bug#49749: 26.3; 26.3 & 27.2: invalid byte compiler warning in short-circuited or form

[Pierre Rouleau]

Paste the following code inside the scratch buffer (nothing else):

(defun f-and ()
  "Use and."
  (when (null (and (boundp 'foo)
                  foo))  ; no warning here, as expected.                        
                    
    (message "foo is not set")))

(defun f-or ()
  "Use or."
  (when (or (null (boundp 'foo))
            (null foo))  ;=> ``Warning: reference to free variable ‘foo’``      
                    
    (message "foo is not set")))

Then do:

- ``M-x compile-defun`` with point over f-and.
  No warning is reported as expected.

- ``M-x compile-defun`` with point over f-or.
  This time the byte compiler (of both Emacs 26.3 and 27.2) reports a
  "Warning: reference to free variable ‘foo’" on the line identified in
  the code.

I believe the byte compiler warning to be invalid.
The symbol `foo' is not bound but the code checks if it is
bound first with the line just above.
The ``or`` form is short-circuiting and will not eval
the (null foo) at that point.

Furthermore the code in `f-or' is a De Morgan equivalent boolean
expression of the one inside the `f-and' function.

Both functions `f-and' and `f-or' will run to completion when called and
both issue the "foo is not set" message.



In GNU Emacs 26.3 (build 1, x86_64-apple-darwin18.6.0)
of 2019-08-30 built on Mojave.local
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Configured using:
'configure --disable-dependency-tracking --disable-silent-rules
--enable-locallisppath=/usr/local/share/emacs/site-lisp
--infodir=/usr/local/Cellar/emacs/26.3/share/info/emacs
--prefix=/usr/local/Cellar/emacs/26.3 --with-gnutls --without-x
--with-xml2 --without-dbus --with-modules --without-ns
--without-imagemagick'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB MODULES THREADS

Important settings:
  value of $LANG: en_CA.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Debbugs

Minor modes in effect:
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  file-name-shadow-mode: t
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  column-number-mode: t
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  transient-mark-mode: t

Load-path shadows:
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/Users/roup/.emacs.d/elpa/benchmark-init-20150905.938/benchmark-init-modes
/Users/roup/.emacs.d/utils/sr-speedbar hides 
/Users/roup/.emacs.d/elpa/sr-speedbar-20161025.831/sr-speedbar
/Users/roup/.emacs.d/elpa/lispy-20210121.926/elpa hides 
/Users/roup/.emacs.d/elpa/ivy-20210311.1638/elpa
/Users/roup/.emacs.d/elpa/lfe-mode-20201007.2214/lfe-indent hides 
/usr/local/share/emacs/site-lisp/lfe/lfe-indent
/Users/roup/.emacs.d/elpa/lfe-mode-20201007.2214/lfe-mode hides 
/usr/local/share/emacs/site-lisp/lfe/lfe-mode
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/Users/roup/.emacs.d/elpa/soap-client-3.2.0/soap-client hides 
/usr/local/Cellar/emacs/26.3/share/emacs/26.3/lisp/net/soap-client
/Users/roup/.emacs.d/elpa/soap-client-3.2.0/soap-inspect hides 
/usr/local/Cellar/emacs/26.3/share/emacs/26.3/lisp/net/soap-inspect
/Users/roup/.emacs.d/elpa/let-alist-1.0.6/let-alist hides 
/usr/local/Cellar/emacs/26.3/share/emacs/26.3/lisp/emacs-lisp/let-alist
/Users/roup/.emacs.d/elpa/seq-2.22/seq hides 
/usr/local/Cellar/emacs/26.3/share/emacs/26.3/lisp/emacs-lisp/seq
/Users/roup/.emacs.d/elpa/nadvice-0.3/nadvice hides 
/usr/local/Cellar/emacs/26.3/share/emacs/26.3/lisp/emacs-lisp/nadvice

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