[cfe-dev] should -Wimplicit-fallthrough require C++11?

Richard Smith richard at metafoo.co.uk
Fri Nov 9 20:41:06 PST 2012

On Fri, Nov 9, 2012 at 8:15 PM, Ted Kremenek <kremenek at apple.com> wrote:
> On Nov 9, 2012, at 7:04 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> The original intention was that the warning could be used in any
> language mode, if you wanted a warning on *all* switch fallthrough,
> and that people who didn't want that could just not turn it on.
> Right.
> I
> guess the complaints you're receiving are for situations where the
> warning can't simply be disabled (or not enabled in the first place)?
> What has happened is that a fair number of users have discovered this
> warning using -Weverything.  When they first discover it they find it
> interesting.  After they discover that they cannot add an annotation (as the
> warning suggests) they get very frustrated.  They are then left with the
> choice of just turning the warning off, or using pragmas.  For those users
> who like the warning in principle, but find that they cannot use it in
> practice because of these limitations (because they are not using C++11)
> they view it is an incomplete feature.  Several people have thus requested
> to not see the warning at all.
> If we really can't find a better solution for these people than
> disabling the warning in languages where we can't annotate
> fallthrough, then we should do so consistently and disable it in C++98
> too.
> Agreed, but I like your suggestion of using something like a builtin.  Right
> now this warnings feels like an incomplete feature.  It has a really great
> workflow in C++11 with the use of the attribute.  Otherwise, unless the user
> wants a zero-tolerance policy with this warning, the workflow is not great.
> What I am afraid of is people turning this warning off (either directly or
> by never turning it on) when they may have found it useful.  Once they make
> that decision, it is unlikely they will give the warning a second try.
> You may have noticed that I reverted my change, pending further discussion
> here.  I still think we should consider what to do for the LLVM 3.2 branch,
> in case this doesn't get enough attention before then.  I myself won't have
> any cycles over the weekend to look at this.  A builtin, however, doesn't
> sound so hard to implement, but I'd prefer some way to sugar coat this to
> avoid the ugly syntax.

Yes, this is where it gets tricky. In C++11 we had a nice syntactic
slot to put something non-ugly. Nonetheless, we expected people to
wrap it in some kind of compatibility macro, and I think we would have
the same expectation of whatever solution we pick for the
outside-C++11 case. Given that, perhaps it's not terrible to pick
something ugly?

If you think the aesthetics of the raw annotation really matters, I
think the least ugly option open to us would be to predefine a
"__fallthrough" macro, which could expand to __builtin_fallthrough(),
or to some keyword (perhaps even a __fallthrough keyword). Having this
be detectable by the preprocessor seems useful, so that people can
easily write portable code:

#ifdef __fallthrough
#define fallthrough __fallthrough
#define fallthrough do {} while (0)

(With the C++11 attributes approach, we were using
__has_warning("-Wimplicit-fallthrough") for this.) That said, I view
the bar for a new keyword or predefined macro as being higher than
that for a new builtin, in part due to the increased risk of breaking
existing code.

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