bug#55743: 28.1.50; No directory error in comp-run-async-workers

[Juri Linkov <juri@linkov.net>]

>>> So here are two problems: why completion sets default-directory to non-existent dir,
>>
>> That's the real problem.
>
> Oh, I see, it's a known problem.  In completion-setup-function:
>
>          (base-dir
>           ;; FIXME: This is a bad hack.  We try to set the default-directory
>           ;; in the *Completions* buffer so that the relative file names
>           ;; displayed there can be treated as valid file names, independently
>           ;; from the completion context.  But this suffers from many problems:
>           ;; - It's not clear when the completions are file names.  With some
>           ;;   completion tables (e.g. bzr revision specs), the listed
>           ;;   completions can mix file names and other things.
>           ;; - It doesn't pay attention to possible quoting.
>           ;; - With fancy completion styles, the code below will not always
>           ;;   find the right base directory.
>           (if minibuffer-completing-file-name
>               (file-name-as-directory
>                (expand-file-name
>                 (buffer-substring (minibuffer-prompt-end) (point)))))))
>       ...
>       (if base-dir (setq default-directory base-dir))

The problem can be solved by this patch that handles both the most
frequent cases: when the completion string is the default directory,
and when a file prefix is added to it.

diff --git a/lisp/simple.el b/lisp/simple.el
index 103e7f33dd..81e04f28e9 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -9803,7 +9802,7 @@ completion-setup-function
           ;; - With fancy completion styles, the code below will not always
           ;;   find the right base directory.
           (if minibuffer-completing-file-name
-              (file-name-as-directory
+              (file-name-directory
                (expand-file-name
                 (buffer-substring (minibuffer-prompt-end) (point)))))))
     (with-current-buffer standard-output