bug#29713: 26.0; Provide completion for `make-frame-on-display'

[Eli Zaretskii <eliz@gnu.org>]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Date: Thu, 18 Mar 2021 14:02:00 +0000
> Cc: 29713@debbugs.gnu.org, Lars Ingebrigtsen <larsi@gnus.org>
> 
> Juri Linkov <juri@linkov.net> writes:
> 
> >>> Subject line says it all.  Gather the displays using, say,
> >>> `x-display-list' and use them as completion candidates.
> >>> (I don't use X Window, so I won't have more to say about this.)
> > Maybe it could use `x-display-list'.  But I don't know how portable it is.
> > Is `x-display-list' available on OSX and Windows?
> 
> It's not available on --without-x builds:
> 
> 0. ./src/emacs -Q
> 1. M-x toggle-debug-on-error RET
> 2. M-x make-frame-on-display RET
>    Debugger entered--Lisp error: (void-function x-display-list)
> 
> What's TRT here?

For which use case?

In a build --without-x asking about available displays makes no sense,
because such a build cannot possibly access any displays.  Right?

In a GUI session, the function is always available, although the list
it returns might disappoint w32 users...

>   (interactive (list (completing-read
>                       (format "Make frame on display: ")
>                       (if (fboundp 'x-display-list)
>                           (x-display-list)
>                         (user-error "No connected displays found")))))
> 
> This will barf before make-frame is called, and pacifies the
> byte-compiler warning about x-display-list not being known.
> 
>   (interactive (list (completing-read
>                       (format "Make frame on display: ")
>                       (and (fboundp 'x-display-list)
>                            (x-display-list)))))
> 
> This will barf when make-frame is called, with e.g.:
> 
>   make-frame: Don’t know how to interpret display ""
>   make-frame: Don’t know how to interpret display ":0"
> 
> And also pacifies the warning.
> 
> Any preferences or alternative suggestions?

make-frame-on-display should signal an error in non-GUI builds.