[PATCH] PR18327: -Wsystem-headers introduces build errors

Marshall Clow mclow.lists at gmail.com
Thu Jan 9 15:18:50 PST 2014

On Jan 9, 2014, at 2:51 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Thu, Jan 9, 2014 at 2:39 PM, Marshall Clow <mclow.lists at gmail.com> wrote:
> On Jan 9, 2014, at 2:20 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> On Thu, Jan 9, 2014 at 7:25 AM, Alp Toker <alp at nuanti.com> wrote:
>> It's not fair on the user or the system header maintainers to bring up unexpected errors with this flag.
>> I don't think this is clear. If the code in the system header is ill-formed, I would expect the system header maintainers to be *grateful* that we flag it as an error and not just as a warning. But I'll wait to hear what they have to say about that.
> A few points, in no particular order:
> * If we have ill-formed code in system headers, I would expect it to fail to compile whether the user specifies -Wsystem-headers or not.
> And yet in some cases we don't, because we deliberately support some flavors of broken system headers.
> Given that, would you want -Wsystem-headers to make such code produce an error or only a warning?


First - is any of that code in libc++?
Second, there’s two audiences that I think are affected here, and they have different needs:
* People trying to use the system headers. These people are, in general, unable/unwilling to change them. Giving them warnings/errors is just noise.
* People who are working on the system headers. They need to see the warnings.

> * If a user has a clean build, and then rebuilds with -Wsystem-headers, I would expect to get warnings - not errors. [ Isn’t that what PR18327 is all about? ]
> Do you expect this because the flag starts with -W?


> (Maybe that's the problem here -- it's *not* a warning flag in the sense that the other -W flags are. But that's not unprecedented -- nor is -Werror.)

> * There are some “interesting” language features which are only enabled for system headers, and cause warnings if used in user code.
> [ User-defined suffixes that do not start with an underscore, for example. ]
> This is a really great example, thanks.

Note that putting something like this in your source file gives you a warning;

    std::complex<long double> operator"" ilx(long double __im)
        return { 0.0l, __im };

$ clang11 junk.cpp
junk.cpp:5:31: warning: user-defined literal suffixes not starting with '_' are
      reserved; no literal will invoke this operator [-Wuser-defined-literals]
    std::complex<long double> operator"" ilx(long double __im)
1 warning generated.

> So it seems there are at least three different classes of errors that we might think about producing in system headers:
>  1) warnings that the user has turned into errors with -Werror or -Werror=foo
>  2) errors for ill-formed code that we suppress by default in system headers as a workaround for a system header bug
>  3) errors for code that is ill-formed outside system headers but valid within system headers (using reserved names, adding names to namespace std, that sort of thing)
> With -Wsystem-headers enabled, I think (1) should be an error, (3) should remain suppressed (not even a warning), and (2) should be either a warning or an error (and your first two bullets don't give me a clear idea of which way these cases should go).

Agreed for #1 and #3.
And for #2, I’d like to see a warning;  “This is code we wouldn’t allow outside a system header, but we do so here”

I don’t see the point of making #2 an error. 
For most people (who can’t fix it), it’s just a hassle.
For system developers, they can read warning output just as well as error output.

— Marshall

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