bug#64928: 29.1; format-decode-buffer fails for de646 and duden

[Ulrich Müller]

The following command results in an error:

M-x format-decode-buffer RET de646 RET
Format decoding failed
recode: Request `iso646-ge:latin1' is erroneous

The "duden" encoding does not work either:

M-x format-decode-buffer RET duden RET
Format decoding failed
/bin/bash: line 1: diac: command not found

A trivial patch is included below.

(Not sure if fixing this is even worth the effort. AFAICS Recode never
recognised a charset "iso646-ge". The ISO 646 variants were added
in 1993 with recode-3.3, and its label for the German variant was
"iso646de" even back then. So nobody has reported this since almost
30 years.)

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>From d5a7b6c0fdc93e3ad74d9798729b3036e30f0b19 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ulrich=20M=C3=BCller?= <ulm@gentoo.org>
Date: Sat, 29 Jul 2023 11:37:45 +0200
Subject: [PATCH] ; Fix conversions in format.el

* lisp/format.el (format-alist): Fix duden and de646 conversions.
---
 lisp/format.el | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/lisp/format.el b/lisp/format.el
index b2dba16659b..20c25b0b937 100644
--- a/lisp/format.el
+++ b/lisp/format.el
@@ -87,11 +87,16 @@ format-alist
           rot13-region rot13-region t nil)
     (duden ,(purecopy "Duden Ersatzdarstellung")
           nil
-          ,(purecopy "diac") iso-iso2duden t nil)
+          ;; FROM-FN used to call the "diac" command which is not widely
+          ;; available and apparently not under a free software license:
+          ;; https://tug.ctan.org/support/umlaut_tex.txt
+          ;; Reliable round-trip conversion is not possible anyway
+          ;; and would be by heuristic method, so use nil for now.
+          nil iso-iso2duden t nil)
     (de646 ,(purecopy "German ASCII (ISO 646)")
           nil
-          ,(purecopy "recode -f iso646-ge:latin1")
-          ,(purecopy "recode -f latin1:iso646-ge") t nil)
+          ,(purecopy "iconv -f iso646-de -t utf-8")
+          ,(purecopy "iconv -f utf-8 -t iso646-de") t nil)
     (denet ,(purecopy "net German")
           nil
           iso-german iso-cvt-read-only t nil)
-- 
2.41.0

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