decode-coding-string on invalid UTF-8 string isn't rejected

[Simon Josefsson]

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I'm trying to use decode-coding-string to "guess" charsets, and
noticed this:

(decode-coding-string "r\xe4k" 'latin-1)
 => "räk"
(decode-coding-string "r\xe4k" 'utf-8)
 => "r"

Wouldn't it be more appropriate if it returned nil (like
`decode-char') or "rk"?

Perhaps I'm looking in the wrong place though.  Is there a elisp
function that takes a unibyte string and decodes it using whatever the
default (process) coding system priorities may be?  I.e., for me that
runs in a UTF-8 locale, first try decoding as utf-8, if it fails,
continue with Latin-1, etc.

In GNU Emacs 21.3.50.19 (i686-pc-linux-gnu)
 of 2003-03-07 on latte.josefsson.org
configured using `configure '--with-gtk''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: en_US.UTF-8
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: sv_SE.UTF-8
  locale-coding-system: utf-8
  default-enable-multibyte-characters: t

Recent input:
<up> <left> <M-left> <M-left> <M-left> <right> C-d 
\ x e 4 C-e C-j <up> <left> <M-left> <M-left> M-d M-d 
l a t i n - 1 C-e C-j <up> <M-left> <M-left> M-d M-d 
u t f - 8 C-e C-j C-x o C-x k <return> n p p q m e 
m a c s - d e v e l @ e m a c s . o r g <M-left> <M-left> 
M-d g n u <down> C-h f <up> <up> <return> <down> <down> 
C-x k <return> y e s <return> M-x r e p o r <tab> 
<return>

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