[Bug c++/25808] New: constant on rhs of conditional assignment

[Bug c++/25808] New: constant on rhs of conditional assignment

[dcb314 at hotmail dot com]

Given the following C++ source code 

void g();
void h();

void f( int a, int b)
{
        if (b = 1)      // case 1 - fixed constant on rhs. g() always executed.
                g();
        if (b = a)      // case 2 - variable on rhs. h() might be executed.
                h();
}

then GNU C++ 4.2 snapshot says

dcb@linux:~/C++/Alphasrc> ~/gnu/42-20060114/results/bin/g++ -c -Wall jan15b.cc
jan15b.cc: In function 'void f(int, int)':
jan15b.cc:6: warning: suggest parentheses around assignment used as truth value
jan15b.cc:8: warning: suggest parentheses around assignment used as truth value
dcb@linux:~/C++/Alphasrc>

We can see the compiler fails to distinguish case 1 and case 2.

Suggest enhance the compiler to say something different for case 1.
Fixed constant on rhs is much more likely to be a programmer error, IMHO.

Here is Intel C++ 9.0 doing what I want

jan15b.cc(6): warning #187: use of "=" where "==" may have been intended
        if (b = 1)
            ^


-- 
           Summary: constant on rhs of conditional assignment
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dcb314 at hotmail dot com
  GCC host triplet: x86_64-suse-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25808

[Bug c++/25808] constant on rhs of conditional assignment

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-01-16 15:27 -------
Both of these are questionable, I don't see why there should be a different
diagnostic.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25808

[Bug c++/25808] constant on rhs of conditional assignment

[dcb314 at hotmail dot com]

------- Comment #2 from dcb314 at hotmail dot com  2006-01-16 22:28 -------
(In reply to comment #1)
> Both of these are questionable, I don't see why there should be a different
> diagnostic.

Because the two different diagnostics make it easier to grep for.

For example, Intel C++ compiler finds case 1, but says nothing
about case 2. Easy to grep for.

GNU C++ produces the same message for case 1 and case 2, leading
to a lot of time wasting manual checking of diagnostics.

Surely case 1 is much more likely to be programmer error than case 2 ?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25808

[Bug c++/25808] constant on rhs of conditional assignment

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2006-01-17 11:51 -------
I agree with Pinskia here - also the detection of a constant RHS is difficult
and
will cause followup PRs that we do not catch all cases.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25808

[Bug c++/25808] constant on rhs of conditional assignment

[bangerth at dealii dot org]

------- Comment #4 from bangerth at dealii dot org  2006-03-08 05:09 -------
I agree with Richard and Andrew. I don't see why we should issue different
diagnostics for essentially the same case. 

W.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at dealii dot org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25808