[Bug c/25509] can't disable __attribute__((warn_unused_result))

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509

--- Comment #29 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Paul Pluzhnikov from comment #28)
> Well, that did expose the 30 bugs above, but unfortunately I can't do that
> permanently, because it also exposed this false positive:
> 
>    assert(v.empty());
> 
> where assert in NDEBUG mode expanded into
> 
>   static_cast<void>(v.empty());

Isn't assert in NDEBUG mode guaranteed to not evaluate its argument? The above
seems to violate that assumption.

In C++ you could do this:

template<typename T>
inline T ignore_result(T x __attribute__((unused)))
{
    return x;
}
extern int foo() __attribute__((warn_unused_result));

int main()
{
   ignore_result(foo());
   return 0;
}

Another alternative is to use #pragma GCC diagnostics push/ignored/pop. Ideally
you could encapsulate that into a macro "ignore_result", but #pragma
diagnostics does not work well in a macro definition yet (I cannot remember the
PR number for this).