[Bug c/39170] cannot silence -Wconversion warnings for bit-fields

[manu at gcc dot gnu dot org]

------- Comment #5 from manu at gcc dot gnu dot org  2009-03-10 19:15 -------
(In reply to comment #4)
> 
> The previous version of "gcc" warned when implicit narrowing of doubles to
> integral values, such as
> 
>     double n = 0.0000000005;
>     int d = n;
> 
> when using the "-Wall" option.

AFAIK, that is not true. I just tried your very example with gcc 4.2.4 and it
doesn't warn with -Wall -Wextra -Wconversion. g++ did warn but not with -Wall,
you still needed to specify -Wconversion, and it did not warn in many cases. So
please, check your facts.

> The behavior of "gcc" has changed. Moving all the conversion warnings to fall
> under "-Wconversion" may make semantic sense, but it alters the behavior of
> "gcc". 

Fixing bugs alters behaviour. The change of behaviour was documented beyond
what is normally expected.

> We can fault ourselves for missing this change in the documentation, but there
> a level of expectation that the fundamental behavior of the compiler is
> consistent from release-to-release. And when fundamental behavior of "gcc"
> changes, ample notice should be given. People need to change Makefiles, alter
> code (if possible), etc... 

The behaviour of the compiler changes *every* release in many aspects. That is
exactly what http://gcc.gnu.org/gcc-X.X/changes.html is for.

> With regard to "-Wtraditional-conversion", it does not work when compiling
> "C++" code.

Of course it doesn't. Have you understood what -Wtraditional-converion (and the
old -Wconversion) actually warned for?

> Absolutely not. It's not a portable solution. There is nothing in the "C++"
> standard (that I'm aware of) that suggests that "anding" an integral value with
> a "constant" value results in a truncated integral value. It's a bad hack.
> 

Really? Then I have no ideas. In any case, someone else would need to take care
of this because I do not have time. http://gcc.gnu.org/faq.html#support

> If you have ideas on how to solve this, please submit them to the "C" or "C++"
> standards group.

bit-field cast operators?

> We've already moved back to the 4.2 "gcc".

Good that you found a solution. I will leave this open in case someone finds a
better solution and decides to submit a patch.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39170