[Bug c++/51755] New: -Wconversion generates false warnings when the ternary operator is used

[Bug c++/51755] New: -Wconversion generates false warnings when the ternary operator is used

[james.kanze at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

             Bug #: 51755
           Summary: -Wconversion generates false warnings when the ternary
                    operator is used
    Classification: Unclassified
           Product: gcc
           Version: 4.4.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: james.kanze@gmail.com


This might be covered by 40752; it's related.  But if -Wconversion is used,
simple code like the following

    int main()
    {
        static uint8_t const k = 0x02;
        bool c = true;
        uint8_t x = 0;
        x |= c ? k : 0;
    }

triggers a warning, although there is absolutely no possibility of overflow.

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-01-04 19:02:33 UTC ---
also PR 51294

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[james.kanze at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

--- Comment #2 from James Kanze <james.kanze at gmail dot com> 2012-01-05 08:45:22 UTC ---
I'd miss PR 51294.  My report is almost certainly a duplicate of this one.

It's a minor bug, in that it only affects warnings, but it does make
-Wconversion unusable in certain contexts.

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |paolo.carlini at oracle dot
                   |                            |com

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-01-05 09:29:02 UTC ---
Hi Jason. Looks like people are really unhappy with the -Wconversion warnings
vs the ternary operator, and I'd like to look into these issues in time for
4.7. At the moment I'm confused, however, because I don't understand if the
patch linked by Manuel in PR51294 (maybe we can consolidate these two issues)
was rejected in a unamendable way, or we (I) can start from it and complete the
work. Can you guide me a bit? Thanks...

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2012-01-05 14:11:19 UTC ---
(In reply to comment #3)
> At the moment I'm confused, however, because I don't understand if the
> patch linked by Manuel in PR51294 (maybe we can consolidate these two issues)
> was rejected in a unamendable way, or we (I) can start from it and complete the
> work. Can you guide me a bit? Thanks...

I'm not the one who rejected it, so I don't know what Joseph's thinking is; I'd
suggest responding to his mail.

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |manu at gcc dot gnu.org
         Resolution|                            |DUPLICATE

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-01-05 15:32:03 UTC ---
(In reply to comment #3)
> Hi Jason. Looks like people are really unhappy with the -Wconversion warnings
> vs the ternary operator, and I'd like to look into these issues in time for
> 4.7. At the moment I'm confused, however, because I don't understand if the
> patch linked by Manuel in PR51294 (maybe we can consolidate these two issues)
> was rejected in a unamendable way, or we (I) can start from it and complete the
> work. Can you guide me a bit? Thanks...

In fact, it would be the opposite. Start from it and cut out what Joseph was
against, which is to not warn for char a,b,c; a = b * c, which is what PR40752
asks for. The ternary operator (PR51294) was orthogonal to this issue but the
patch fixed both.

(I wouldn't send a patch like that now, but I was a newbie then.)

I think this is a clearly a duplicate of 51294.

*** This bug has been marked as a duplicate of bug 51294 ***

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-01-06 13:13:41 UTC ---
Thus by 'opposite' of 'complete' you mean that nothing should be *added* to the
patch, instead something removed from it? Great. You see I meant completing
*the work*, thus committing and closing the PR. Anyway, Manuel, would you be
willing to send over a patch answeing Joseph' concerns? Otherwise I'll try to
do it myself, but probably it will take more time because I have start from
scratch on this. Thanks!

[Bug c++/51755] -Wconversion generates false warnings when the ternary operator is used

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51755

--- Comment #7 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-01-07 13:03:54 UTC ---
(In reply to comment #6)
> Thus by 'opposite' of 'complete' you mean that nothing should be *added* to the
> patch, instead something removed from it? Great. You see I meant completing

Yes. But probably the patch needs to be substantially changed. And you should
check with Joseph whether he is happy with the additional complexity added.

> *the work*, thus committing and closing the PR. Anyway, Manuel, would you be
> willing to send over a patch answeing Joseph' concerns? Otherwise I'll try to
> do it myself, but probably it will take more time because I have start from
> scratch on this. Thanks!

Sorry, I don't have enough free time to do this. 

Also, I am not sure anymore that it is worth to add these checks to gcc, which
is not meant to be a static analysis tool or be used to build one. GCC FEs are
very limited in the kind of analysis they can make, and this seems to be on
purpose. It would be better to focus on scan-build
http://clang-analyzer.llvm.org/scan-build.html or build a GPL static analyzer
on top of clang.