master 0a36671f41: pcomplete.el: Fix part of bug#50470

[Stefan Monnier]

branch: master
commit 0a36671f415bd681ddca0bad8612aca031fd407d
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    pcomplete.el: Fix part of bug#50470
    
    Try and handle the case where `pcomplete-parse-arguments-function`
    directly returns a list of completions.
    
    * lisp/pcomplete.el (pcomplete-parse-arguments): Don't modify the
    buffer if we're not allowed to.  Instead use the buffer's current
    content as the "pattern to be completed" and return the list
    of completions as is.  Also, use `try-completions` to simplify the
    previous code.
---
 lisp/pcomplete.el | 43 ++++++++++++++++++++++++-------------------
 1 file changed, 24 insertions(+), 19 deletions(-)

diff --git a/lisp/pcomplete.el b/lisp/pcomplete.el
index a1492af89d..3393c322e3 100644
--- a/lisp/pcomplete.el
+++ b/lisp/pcomplete.el
@@ -802,25 +802,30 @@ this is `comint-dynamic-complete-functions'."
       (let ((begin (pcomplete-begin 'last)))
        (if (and (listp pcomplete-stub) ;??
                 (not pcomplete-expand-only-p))
-           (let* ((completions pcomplete-stub) ;??
-                  (common-stub (car completions))
-                  (c completions)
-                  (len (length common-stub)))
-             (while (and c (> len 0))
-               (while (and (> len 0)
-                           (not (string=
-                                 (substring common-stub 0 len)
-                                 (substring (car c) 0
-                                            (min (length (car c))
-                                                 len)))))
-                 (setq len (1- len)))
-               (setq c (cdr c)))
-             (setq pcomplete-stub (substring common-stub 0 len)
-                   pcomplete-autolist t)
-             (when (and begin (> len 0) (not pcomplete-show-list))
-               (delete-region begin (point))
-               (pcomplete-insert-entry "" pcomplete-stub))
-             (throw 'pcomplete-completions completions))
+           ;; If `pcomplete-stub' is a list, it means it's a list of
+            ;; completions computed during parsing, e.g. Eshell uses
+            ;; that to turn globs into lists of completions.
+           (if (not pcomplete-allow-modifications)
+               (progn
+                 ;; FIXME: The mapping from what's in the buffer to the list
+                  ;; of completions can be arbitrary and will often fail to be
+                  ;; understood by the completion style.  See bug#50470.
+                  ;; E.g. `pcomplete-stub' may end up being "~/Down*"
+                  ;; while the completions contain entries like
+                  ;; "/home/<foo>/Downloads" which will fail to match the
+                  ;; "~/Down*" completion pattern since the completion
+                  ;; is neither told that it's a file nor a global pattern.
+                 (setq pcomplete-stub (buffer-substring begin (point)))
+                  (throw 'pcomplete-completions pcomplete-stub))
+             (let* ((completions pcomplete-stub)
+                    (common-prefix (try-completion "" completions))
+                    (len (length common-prefix)))
+               (setq pcomplete-stub common-prefix
+                     pcomplete-autolist t)
+               (when (and begin (> len 0) (not pcomplete-show-list))
+                 (delete-region begin (point))
+                 (pcomplete-insert-entry "" pcomplete-stub))
+               (throw 'pcomplete-completions completions)))
          (when expand-p
            (if (stringp pcomplete-stub)
                (when begin