Re: Possible __VA_OPT__ bug

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On Thu, Oct 31, 2019 at 10:27 PM Jędrzej Dudkiewicz
<jedrzej.dudkiewicz@xxxxxxxxx> wrote:
>
> On Thu, Oct 31, 2019 at 9:28 PM Edward Diener
> <eldlistmailingz@xxxxxxxxxxxxxx> wrote:
> >
> > 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.
>
> #define T(a) #a
> T(NO_DATA)
>
> Expands to "NO_DATA", which means that it is:
>
> T(NO_DATA) -> #NO_DATA -> "NO_DATA"
>
> and not:
>
> T(NO_DATA) -> T() -> <error, no argument>
>
> Which explains why __VA_OPT__ behaves this way. According to godbolt
> T(NO_DATA) also expands to "NO_DATA", which would mean that difference
> between compilers boils down to checking if list is empty - gcc checks
> it before macro expansion, and clang after expansion?

Ugh, I just realized that this is what you wrote. Sorry for the noise.

-- 
Jędrzej Dudkiewicz

I really hate this damn machine, I wish that they would sell it.
It never does just what I want, but only what I tell it.




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