bug#67210: 30.0.50; completing-read with REQUIRE-MATCH=t can sometimes return a non-match

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Wed, 15 Nov 2023 15:25:15 -0500
> 
> 
> 1. emacs -Q
> 2. (completing-read ":" '("foo") nil t '("foobar" . 3))
> 3. RET
> 4. See "foobar", i.e. "foobar" was accepted as input despite not being
>    in the collection we're completing over.
> 
> I believe this is because
> (completion-try-completion "foobar" '("foo") nil 3)
> returns t, which completion--do-completion interprets as meaning that
> the input string is an "Exact and unique match."
> 
> That t is returned because
> (completion-emacs22-try-completion "foobar" '("foo") nil 3)
> returns t.
> 
> So perhaps completion--do-completion is wrong in assuming that a t
> return value from completion-try-completion indicates an "Exact and
> unique match."?
> 
> Or perhaps completion-emacs22-try-completion is violating the spec of
> completion-try-completion, and should be returning nil in this case?

Adding Stefan to the discussion, as he probably has an opinion about
this.