Re: Re: typeof and operands in named address spaces

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Nov 17, 2020 at 11:13 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> > +#define __unqual_typeof(type)  typeof( (typeof(type))type )
>
> that's certainly a much nicer version than the existing pre-processor
> expansion from hell.

Oh, and sparse doesn't handle this, and doesn't remove any qualifiers
for the above. And so this horror-test-case fails:

    #define __unqual_typeof(type)  typeof( (typeof(type))(type) )

    int *function(volatile int x)
    {
        extern __unqual_typeof(x) y;
        return &y;
    }

with

  t.c:6:17: warning: incorrect type in return expression (different modifiers)
  t.c:6:17:    expected int *
  t.c:6:17:    got int volatile *
  t.c:3:5: warning: symbol 'function' was not declared. Should it be static?

adding Luc and the sparse mailing list to the participants list.

But it does work with both gcc and clang for me.

For Luc, quoting some earlier emails:

> > lvalue conversion drops qualifers in C.  In GCC, this is not
> > implemented correctly as it is unobservable in standard C
> > (but it using typeof).
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97702
> >
> > A have a working patch in preparation to change this. Then you
> > could use
> >
> > typeof( ((void)0, x) )

on another thing that fails with sparse. But since gcc gets that wrong
too and it only works with clang, that's not so relevant for the
kernel.

Luc - same test-case as above, just

    #define __unqual_typeof(x) typeof( ((void)0, (x)) )

instead.

           Linus