[ Reported to the Debian BTS as report #33975.
  Please CC 33975@bugs.debian.org on replies.
  Log of report can be found at http://bugs.debian.org/33975 ]
	

This is a suggestion for a warning which would greatly help those
converting old C++ programs to g++ and ANSI/ISO standard C++, by
helping to detect code which gets a clean compile from a strict
compiler, but whose behaviour is different at run time as a result of
a change in defined semantics imposed by the standard. 

An example which g++ already handles very well is the change in the
scope of variables declared in a for-init-statement causes g++ to
issue clear warnings if an ambiguous case is detected (see the
documentation for the `-ffor-scope' option).

This case is similar, except that g++ has enforced the now standard
interpretation since at least gcc 2.7.2 - namely the change in the
scope of a conditional expression with respect to a trailing
assignment.  This change means that, for example, an expression such
as (a ? b : c = d) which originally meant ((a ? b : c) = d) will now
be interpreted to mean (a ? b : (c = d)) instead.

I suggest that expressions like this, when written without parentheses
to assert the author's intent, should trigger a warning if the option
`-Wparentheses' is active (whether or not `-ansi' was specified),
since it seems to match the description `when operators are nested
whose precedence people often get confused about'. Alternatively,
since the erroneous interpretation is an anachronism, perhaps
`-Wtraditional' would be the appropriate option.


An example program follows, with output. I'm using

   gcc version gcc-3.1

on Debian 3.0.

Thanks,

Patrick

// --------------------------------------------------------------------
#include <stdio.h>
int main (void)
{
    int x = 0;
    if (x = 3) // this assignment will raise a parentheses warning...
        ;

    // ...but this dubious conditional expression will not do so
    int a = 1, b = 2, c = 3, d = 4;
    x = (a ? b : c = d);

    // just to prove g++ obeys the standard:
    printf ("(a ? b : c = d) means %s\n", 
            ((b != 4) ? 
             "(a ? b : (c = d))" :   // ansi
             "((a ? b : c) = d)"));  // ancient
    exit(0);
}
// --------------------------------------------------------------------


$ g++.3,1 -Wparentheses a.cc && ./a.out
a.cc: In function `int main()':
a.cc:5: warning: suggest parentheses around assignment used as truth value
(a ? b : c = d) means (a ? b : (c = d))

Release:
3.1 (Debian) (Debian unstable)

Environment:
System: Debian GNU/Linux (unstable)
Architecture: i686
	
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  gcc-3.1        3.1-2          The GNU C compiler.
ii  g++-3.1        3.1-2          The GNU C++ compiler.
ii  libstdc++4     3.1-2          The GNU stdc++ library version 3
ii  libstdc++4-dev 3.1-2          The GNU stdc++ library version 3 (developmen
ii  binutils       2.12.90.0.7-1  The GNU assembler, linker and binary utiliti
ii  libc6          2.2.5-6        GNU C Library: Shared libraries and Timezone
host: i386-linux
configured with: /mnt/data/gcc-3.1/gcc-3.1-3.1ds2/src/configure -v --enable-languages=c,c++,java,f77,proto,objc,ada --prefix=/usr --mandir=$\(prefix\)/share/man --infodir=$\(prefix\)/share/info --with-gxx-include-dir=$\(prefix\)/include/g++-v3-3.1 --enable-shared --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --enable-clocale=gnu --enable-threads=posix --enable-java-gc=boehm --enable-objc-gc i386-linux