Subject: Re: [PATCH] rebase -i: permit 'drop' of a merge commit

From: Junio C Hamano <[email-masked]>
Date: Wed, 06 Aug 2025 14:04:31 -0700

Johannes Sixt <[email-masked]> writes:

> 4c063c82e9 (rebase -i: improve error message when picking merge,
> 2024-05-30) added advice texts for cases when a merge commit is
> passed as argument of sequencer command that cannot operate with
> a merge commit. However, it forgot about the 'drop' command, so
> that in this case the BUG() in the default branch is reached.
>
> Handle 'drop' like 'merge', i.e., permit it without a message.
>
> Signed-off-by: Johannes Sixt <[email-masked]>
> ---
>  sequencer.c                   | 1 +
>  t/t3404-rebase-interactive.sh | 1 +
>  2 files changed, 2 insertions(+)

Thanks.  Now I understand why some people are sometimes tempted to
omit the default arm in switch() and allow compilers complain when
explicit case arms are not exhaustive.  I am not saying we should do
so, and I am not convinced that it is a good idea (there are cases
you cannot afford to be exhausitive, yet the cases your particular
switch must care about are multiple to make an if/else if cascade
impractical).  But this is one of the case it might make sense.

> diff --git a/sequencer.c b/sequencer.c
> index aaf2e4df64..9ae40a91b2 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -2720,8 +2720,9 @@ static int check_merge_commit_insn(enum todo_command command)
>  	case TODO_SQUASH:
>  		return error(_("cannot squash merge commit into another commit"));
>  
>  	case TODO_MERGE:
> +	case TODO_DROP:
>  		return 0;
>  
>  	default:
>  		BUG("unexpected todo_command");
> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
> index 6bac217ed3..34d6ad0770 100755
> --- a/t/t3404-rebase-interactive.sh
> +++ b/t/t3404-rebase-interactive.sh
> @@ -2262,8 +2262,9 @@ rebase_setup_and_clean () {
>  	reword $oid
>  	edit $oid
>  	fixup $oid
>  	squash $oid
> +	drop $oid # acceptable, no advice
>  	EOF
>  	(
>  		set_replace_editor todo &&
>  		test_must_fail git rebase -i HEAD 2>actual