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

[Bruno Haible]

Hi Sam,

> 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.

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.

> /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.

Bruno

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