bug#12079: 23.3; erroneous byte-compile-file warning

[Winston]

GNU Emacs 23.3.1 (amd64-portbld-freebsd9.0, GTK+ Version 2.24.6) of 2011-12-05

It looks like byte-compile-file is issuing a bogus warning for
(set-buffer) inside (save-excursion).

1) The internal description of (save-excursion) says it saves and
   restores point, mark, and the current buffer (just as it's always
   done).  Here's the relevant part of the description (with -->).

    save-excursion is a special form in `C source code'.

    (save-excursion &rest BODY)

    Save point, mark, and current buffer; execute BODY; restore those things.
    Executes BODY just like `progn'.
--> The values of point, mark and the current buffer are restored
    even in case of abnormal exit (throw or error).
    The state of activation of the mark is also restored.

2) This trivial example, run from buffer *scratch*, agrees with that
   description, that the current buffer gets restored:

    (progn
      (save-excursion
        (set-buffer "foo")      ; I'd already created this
        (barf-if-buffer-read-only)
        )
      (current-buffer)
    )
    #<buffer *scratch*>

3) However, when I write that code to foo.el and run byte-compile-file
   on it, I get the following error:

    Compiling file /home/wbe/foo.el at Sat Jul 28 09:07:28 2012
    Entering directory `/home/wbe/'
--> foo.el:1:1:Warning: `save-excursion' defeated by `set-buffer'

It looks to me like the Warning message is untrue.
 -WBE

[The rest is from (report-emacs-bug).  I removed some clearly irrelevant
parts.]


In GNU Emacs 23.3.1 (amd64-portbld-freebsd9.0, GTK+ Version 2.24.6)
 of 2011-12-05 on jail-15
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
configured using `configure  '--localstatedir=/var' '--with-x-toolkit=gtk' 
'--x-libraries=/usr/local/lib' '--x-includes=/usr/local/include' 
'--prefix=/usr/local' '--mandir=/usr/local/man' '--infodir=/usr/local/info/' 
'--build=amd64-portbld-freebsd9.0' 'build_alias=amd64-portbld-freebsd9.0' 
'CC=cc' 'CFLAGS=-O2 -pipe -fno-strict-aliasing' 'LDFLAGS= -L/usr/local/lib 
-rpath=/usr/lib:/usr/local/lib' 'CPPFLAGS=-I/usr/local/include' 'CPP=cpp''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: iso_8859_1
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  value of $XMODIFIERS: nil
  locale-coding-system: iso-latin-1-unix
  default enable-multibyte-characters: t

Major mode: Message

Minor modes in effect:
  mml-mode: t
  gnus-message-citation-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

[...]

Load-path shadows:
/home/wbe/lib/info hides /usr/local/share/emacs/23.3/lisp/info

Features:
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hashcash gnus-win gnus gnus-ems nnheader gnus-util netrc time-date
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overlay md5 base64 format env code-pages mule custom widget
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system-font-setting font-render-setting gtk x-toolkit x multi-tty emacs)