Re: -Wint-conversion, -Wincompatible-pointer-types, -Wpointer-sign: Are they hiding constraint C violations?

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

On Thu, 10 Nov 2022 at 19:17, Florian Weimer via Gcc <gcc@gcc.gnu.org> wrote:
>
> * Marek Polacek:
>
> > On Thu, Nov 10, 2022 at 07:25:21PM +0100, Florian Weimer via Gcc wrote:
> >> GCC accepts various conversions between pointers and ints and different
> >> types of pointers by default, issuing a warning.
> >>
> >> I've been reading the (hopefully) relevant partso f the C99 standard,
> >> and it seems to me that C implementations are actually required to
> >> diagnose errors in these cases because they are constraint violations:
> >> the types are not compatible.
> >
> > It doesn't need to be a hard error, a warning is a diagnostic message,
> > which is enough to diagnose a violation of any syntax rule or
> > constraint.
> >
> > IIRC, the only case where the compiler _must_ emit a hard error is for
> > #error.
>
> Hmm, you could be right.
>
> The standard says that constraint violations are not undefiend behavior,
> but of course it does not define what happens in the presence of a
> constraint violation.  So the behavior is undefined by omission.  This
> seems to be a contradiction.

As long as a diagnostic is issued, the invalid program can be
successfully translated. Presumably the implementation defines what it
means in that case, because it's not a valid C program, so the
standard no longer applies. As you say, that's undefined by omission.


> I assumed that there was a rule similar to the the rule for #error for
> any kind of diagnostic, which would mean that GCC errors are diagnostic
> messages in the sense of the standard, but GCC warnings are not.
>
> I wonder how C++ handles this.

Similarly. We don't have constraint violations, we have "diagnosable
rules", see [intro.compliance.general]. If a program contains a
violation of a diagnosable rule, the implementation must issue a
diagnostic message. What it does after that is up to the
implementation, the program has left the realm of valid C++ code.

It's not exactly easy for a user to know whether a given compiler
warning was issued because of a constraint (or diagnosable rule)
violation, or because e.g. the compiler thinks extra parens or more
consistent indentation would be a good idea. In the former case, the
program has undefined behaviour (strictly speaking, but in practice
the implementation might "define" its behaviour) and in the latter
case it doesn't. -pedantic-errors can help, as that turns the former
class into errors.