Re: A: a new bug to old plain C

On Fri, 21 Nov 2025 at 13:18, <[email protected]> wrote:
>
> > -----Ursprüngliche Nachricht-----
> > Von: Gcc-help <[email protected]> Im Auftrag von
> > Jonathan Wakely via Gcc-help
> > Gesendet: Freitag, 21. November 2025 12:47
> > An: Chris S <[email protected]>
> > Cc: Александр Поваляев <[email protected]>; gcc-help <gcc-
> > [email protected]>
> > Betreff: Re: A: a new bug to old plain C
> >
> > On Fri, 21 Nov 2025 at 03:25, Chris S via Gcc-help <[email protected]>
> > wrote:
> > >
> > > You're applying two consts to the same thing, so it's an error.
> >
> > No, that's not happening here.
> >
> > The explanation is
> > https://isocpp.org/wiki/faq/const-correctness#constptrptr-conversion
> >
> > >
> > > "Const int" and "int const" are the same (look up East vs West const),
> > > but "const int const" is an error.  Declaring a pointer to that
> > > invalid type declaration is still invalid.
> > >
> > > On Thu, Nov 20, 2025, 5:27 PM Александр Поваляев via Gcc-help <
> > > [email protected]> wrote:
> > >
> > > > struct zzz {
> > > > unsigned x, y;
> > > > };
> > > >
> > > > void square(struct zzz ** arr_of_ptr, unsigned count) {
> > > >
> > > > *// The first "const" produce an error (not a warning) within some
> > > > GCC toolchains, for example ARM GCC trunk (linux), see goldbolt.org
> > > > <http://goldbolt.org>*
> > > > struct zzz const * const * const end_of_arr = arr_of_ptr + count;
> > > >
> > > > *// Without the first const "struct zzz * const * const end_of_arr =
> > > > arr_of_ptr + count;" it works fine!!!*
> > > >
> > > > }
> > > >
> > > > Respectfully,
> > > > Aleksandr G Povaliaev.
> > > >
>
> Consider:
>
> char const * foo1(char * x) {
>     char const * y = x;

This is valid.

>     *x = 'a';
>     return y;
> }
>
> char const ** foo2(char ** x) {
>     char const ** y = x;

This doesn't compile for the reasons explained in the link I gave.

>     **x = 'a';
>     return y;
> }
>
> typedef char * cp;
> cp const * foo3(cp * x) {
>     cp const * y = x;

This is completely different, this is:

char * const * y = x;

>     **x = 'a';
>     return y;
> }
>
> All three should compile fine, but one does not.

No, one should not compile.

> It is not consistent.

They are three completely different things, with different semantics.
Why should things that are not the same be consistent? That makes no
sense.

The link I gave explains why the language standards are defined this way.