On Wed, 9 Jul 2025 at 11:49, Jens Gustedt via Core <core_at_[hidden]>
wrote:

> Hi,
>
> Am 9. Juli 2025 20:15:57 MESZ schrieb Corentin via Liaison <
> liaison_at_[hidden]>:
> > On Mon, Jul 7, 2025 at 7:38 PM Jens Maurer <jens.maurer_at_[hidden]> wrote:
> >
> > >
> > >
> > > On 07.07.25 17:41, Corentin via Core wrote:
> > > > Hey folks,
> > > >
> > > > [[foo(#)]] is a valid attribute in C, but invalid in C++, as # is
> > > considered a token in C and a preprocessing-operator in C++
> > > > (The grammar of attributes expects tokens in both languages)
> > > >
> > > > Is this divergence intended? It does not seem desirable.
> > > >
> > > > Note that there is implementation divergence amongst C++ compilers
> here,
> > > Clang accepts in all language modes, GCC rejects in all language modes
> > > https://godbolt.org/z/Wj5fqdd1x <https://godbolt.org/z/Wj5fqdd1x>
> > >
> > > Looks like ## should have the same divergence.
> > >
> >
> > yes, the same question applies to all of #, ##, %:, %:%:
>
> Actually to all preprocessor tokens, that have no equivalent
> in translation phase 7. these are probably a lot more than the ones
> listed.
>

In addition to # and ##, in the basic source character set there's also ` @
$ \ which satisfy the "single character that's not the start of another
preprocessing-token" rule. EDG rejects all of those, GCC and MSVC
reject all but $, clang accepts all.

Then there are pp-numbers that don't correspond to tokens, such as 0xhello.
GCC and Clang accept that in an attribute, EDG and MSVC reject.

(Summary: EDG rejects all pp tokens that aren't tokens. Clang accepts all
of them. GCC and MSVC are inconsistent but mostly reject.)

It seems like it might be useful to accept pp-numbers that don't match
numeric literal syntax. But the rest seem a little questionable: I don't
think we should accept a stray \ -- even in a macro argument, that seems
like it should be reserved for metasyntactic uses, and notably you wouldn't
be able to wrap a line after it -- and at least MSVC produces a good
diagnostic for a stray # or ## that would presumably suffer if we accepted
those as attribute arguments. (But maybe someone has use for an attribute
that contains a macro definition?)

Do we want to reserve tokens starting with ` @ or \ for future extension?
If so, that might be a reason to not allow them as macro arguments.

Jens G
> >
> >
> > >
> > > Jens
> > >
> > >
> > > > https://eel.is/c++draft/lex.operators#1 <
> > > https://eel.is/c++draft/lex.operators#1>
> > > > https://eel.is/c++draft/lex.token#nt:token <
> > > https://eel.is/c++draft/lex.token#nt:token>
> > > > https://eel.is/c++draft/dcl.attr#nt:balanced-token <
> > > https://eel.is/c++draft/dcl.attr#nt:balanced-token>
> > > >
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> > >
> > >
>
>
> --
> Jens Gustedt - INRIA & ICube, Strasbourg, France
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