[PATCH] assert: Suppress pedantic warning caused by statement expression [BZ# 21242]

[Florian Weimer <fweimer at redhat dot com>]

On 06/26/2017 12:56 PM, Joseph Myers wrote:
> 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).

This is what I came up with as a patch.

I would consider this still appropriate during the freeze.  It's
something we'd backport to glibc 2.25, too.

Thanks,
Florian