[Bug c/25733] missed diagnostic about assignment used as truth value.

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

--- Comment #13 from Vincent Lefèvre  ---
Another example:

int f1 (int a, int b, int c, int d)
{
  if (a = c && b == d)
return 0;
  else
return 1;
}

int f2 (int a, int b, int c, int d)
{
  if (a == b || (a = c && b == d))
return 0;
  else
return 1;
}

int f3 (int a, int b, int c, int d)
{
  if (a == b && (a = c && b == d))
return 0;
  else
return 1;
}

GCC emits a warning for f1 (this is the usual case), but neither for f2, nor
for f3. Note that for f2, the parentheses around what should be a == c && b ==
d are optional, but strongly recommended (GCC has a warning for that), so one
would really expect

  if (a == b || (a == c && b == d))

or

  if (a == b || ((a = c && b == d)))

For f3, the parentheses around what should be a == c && b == d are not
specially recommended (but may be useful to emphasize on some symmetry in some
context), but I think that there should be the same rule as f2.

FYI, MPFR had an undetected bug corresponding to f3 and a warning would have
really been useful:
 
https://gforge.inria.fr/scm/viewvc.php/mpfr/trunk/src/sqrt.c?r1=11021=11022

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

Manuel López-Ibáñez manu at gcc dot gnu.org changed:

   What|Removed |Added

 CC||aravindvijayan224185 at
   ||gmail dot com

--- Comment #12 from Manuel López-Ibáñez manu at gcc dot gnu.org 2012-01-19 
13:48:48 UTC ---
*** Bug 51894 has been marked as a duplicate of this bug. ***

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu dot org]

--- Comment #7 from manu at gcc dot gnu dot org  2010-02-21 00:30 ---
(In reply to comment #3)
 As another data-point,
 
 if ( (a=10) ) ;
 
 also doesn't warn.  I'm not sure what the standard says on that, but other
 contemporary compilers do give the an assignment used as truth value warning
 for the example above.
 

How do other compilers deal with false positives? That is, how can a programmer
specify that they really want to do an assignment?

We could use a cast to bool. 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu dot org]

--- Comment #8 from manu at gcc dot gnu dot org  2010-02-21 00:33 ---
*** Bug 32587 has been marked as a duplicate of this bug. ***


-- 

manu at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||fritz at intrinsity dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu dot org]

--- Comment #9 from manu at gcc dot gnu dot org  2010-02-21 00:34 ---
Another testcase:

int bar(int a, int b, int c)
{
if ((b = c)  (a == 0))
{
return 0;
}
return 1;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[bangerth at gmail dot com]

--- Comment #10 from bangerth at gmail dot com  2010-02-21 01:25 ---
(In reply to comment #7)
 (In reply to comment #3)
  As another data-point,
  
  if ( (a=10) ) ;
  
  also doesn't warn.  I'm not sure what the standard says on that, but other
  contemporary compilers do give the an assignment used as truth value 
  warning
  for the example above.
  
 
 How do other compilers deal with false positives? That is, how can a 
 programmer
 specify that they really want to do an assignment?
 
 We could use a cast to bool. 

I think at least in C++ the warning is useful. Conditions in if(...) statements
have type bool, and things like
  if (a=10)
use the implicit conversion from int to bool. If a programmer wants to avoid
the warning, one can always be explicit and write
  if ( (a=10) != 0)

W.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu dot org]

--- Comment #11 from manu at gcc dot gnu dot org  2010-02-21 01:43 ---
I am not saying that the warning is not useful. I am saying that the current
behaviour of not warning if there are parenthesis is not ideal. 

So either:

if ((bool) a = 0)

or 

if ( (a=0) != 0)

seem much better choices for avoiding warning.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[ianw at vmware dot com]

--- Comment #3 from ianw at vmware dot com  2008-04-28 22:14 ---
As another data-point,

if ( (a=10) ) ;

also doesn't warn.  I'm not sure what the standard says on that, but other
contemporary compilers do give the an assignment used as truth value warning
for the example above.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[ianw at vmware dot com]

--- Comment #4 from ianw at vmware dot com  2008-04-28 22:16 ---
Oh, just to be clear, my point was if (a=10) warns, but the extra parenthesis
hide the warning.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[pinskia at gcc dot gnu dot org]

--- Comment #5 from pinskia at gcc dot gnu dot org  2008-04-28 22:17 ---
(In reply to comment #4)
 Oh, just to be clear, my point was if (a=10) warns, but the extra parenthesis
 hide the warning.

That is by design.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[ianw at vmware dot com]

--- Comment #6 from ianw at vmware dot com  2008-04-28 22:28 ---
(In reply to comment #5)
 (In reply to comment #4)
  Oh, just to be clear, my point was if (a=10) warns, but the extra 
  parenthesis
  hide the warning.
 
 That is by design.

Ok, I did try looking but is that documented anywhere?

It would seem to have ramifications for people that do things like make an
ASSERT statement something like

#define ASSERT(v) {if (!(v))}

A quick search on google codesearch showed this was a very common idiom...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[manu at gcc dot gnu dot org]

--- Comment #2 from manu at gcc dot gnu dot org  2008-04-27 22:13 ---
*** Bug 36050 has been marked as a duplicate of this bug. ***


-- 

manu at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||ianw at vmware dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733

[Bug c/25733] missed diagnostic about assignment used as truth value.

[rguenth at gcc dot gnu dot org]

--- Comment #1 from rguenth at gcc dot gnu dot org  2006-01-10 10:17 ---
Confirmed.  Same in the C++ frontend.


-- 

rguenth at gcc dot gnu dot org changed:

   What|Removed |Added

   Severity|normal  |enhancement
 Status|UNCONFIRMED |NEW
 Ever Confirmed|0   |1
  GCC build triplet|i686-pld-linux  |
   GCC host triplet|i686-pld-linux  |
 GCC target triplet|i686-pld-linux  |
   Keywords||diagnostic
  Known to fail|3.4.5 4.1.0 |2.95.3 3.3.6 3.4.5 4.0.2
   ||4.1.0
   Last reconfirmed|-00-00 00:00:00 |2006-01-10 10:17:07
   date||


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733