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

[Spencer Baugh <sbaugh@janestreet.com>]

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?


In GNU Emacs 30.0.50 (build 5, x86_64-pc-linux-gnu, GTK+ Version
 3.22.30, cairo version 1.15.12) of 2023-11-15 built on igm-qws-u22796a
Repository revision: 1a1f47e4a1fb70e6810f9eabd0f1826b71a2bcb0
Repository branch: project-history
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.8 (Green Obsidian)

Configured using:
 'configure --enable-checking=yes,glyphs --enable-check-lisp-object-type
 'CFLAGS=-O0 -g3' -C --with-gif=ifavailable'

Configured features:
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LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM
XINPUT2 XPM GTK3 ZLIB

Important settings:
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Major mode: Lisp Interaction

Minor modes in effect:
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  electric-indent-mode: t
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  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
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Memory information:
((conses 16 64205 13453) (symbols 48 9538 0) (strings 32 22015 1421)
 (string-bytes 1 665593) (vectors 16 10318)
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