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

[Spencer Baugh]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>  (defun completion-emacs22-try-completion (string table pred point)
>>    (let ((suffix (substring string point))
>>          (completion (try-completion (substring string 0 point) table pred)))
>> -    (if (not (stringp completion))
>> -        completion
>> +    (cond
>> +     ((eq completion t)
>> +      ;; The prefix is an exact and unique completion, but STRING
>> +      ;; might not be a completion.
>> +      (if (test-completion string table)
>> +          t
>> +        (cons string point)))
>
> I think we should test (equal "" suffix) instead.
> The cases where (equal "" suffix) is false but (test-completion string table)
> are sufficiently weird that returning t seems risky and I can't think of
> any reason why it's worthwhile to take this risk.

True.  I was thinking about the case where STRING is a completion, so it
would be nice to just return t.  But that would be caught by other
completion styles before emacs22 anyway.

Fixed.

>From b196a456430ff6ddc4705bc012a14f615b360e37 Mon Sep 17 00:00:00 2001
From: Spencer Baugh <sbaugh@janestreet.com>
Date: Thu, 16 Nov 2023 11:34:08 -0500
Subject: [PATCH] Return t from completion-emacs22-try-completion only for
 completions

The emacs22 completion style ignores the text after point when
computing completions.  However, it still needs to take into account
the entire string it's given, to avoid returning incorrect values.

Previously, completion-emacs22-try-completion would return t if the
text before point was an exact completion.  But this is effectively
saying that the entire input string was an exact completion, which may
not be correct.  This would cause completing-read with REQUIRE-MATCH=t
to return a non-completion.

Now, completion-emacs22-try-completion only returns t if the entire
input string is an exact completion.

* lisp/minibuffer.el (completion-emacs22-try-completion): Return t
only if the entire input string is an exact completion.  (Bug#67210)
---
 lisp/minibuffer.el | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 07a284134d6..5bba202603a 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -3526,8 +3526,13 @@ completion-emacs21-all-completions
 (defun completion-emacs22-try-completion (string table pred point)
   (let ((suffix (substring string point))
         (completion (try-completion (substring string 0 point) table pred)))
-    (if (not (stringp completion))
-        completion
+    (cond
+     ((eq completion t)
+      (if (equal "" suffix)
+          t
+        (cons string point)))
+     ((not (stringp completion)) completion)
+     (t
       ;; Merge a trailing / in completion with a / after point.
       ;; We used to only do it for word completion, but it seems to make
       ;; sense for all completions.
@@ -3541,7 +3546,7 @@ completion-emacs22-try-completion
                (eq ?/ (aref suffix 0)))
           ;; This leaves point after the / .
           (setq suffix (substring suffix 1)))
-      (cons (concat completion suffix) (length completion)))))
+      (cons (concat completion suffix) (length completion))))))
 
 (defun completion-emacs22-all-completions (string table pred point)
   (let ((beforepoint (substring string 0 point)))
-- 
2.39.3