bug#48740: 28.0.50; Composition text property is not always honoured

[Stefan Kangas <stefan@marxist.se>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> I understand. However, the problem is quite unexpected. I do not see how
>> application can anticipate it. Probably, adding a note to docstring
>> would be useful? Something like in the attached patch.
>
> Fine with me, thanks.

The patch here was never installed.  Should it be?

I've attached a version below with some minor touch-ups.

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From d2b1f2125b98ac8b496712ed133cda27757437b5 Mon Sep 17 00:00:00 2001
From: Ihor Radchenko <yantar92@gmail.com>
Date: Thu, 24 Jun 2021 22:33:08 +0800
Subject: [PATCH] Doc fix for concat

* src/fns.c (Fconcat): Note that composition values may not remain eq
in return value of concat.  (Bug#48740)
---
 src/fns.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/fns.c b/src/fns.c
index a72e41aee5..6f358dd1ba 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -672,6 +672,9 @@ DEFUN ("concat", Fconcat, Sconcat, 0, MANY, 0,
        doc: /* Concatenate all the arguments and make the result a string.
 The result is a string whose elements are the elements of all the arguments.
 Each argument may be a string or a list or vector of characters (integers).
+
+Values of the `composition' property of the result are not guaranteed
+to be `eq'.
 usage: (concat &rest SEQUENCES)  */)
   (ptrdiff_t nargs, Lisp_Object *args)
 {
-- 
2.30.2