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.
