Hi Chris, On Sun, Sep 07, 2025 at 02:38:20PM +0100, Christopher Bazley wrote: > > alx@debian:~/tmp$ cat fwd.c | nl > > 1 void f(int n, int n; int n); > > 2 void g(int n; int n; int n); > > 3 void h(int n, int m; int n, int m); > > 4 void i(int n; int m; int n, int m); > > alx@debian:~/tmp$ gcc -S -Wall -Wextra fwd.c > > fwd.c:1:19: error: redefinition of parameter ‘n’ > > 1 | void f(int n, int n; int n); > > | ~~~~^ > > fwd.c:1:12: note: previous definition of ‘n’ with type ‘int’ > > 1 | void f(int n, int n; int n); > > | ~~~~^ > > fwd.c:1:12: error: parameter ‘n’ has just a forward declaration > > alx@debian:~/tmp$ gcc -S -Wall -Wextra -Wpedantic fwd.c > > fwd.c:1:19: error: redefinition of parameter ‘n’ > > 1 | void f(int n, int n; int n); > > | ~~~~^ > > fwd.c:1:12: note: previous definition of ‘n’ with type ‘int’ > > 1 | void f(int n, int n; int n); > > | ~~~~^ > > fwd.c:1:1: warning: ISO C forbids forward parameter declarations > > [-Wpedantic] > > 1 | void f(int n, int n; int n); > > | ^~~~ > > fwd.c:1:12: error: parameter ‘n’ has just a forward declaration > > 1 | void f(int n, int n; int n); > > | ~~~~^ > > fwd.c:2:1: warning: ISO C forbids forward parameter declarations > > [-Wpedantic] > > 2 | void g(int n; int n; int n); > > | ^~~~ > > fwd.c:3:1: warning: ISO C forbids forward parameter declarations > > [-Wpedantic] > > 3 | void h(int n, int m; int n, int m); > > | ^~~~ > > fwd.c:4:1: warning: ISO C forbids forward parameter declarations > > [-Wpedantic] > > 4 | void i(int n; int m; int n, int m); > > | ^~~~ > > > > - We want to keep the error in line 1, right? > > Yes, although "parameter ‘n’ has just a forward declaration" is misleading. > I am not sure exactly what is causing that. It seems critical to understand > it even if it proves difficult to remove.
I'll check it.
> > - The diagnostic in lines 2 & 4 should be reworded, and enabled by -Wextra.
> > - I should remove the diagnostic in line 3, at least for >C23.
> >
> > Right? I can do that.
>
> Sorry, when I wrote "implement n3681", I didn't mean that literally because
> it isn't possible to remove diagnostics for >C23 unless/until WG14 accepts
> forward parameter declarations into C2Y. I expect GCC's maintainers to
> reject any change that removes "warning: ISO C forbids forward parameter
> declarations" under any circumstance in which it is currently produced --
Yup; I'll have that part prepared for the future, but not present it now
for obvious reasons. It'll remain under -Wpedantic.
> including in the presence of any new warning about deprecation of support
> for multiple forward parameter declaration lists.
>
> What I was really thinking about was changes like the proposed extended
> syntax, which could be accepted (but isn't):
>
> void g( ; int x);
> void h( ; );
> void i( ; ...);
Ahhh, thanks for the reminder!
BTW, I'm wondering about the following:
alx@debian:~/tmp$ cat fwd.c
void j(void; int x);
void k(void; void);
void l(void; ...);
alx@debian:~/tmp$ gcc fwd.c
fwd.c:1:8: error: parameter ‘({anonymous})’ has just a forward
declaration
1 | void j(void; int x);
| ^~~~
fwd.c:2:14: error: ‘void’ must be the only parameter
2 | void k(void; void);
| ^~~~
fwd.c:2:8: error: parameter ‘({anonymous})’ has just a forward
declaration
2 | void k(void; void);
| ^~~~
These should be supported as well (even with more priority than empty
lists). (I never liked C23's decission of allowing () as synonyn for
(void).)
> Also "Both declarations shall specify compatible types before adjustment and
> have the same storage-class specifiers". For example, the following examples
> could be rejected (but aren't):
>
> void l(int x[3]; int x[4]); // incompatible types before adjustment
> void m(int *x; int x[]); // incompatible types before adjustment
> void p(register int x; int x); // different storage class
>
> I think this second point is the one of most relevance to your proposed
> extension to _Countof, isn't it?
Yup, I think so.
> > > since one of their
> > > objections was that they do not have enough resources to implement C
> > > language features that are not useful in C++. (This particular feature
> > > is not useful because C++ does not support dependent types, so
> > > obviously they are going to resist it.)
> > >
> > > I do not know whether Alejandro has time to do this work, but it seems
> > > like the best way to determine whether Clang's objections have any
> > > real substance, and to prove that the feature is feasible and useful
> > > as specified in N3681.
> > I have time; and Aaron would probably help me with doubts if I try.
>
> He is against the feature, but there may be others in the Clang community
> who could help.
No, I talked with Aaron in Brno, and he seemed happy to help me. While
he doesn't like the feature, he likes having me as a contributor, which
might be enough for him to help with learn Clang internals.
> > I just need energy to resist the pain of reading and writing C++ code.
>
> Of course GCC is also written in C++, although less noticeably.
Yeah, in theiry. So far, I didn't notice, other than in the .cc file
extensions. It's the kind of C++ I'd write, if I were forced to write
C++. :-)
Have a lovely night!
Alex
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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