[PATCH] Fix assert() warning in gcc < 4.8 [BZ# 21242]

[Joseph Myers]

On Sun, 25 Jun 2017, Florian Weimer wrote:

> I think we can use __extension__ if we also expand the expression
> without __extension__ in an unevaluated context.  The tricky part is to
> find one that is independent of GNU extensions.
> Perhaps this would work?
> 
> #  define assert(expr)                                           \
>   ((void) sizeof ((expr) == 0), __extension__ ({                 \
>       if (expr)                                                  \
>         ; /* empty */                                            \
>       else                                                       \
>         __assert_fail (#expr, __FILE__, __LINE__, __FUNCTION__); \
>     }))
> 
> sizeof suppresses the evaluation of the first occurrence of expr.  The
> comparison is needed because sizeof cannot be applied to function
> pointers and bitfields.  C11 says that expr is compared to zero, so the
> (expr) == 0 expression is well-formed.
> 
> What do you think?  Should we make this change?

I think that's reasonable (appropriately commented to explain why it's 
done that way).

-- 
Joseph S. Myers
joseph@codesourcery.com