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Re: Did CWG issues ever get created for the NB comments about preprocessor UB?

From: Jonathan Wakely <cxx_at_[hidden]>
Date: Thu, 28 Apr 2022 20:18:42 +0100
On Thu, 28 Apr 2022 at 20:06, Jonathan Wakely <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>
>> >> 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> 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
> Great, thanks!

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".

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?

The reason I started looking at this is that I was asked about this case:

#if 0
Can't be tokenized.

I think that's the subject of
https://cplusplus.github.io/CWG/issues/2574.html - is that right?

Received on 2022-04-28 19:18:55