bug#30668: 27.0.50; Error in `completing-read'

[Lars Ingebrigtsen <larsi@gnus.org>]

p.stephani2@gmail.com writes:

> emacs -Q --eval='(completing-read "p: " nil)'
>
> Hit <left> <SPC>.  An error will be signaled:
>
> completion--some: Args out of range: "", 0, -1
>
> Backtrace is
>
> Debugger entered--Lisp error: (args-out-of-range "" 0 -1)
>   completion-basic-try-completion("" nil nil -1)
>   #f(compiled-function (style) #<bytecode 0x3aabd9>)(basic)
>   completion--some(#f(compiled-function (style) #<bytecode 0x3aabd9>)
>   (basic partial-completion emacs22))
>   completion--nth-completion(1 "" nil nil -1 (metadata))
>   completion-try-completion("" nil nil -1 (metadata))
>   completion--try-word-completion("" nil nil -1 (metadata))
>   completion--do-completion(4 4 completion--try-word-completion)

This problem is still present in Emacs 28.  I'm not quite sure what the
correct fix is -- just change `completion--do-completion' to not do
completion if `(- (point) beg)' is negative?

Perhaps Stefan has an opinion here; added to the CCs.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no