Re: status of C forward declarations (was: [PATCH 1/1] man/man2/request_key.2: Add missing GNU size forward-declarations)

[Alejandro Colomar <alx@kernel.org>]

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Hi Branden,

> Date: 2026-09-14 13:37:50-0500
> From: "G. Branden Robinson" <g.branden.robinson@gmail.com>
>
> At 2026-09-14T10:19:13-0700, Mark Harris wrote:
> > Alejandro Colomar wrote:
> > > > Anyway N3207 (spec mentioned in d2c2db883) is something I'll need
> > > > to get used to :).
> > >
> > > There's a more recent paper with a slightly better spec (also
> > > compatible with GCC): N3681.
> > > <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3681.pdf>
> > 
> > There is an even more recent paper addressing this subject, N3923
> > <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3923.pdf>, and the
> > outcome from the last WG14 meeting was strong direction to proceed
> > along these lines.  Hopefully this much more human-readable approach
> > will prevail and we can do away with these confusing forward
> > declarations in parameter lists.
> 
> This sort of naked triumphalism when reporting on working group
> deliberations would be better replaced by a citation of the minutes of
> the WG14 meeting in question.  Since you didn't do that, I will, and
> further will quote the relevant portion.
> 
> At this point, the meeting minutes for summer 2026 are available in
> draft form only.
> 
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3953.htm
> 
> ---snip---
> Na/, Forward Parameter References through Retroactive Scoping, v2
> (Updates N3830) [N3923]
> 
> Celeste: I like this. But I am worried that "constraint" is too strong.
> Unrelated libraries can collide with each other. I would recommended
> practice be strong enough.
> 
> Bhakta: I like the changes. I also like that there is no new syntax for
> it. Some compilers still have issues with far-enough look-ahead; that is
> my only concern.
> 
> Gustedt: I am sympathetic to this approach; we need a solution. I am
> still baffled that we did not get any other solutions. I am scared of
> the complexity. I would still prefer forward declarations.
> 
> Celeste: This is similar for pre-C90 where you had to hook up with a
> declaration later.
> 
> Adams: How does this integrate with the type system? Is it separated?
> 
> Na: Your question is about the attributes, right? This is not about
> attributes. We have other proposals about intended attributes. What is
> your proposal number?
> 
> Adams: My example is for the noreturn attribute. What is the current
> status for the C type system?
> 
> Na: This proposal is for core language array size.
> 
> Bazley: SDCC already supports forward parameter declarations, but is
> more limited than GCC. I wonder if this is an over-engineered solution
> for a problem that does not need to exist at all?
> 
> Celeste: Is this ready for the Wording Group?
> 
> Gustedt: No, there are too many unanswered questions.
> 
> Columnar: I strongly prefer forward declarations. I like this proposal
> more. Given that small implementors cannot implement this, I am not
> comfortable accepting this.

This lacks some context.  "I like this proposal more [compared to the
previous version of it]."

> Uecker: The SlimCC developer and another compiler developer wrote in a
> discussion about general features that they are worried about parameters
> for active declaration scoping. It is difficult for them to implement
> this. They would rather implement template C++ functions.
> 
> Na: I need to discuss with that person offline the nature of their
> problem.
> 
> Colomar: Would it be acceptable for Clang to propose forward
> declarations, and then allow compilers to decide whether they want to do
> this retroactive parsing?
> 
> Tarditi: You could use that rationale to deny many changes. I do not
> think this is a serious issue. We could add implementation-defined
> limits if forward tokens is an issue.
> 
> Celeste: This could potentially be optional. I do not think that is
> possible. It does not give any cues to the compiler. Which bits would be
> optional? If the expression in the array bounds is limited to an
> identifier, there is no parsing problem.
> 
> Meneide: We do have an "implementation limit" section in the standard
> for such things. Limits on look-ahead for this feature can go there.
> 
> Straw Poll: Would WG14 like to adopt something along the lines of N3923
> for C2y? 3-0-0+12-7-4=15-7-4 strong direction to proceed
> 
> Gustedt: If we want this into C2y, we should have a paper with the
> semantics cleared by November, so it can go to the Wording Group.
> ---end snip---
> 
> I assume the "Columnar" listed above is a typo for "Colomar".

Yes, it is.


Have a lovely night!
Alex

> 
> Regards,
> Branden



-- 
<https://www.alejandro-colomar.es>

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