bug#60464: 29.0.60; Regression - pcomplete-arg fails with argument 'last

[Stefan Monnier]

>    (let ((arg
>           (nth (+ (pcase index
>                    ('first 0)
> @@ -659,11 +660,11 @@ pcomplete-arg
>                    (_      (- pcomplete-index (or index 0))))
>                  (or offset 0))
>                pcomplete-args)))
> -    (if (stringp arg)
> +    (if (or (stringp arg)
> +            (eq index 'last))
>          arg
>        (propertize
> -       (buffer-substring (pcomplete-begin index offset)
> -                         (pcomplete-begin (1- (or index 0)) offset))
> +       (car (split-string (pcomplete-actual-arg index offset)))
>         'pcomplete-arg-value arg))))

I'm not sure what specific problem this is trying to solve (is it the
choice of the "index" or is it the precise buffer positions of the
bounds)?

For the first, would the patch below help?
[ For the second, I suspect we can't provide 100% reliably correct
  information anyway, so I think we'll need to find concrete usecases
  where it matters before we can judge how much effort is warranted.  ]


        Stefan


diff --git a/lisp/pcomplete.el b/lisp/pcomplete.el
index 2d3730e294a..815ad252fbd 100644
--- a/lisp/pcomplete.el
+++ b/lisp/pcomplete.el
@@ -1,6 +1,6 @@
 ;;; pcomplete.el --- programmable completion -*- lexical-binding: t -*-
 
-;; Copyright (C) 1999-2022 Free Software Foundation, Inc.
+;; Copyright (C) 1999-2023 Free Software Foundation, Inc.
 
 ;; Author: John Wiegley <johnw@gnu.org>
 ;; Keywords: processes abbrev
@@ -652,13 +652,12 @@ pcomplete-arg
 representation of the argument, namely what the user actually
 typed in, is returned, and the value of the argument is stored in
 the pcomplete-arg-value text property of that string."
-  (let ((arg
-         (nth (+ (pcase index
+  (let* ((index (+ (pcase index
                   ('first 0)
                   ('last  pcomplete-last)
                   (_      (- pcomplete-index (or index 0))))
-                (or offset 0))
-              pcomplete-args)))
+                (or offset 0)))
+         (arg (nth index pcomplete-args)))
     (if (stringp arg)
         arg
       (propertize