Re: How can Autoconf help with the transition to stricter compilation defaults?

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 16 Nov 2022 at 15:59, Michael Matz <matz@suse.de> wrote:
>
> Hello,
>
> On Wed, 16 Nov 2022, Sam James wrote:
>
> > Unrelated but I was a bit tempted to ask for throwing in
> > -Wbuiltin-declaration-mismatch to default -Werror while Clang 16 was at
> > it, but I suppose we don't want the world to burn too much,
>
> :-)  It's IMHO a bug in the standard that it misses "if any of its
> associated headers are included" in the item for reservation of external
> linkage identifiers; it has that for all other items about reserved
> identifiers in the Library clause.  If that restriction were added you
> couldn't justify erroring on the example at hand (because it doesn't
> include e.g. <stdio.h> and then printf wouldn't be reserved).  A warning
> is of course always okay and reasonable.  As is, you could justify
> erroring out, but I too think that would be overzealous.


I think that's very intentional and not a defect in the standard.

If one TU was allowed to define:

void printf() { }

and have that compiled into the program, then that would cause
unexpected behaviour for every other TU which includes <stdio.h> and
calls printf. They would get the non-standard rogue printf.