2020-checkout-detach.sh
> index bce284c297..8d90d02850 100755
> --- a/t/t2020-checkout-detach.sh
> +++ b/t/t2020-checkout-detach.sh
> @@ -176,7 +176,10 @@ test_expect_success 'tracking count is accurate after orphan check' '
>         git config branch.child.merge refs/heads/main &&
>         git checkout child^ &&
>         git checkout child >stdout &&
> -       test_cmp expect stdout
> +       test_cmp expect stdout &&
> +
> +       git checkout --detach child >stdout &&
> +       test_grep ! "can be fast-forwarded\." stdout
>  '
>
>  test_expect_success 'no advice given for explicit detached head state' '
> --
> 2.44.0-413-gd6fd04375f
>
>
>