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

[Drew Adams]

> >> 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.)
> >
> > It provided completion for me when I tried this (":1")...
> > Yup, fixed in 57d2f24005a earlier this year.
> 
> 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?

Yes, on Windows (10), `C-h f x-display-list':

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x-display-list is a built-in function in 'C source code'.

(x-display-list)

Return the list of display names that Emacs has connections to.

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And it returns this list: ("w32").