bug#59960: 30.0.50; Eshell's $* variable (used to define aliases) doesn't work as expected

[Eli Zaretskii]

> Date: Sat, 10 Dec 2022 22:13:39 -0800
> From: Jim Porter <jporterbugs@gmail.com>
> 
> This bug report originally comes from a Reddit post[1] where a user 
> found that aliasing the Eshell built-in "mv" and "cp" commands didn't 
> work properly. I'll show a slightly simpler example though. Starting 
> from "emacs -Q -f eshell":
> 
>    ~ $ alias my-echo "echo A $* Z"
>    ~ $ my-echo b c d
>    ("A"
>     ("b" "c" "d")
>     "Z")
> 
>    ~ $ echo A b c d Z
>    ("A" "b" "c" "d" "Z")
> 
> As you can see above, the "$*" special variable inserts the list of 
> arguments passed to the alias *as a sublist*. That might be what you 
> want sometimes, but more likely, you'd want to insert the arguments 
> in-place, like with ",@" in Emacs Lisp backquote forms. (Note: for 
> external processes, this isn't an issue because Eshell flattens all the 
> arguments to external commands.)
> 
> Attached is a draft patch to fix this. It adds a new "$@expr" variable 
> expansion to Eshell which lets you splice a list in-place:
> 
>    ~ $ alias my-echo 'echo A $@* Z'
>    ~ $ my-echo b c d
>    ("A" "b" "c" "d" "Z")
> 
> Currently, this only works in simple situations like the above. It 
> should work for any variable, but likely won't work with Eshell for 
> loops, inside double quotes, etc. It would be nice to get this working 
> in a wider variety of places in Eshell command forms before merging. 
> Still, even what's here is already pretty useful, I think. I'm posting 
> this now mainly to get feedback on whether this is the right direction 
> to go in the first place.

AFAICT, the fact that $* produces a sub-list is not mentioned in the
documentation part of the patch.  I think it should, if only to
explain why a special expansion is needed.