C89 question: Do we need to accept -Wint-conversion warnings

[Florian Weimer]

* Jakub Jelinek:

> On Tue, Oct 10, 2023 at 12:30:52PM -0400, Jason Merrill via Gcc wrote:
>> On Tue, Oct 10, 2023 at 7:30 AM Florian Weimer via Gcc <gcc@gcc.gnu.org>
>> wrote:
>> 
>> > Are these code fragments valid C89 code?
>> >
>> >   int i1 = 1;
>> >   char *p1 = i;
>> >
>> >   char c;
>> >   char *p2 = &c;
>> >   int i2 = p2;
>> >
>> > Or can we generate errors for them even with -std=gnu89?
>> >
>> > (It will still be possible to override this with -fpermissive or
>> > -Wno-int-conversion.)
>> >
>> 
>> Given that C89 code is unlikely to be actively maintained, I think we
>> should be permissive by default in that mode.  People compiling with an old
>> -std flag are presumably doing it to keep old code compiling, and it seems
>> appropriate to respect that.
>
> Yeah, complete agreement here.

Okay.  It helps with the test suite conversation because -std=gnu89 -w
is available today, so I can post such patches separately.

>> I'm also (though less strongly) inclined to be permissive in C99 mode, and
>> only introduce the new strictness by default for C11/C17 modes.
>
> Especially when the default is -std=gnu17 that can be an option as well.
>
> There might be some code in the wild compiled with -std=gnu99 or -std=c99 just
> because it wanted to use C99 features back 15-20 years ago and hasn't been
> adjusted since then, but it might be better to adjust that if needed and keep
> using those flags only when they are needed because the code isn't C11/C17/C2X
> ready.

Linux currently uses -std=gnu99 deliberately in a few places, I believe.
It would be a shame if we defaulted to permissive mode over there.  I
would certainly prefer to restrict permissive mode to the C89/C90
language levels.  We can have this discussion once I post my patch
(which depends on Jason's permerror enhancement in several ways).

Thanks,
Florian