On Wed, Jul 9, 2025 at 9:49 PM Richard Smith <richardsmith_at_[hidden]>
wrote:

> 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.
>

Interestingly, GCC also tries to form UDLs https://godbolt.org/z/r6K3Thd1d


>
> (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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>> > > > Core_at_[hidden]
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>> > > > Link to this post: http://lists.isocpp.org/core/2025/07/18326.php
>> > >
>> > >
>>
>>
>> --
>> Jens Gustedt - INRIA & ICube, Strasbourg, France
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>>
>