On Thu, 28 Apr 2022, 22:04 Jens Maurer, <Jens.Maurer_at_[hidden]> wrote:

> On 28/04/2022 21.18, Jonathan Wakely wrote:
> >
> >
> > On Thu, 28 Apr 2022 at 20:06, Jonathan Wakely <cxx_at_[hidden] <mailto:
> cxx_at_[hidden]>> wrote:
> >
> > On Thu, 28 Apr 2022 at 18:52, Jens Maurer wrote:
> >
> > On 28/04/2022 15.10, Jens Maurer via SG12 wrote:
> > > On 28/04/2022 14.24, Jonathan Wakely via SG12 wrote:
> > >> There were a number of NB comments for C++20 related to UB in
> the preprocessor:
> > >> https://wiki.edg.com/bin/view/Wg21belfast/SG12 <
> https://wiki.edg.com/bin/view/Wg21belfast/SG12> <
> https://wiki.edg.com/bin/view/Wg21belfast/SG12 <
> https://wiki.edg.com/bin/view/Wg21belfast/SG12>>
> > >> The response was to reject them, but create issues. Did the
> issues never get created?
> > >
> > > It doesn't seem so.
> > >
> > >> e.g. https://github.com/cplusplus/nbballot/issues/26 <
> https://github.com/cplusplus/nbballot/issues/26> <
> https://github.com/cplusplus/nbballot/issues/26 <
> https://github.com/cplusplus/nbballot/issues/26>> doesn't seem to have a
> corresponding CWG issue. Is there some other issues list I should be
> looking at, or did these get dropped on the floor?
> > >
> > > I think that action item got dropped on the floor.
> > >
> > > Working on it now.
> >
> > Shiny new core issues are now available:
> >
> > CWG2573 through CWG2581
> >
> > at https://cplusplus.github.io/CWG/issues/2573.html <
> https://cplusplus.github.io/CWG/issues/2573.html>
> >
> >
> > Great, thanks!
> >
> >
> > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3801.pdf <
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3801.pdf> has
> resolutions for some of them.
> >
> > I see some more UB in [cpp] which wasn't covered by those NB comments.
> >
> > [cpp.cond] p7 says "The identifiers __has_include and
> __has_cpp_attribute shall not appear in any context not mentioned in this
> subclause."
> >
> > IIUC failing to meet that requirement produces UB, even though it
> doesn't say the words "undefined behaviour".
>
> That's not my reading. The core language uses "shall" for requirements
> on user programs, the violation of which makes the program ill-formed.
>
> > And similarly for p8, which says "Each preprocessing token that remains
> (in the list of preprocessing tokens that will become the controlling
> > expression) after all macro replacements have occurred shall be in the
> lexical form of a token".
> >
> > Are these covered by any of the new issues?
>
> They already say what we want them to say.
>

Ah good, I'm glad I misunderstood that then.

> The reason I started looking at this is that I was asked about this case:
> >
> > #if 0
> > Can't be tokenized.
> > #endif
> >
> > I think that's the subject of
> https://cplusplus.github.io/CWG/issues/2574.html <
> https://cplusplus.github.io/CWG/issues/2574.html> - is that right?
>
> Yes. That one, and the related example without #if 0.
>
> Jens
>
>