[Bug c++/55077] New: implement and enable by default -Wliteral-conversion

[Bug c++/55077] New: implement and enable by default -Wliteral-conversion

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

             Bug #: 55077
           Summary: implement and enable by default -Wliteral-conversion
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: manu@gcc.gnu.org


>From clang:

test.cc:4:23: warning: implicit conversion from 'double' to 'int' changes value
from 3.1415926 to 3 [-Wliteral-conversion]
 static const int c = 3.1415926;
                  ~   ^~~~~~~~~

This should be fairly easy, since we warn already with -Wconversion:

1) Change the warning flag.
2) Show the values before/after.

[Bug c++/55077] implement and enable by default -Wliteral-conversion

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-10-27 19:48:25 UTC ---
Problem: We do not want to warn for expressions. From the clang testcases:

  // Expressions, such as those that indicate rounding-down, should NOT produce
warnings.
  int x = 24 * 0.5;
  int y = (24*60*60) * 0.25;
  int pennies = 123.45 * 100;

But gcc folds those before conversion_warning, so we cannot know they are
expressions... :-(

[Bug c++/55077] implement and enable by default -Wliteral-conversion

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-10-28 18:02:07 UTC ---
On Sat, 27 Oct 2012, manu at gcc dot gnu.org wrote:

>   // Expressions, such as those that indicate rounding-down, should NOT produce
> warnings.
>   int x = 24 * 0.5;
>   int y = (24*60*60) * 0.25;
>   int pennies = 123.45 * 100;

The last of those seems pretty suspicious (123.45 isn't an exact 
floating-point value, but the user probably wants 12345 independent of 
whether the floating-point value is above or below the exact decimal 
value).  Are you *sure* you don't want a warning in such a case?

[Bug c++/55077] implement and enable by default -Wliteral-conversion

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-11-10
     Ever confirmed|0                           |1

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/55077] implement and enable by default -Wliteral-conversion

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #9 from David Binderman <dcb314 at hotmail dot com> ---
-Wfloat-conversion does the deed: any chance of getting it someplace useful
like -Wall or -Wextra anytime soon ?

I will put it into my local compiler.

[Bug c++/55077] implement and enable by default -Wliteral-conversion

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55077

--- Comment #10 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to David Binderman from comment #9)
> -Wfloat-conversion does the deed: any chance of getting it someplace useful
> like -Wall or -Wextra anytime soon ?
> 
> I will put it into my local compiler.

I think the point here is that the proposed -Wliteral-conversion warns for a
smaller number of cases than -Wfloat-conversion does, and thus would be safer
to enable more widely than -Wfloat-conversion is.