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

[Eli Zaretskii]

> Cc: arstoffel@gmail.com
> Date: Sat, 10 Dec 2022 17:25:53 -0800
> From: Jim Porter <jporterbugs@gmail.com>
> 
> X-Debbugs-CC: arstoffel@gmail.com
> 
> Starting from "emacs -Q -f -eshell", type "echo $exec-path " (note the 
> trailing space), and then hit TAB. The result is this error:
> 
>    pcomplete-match: Wrong type argument: stringp, ("/usr/bin" ...)
> 
> This is a regression from Emacs 28, and it looks like it's due to 
> 'pcomplete-here-using-help' assuming that all the pcomplete args are 
> strings. However, 'exec-path' is a list (and Eshell reports it this way 
> to pcomplete), so the completion fails. I think all that's necessary is 
> checking that the pcomplete args are strings in 
> 'pcomplete-here-using-help', but I know next to nothing about pcomplete...

Adding Stefan, who should know more about pcomplete.

Stefan, any suggestions?