Re: Configure warning/error in m4/frexp.m4

[Sam James]

Sam James <sam@gentoo.org> writes:

> Bruno Haible <bruno@clisp.org> writes:
>
>> Hi Sam,
>
> Hi Bruno,
>
>>
>>> The configure test in gl_FUNC_FREXP_WORKS within m4/frexp.m4 triggers
>>> a Clang warning/error when Clang is passed
>>> -Werror=incompatible-pointer-types.
>>
>> Gnulib does not support CC or CPPFLAGS or CFLAGS values with -Werror at
>> configure time. Neither with gcc nor with clang.
>
> Yes, I understand that. But Clang is likely to make this change to its
> default and -Werror=... was just a way of emulating that. gnulib _does_
> have to cater to the default strictness of compilers.
>
>>
>> The reason is that [1]
>>   "Many GCC warning options usually don’t point to mistakes in the code;
>>    these warnings enforce a certain programming style. It is a project
>>    management decision whether you want your code to follow any of these
>>    styles. Note that some of these programming styles are conflicting.
>>    You cannot have them all; you have to choose among them."
>>
>> For example, there is a warning option that attempts to enforce
>> explicit casts (no implicit conversions), and there is a warning option
>> that attempts to enforce no explicit casts. When you use such a warning
>> option together with -Werror, you are attempting to enforce a certain
>> programming style on the configure tests. Obviously the configure test
>> snippets cannot obey different, conflicting programming styles.
>
> Yep, completely on board with that & I get it. The only reason I
> reported this is because there's a reasonable chance this will be Clang's
> new behaviour by default (see below).
>
>>
>>> /tmp/foo.c: In function ‘main’:
>>> /tmp/foo.c:59:21: warning: passing argument 2 of ‘memcmp’ discards 
>>> ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>>>    59 |     if (memcmp (&y, &x, sizeof x))
>>>       |                     ^~
>>
>> There is nothing wrong with this code (m4/frexp.m4:159, m4/frexpf.m4:83).
>>
>> We don't want to add a cast here, because — as Paul argues — casts can make
>> code more difficult to maintain in the long run.
>
> Yes, I agree it's pedantic, but it's still going to be an issue if (as
> planned) Clang flips '-Wincompatible-pointer-types' to error out by
> default unless they carve out an exception for qualifiers here.
>
> I'll advocate that they do, of course. I just came across it while
> checking for configure changes w/ recent GCC changes.

I should add: if your position is, reasonably, "clang really shouldn't
do that unless they carve out an exception for this kind of case",
then that's fine. But the report was not about me just doing -Werror=x
for the sake of it and misunderstands the motivation here.

>
>>
>> Bruno
>>
>> [1] https://www.gnu.org/software/gnulib/manual/html_node/manywarnings.html