bug#62700: 29.0.60; minibuffer-{previous,next,choose}-completion behave unintuitively when point is not at end of buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: sbaugh@catern.com
> Date: Fri, 21 Apr 2023 18:56:35 +0000 (UTC)
> Cc: Spencer Baugh <sbaugh@janestreet.com>, 62700@debbugs.gnu.org,
> 	juri@linkov.net
> 
> >> Ah, I thought Eli still wanted a backport version because this changes
> >> code which has been on Emacs 29 for over a year.
> >
> > Indeed, that's what I would like to see on the release branch.  Mainly
> > because even if this is deemed a bug, it happens in a relatively rare
> > situation, so I'd like to avoid risking breakage in code which affects
> > other situations.
> 
> Here's the backport for the release branch.

Thanks, but I'd like to make this still safer for the release branch:

> --- a/lisp/minibuffer.el
> +++ b/lisp/minibuffer.el
> @@ -4464,13 +4464,21 @@ minibuffer-next-completion
>  When `minibuffer-completion-auto-choose' is non-nil, then also
>  insert the selected completion to the minibuffer."
>    (interactive "p")
> -  (let ((auto-choose minibuffer-completion-auto-choose))
> +  (let* ((auto-choose minibuffer-completion-auto-choose)
> +         ;; Backported fix for bug#62700
> +         (md (completion--field-metadata (minibuffer--completion-prompt-end)))
> +         (base-suffix
> +          (if (eq (alist-get 'category (cdr md)) 'file)
> +              (buffer-substring (save-excursion (search-forward "/" nil t) (point))
> +                                (point-max))
> +            "")))
>      (with-minibuffer-completions-window
>        (when completions-highlight-face
>          (setq-local cursor-face-highlight-nonselected-window t))
>        (next-completion (or n 1))
>        (when auto-choose
> -        (let ((completion-use-base-affixes t))
> +        (let ((completion-use-base-affixes t)
> +              (completion-base-affixes (list (car completion-base-affixes) base-suffix)))
>            (choose-completion nil t t))))))

Here, the values used only when minibuffer-completion-auto-choose is
non-nil should be computed only when that variable is non-nil,
preferably inside the '(when auto-choose' clause.

> @@ -4489,9 +4497,17 @@ minibuffer-choose-completion
>  If NO-QUIT is non-nil, insert the completion at point to the
>  minibuffer, but don't quit the completions window."
>    (interactive "P")
> -  (with-minibuffer-completions-window
> -    (let ((completion-use-base-affixes t))
> -      (choose-completion nil no-exit no-quit))))
> +  ;; Backported fix for bug#62700
> +  (let* ((md (completion--field-metadata (minibuffer--completion-prompt-end)))
> +         (base-suffix
> +          (if (eq (alist-get 'category (cdr md)) 'file)
> +              (buffer-substring (save-excursion (search-forward "/" nil t) (point))
> +                                (point-max))
> +            "")))
> +    (with-minibuffer-completions-window
> +      (let ((completion-use-base-affixes t)
> +            (completion-base-affixes (list (car completion-base-affixes) base-suffix)))
> +        (choose-completion nil no-exit no-quit)))))

And here we seem to be modifying code that is not only for when
minibuffer-completion-auto-choose is non-nil, or what am I missing?