bug#59956: 29.0.60: Failure when completing arguments in Eshell after variable interpolation

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Still, like Stefan and Augusto mentioned, there's probably a larger issue
> here: should Eshell be allowed to feed Pcomplete non-strings? Since
> Pcomplete was written for Eshell initially, there's some basis for why it
> *might* support non-string values, but actually requiring that is an awful
> lot to ask of every programmer who ever wants to write a pcomplete function.

A "general" solution might be the one below, tho it looks more like
a general workaround, I think.

John?


        Stefan


diff --git a/lisp/pcomplete.el b/lisp/pcomplete.el
index 4e3a88bbda8..6a4d754b8c0 100644
--- a/lisp/pcomplete.el
+++ b/lisp/pcomplete.el
@@ -646,12 +646,14 @@ pcomplete-arg
 The OFFSET argument is added to/taken away from the index that will be
 used.  This is really only useful with `first' and `last', for
 accessing absolute argument positions."
-  (nth (+ (pcase index
-	   ('first 0)
-	   ('last  pcomplete-last)
-	   (_      (- pcomplete-index (or index 0))))
-	  (or offset 0))
-       pcomplete-args))
+  (let ((arg (nth (+ (pcase index
+	               ('first 0)
+	               ('last  pcomplete-last)
+	               (_      (- pcomplete-index (or index 0))))
+	             (or offset 0))
+	          pcomplete-args)))
+    (when arg
+      (if (stringp arg) arg (format "%S" arg)))))
 
 (defun pcomplete-begin (&optional index offset)
   "Return the beginning position of the INDEXth argument.