Re: Possible __VA_OPT__ bug

[Edward Diener <eldlistmailingz@tropicsoft.com>]

On 10/31/2019 4:20 PM, Segher Boessenkool wrote:
> On Thu, Oct 31, 2019 at 02:50:07PM -0400, Edward Diener wrote:
>> On 10/31/2019 1:14 PM, Segher Boessenkool wrote:
>>> On Thu, Oct 31, 2019 at 12:42:54PM -0400, Edward Diener wrote:
>>>> Given:
>>>>
>>>> #define NO_DATA
>>>> #define TRY_VA_OPT(...)  __VA_OPT__ (0) 1
>>>>
>>>> TRY_VA_OPT() -> expands to 1 as expected
>>>> TRY_VA_OPT(NO_DATA) -> expands to 0 1 which is not expected
>>>>
>>>> when compiled with gcc-9.2 with -std=c++2a.
>>>
>>> Why is that not expected?  The variadic macro TRY_VA_OPT does get tokens
>>> in its variable argument (namely, NO_DATA), so __VA_OPT__ expands to its
>>> argument (which is 0).
>>>
>>> https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
>>
>> The wording of 15.6.1 paragraph 3 is confusing to me. I had thought that
>> the arguments to the variadic ... parameter were completely macro
>> expanded before considering whether the __VA_OPT__ ( pp-tokens ) would
>> expand to 'pp_tokens' or a single placemarker token. Evidently you are
>> saying that the correct interpretation with the __VA_OPT__ construct is
>> that the variadic ... arguments are not macro expanded before
>> consideration of the __VA_OPT__ construct processing . This seems to me
>> very odd because the arguments to the variadic ... parameter are always
>> completely macro expanded before being replaced by any __VA_ARGS__
>> parameter in the replacement list. I wonder why the C++ standard
>> committee decided to treat __VA_OPT__ differently from __VA_ARGS__ in
>> this regard ?
> 
> I don't know.  I don't know if the GCC implementation is correct, either.

Clang in its C++20 mode treats __VA_OPT__ as I thought it should be, 
where the determination of whether __VA_OPT__ ( pp-tokens ) expands to a 
placeholder token or pp-tokens is determined by whether the arguments 
for the variadic data parameter ... are empty or not after the arguments 
have been fully macro replaced. In other words clang expands to 1 in 
both my examples above.

> 
> 
> Segher
>