On Thu, Mar 15, 2018 at 10:46:56PM +0100, Szabolcs Nagy wrote:
> * Rich Felker <dalias@libc.org> [2018-03-15 15:32:44 -0400]:
> > Perhaps this should be documented more explicitly, but there is no
> > guarantee that building with -Werror[=anything except warnings which
> > are constraint violations in C] will succeed, especially when GCC is
> > not honoring its usual promise not to produce warnings for code
> > expanded from macros from -isystem paths. I did just test and indeed
> > the warning is produced with gcc 6.3.0.
> > 
> 
> how did you reproduce it? -Wsign-conversion (or -Wconversion)
> is not even enabled by

Manually using -Wsign-conversion with the attached program.

> what would be acceptable is some form of assertion that the
> fd must be positive, however it seems nobody cares about this
> warning so neither gcc nor clang tries to use available range
> information to see if conversion can ever change the result:
> 
>   if (d > 0) FD_SET(d,s);
> 
> still warns.

This basically settles it that it's a buggy warning option.

> if users care about this warning they should first make
> sure the compilers dont emit false positives for such
> trivial cases.

Agreed.

Rich