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

Richard Smith richard at metafoo.co.uk
Wed Jan 8 12:36:37 PST 2014

On Tue, Jan 7, 2014 at 8:33 PM, Alp Toker <alp at nuanti.com> wrote:

> On 08/01/2014 04:21, Argyrios Kyrtzidis wrote:
>> On Jan 7, 2014, at 7:56 PM, Alp Toker <alp at nuanti.com> wrote:
>>  On 08/01/2014 01:48, Argyrios Kyrtzidis wrote:
>>>> On Jan 6, 2014, at 1:47 PM, Richard Smith <richard at metafoo.co.uk<mailto:
>>>> richard at metafoo.co.uk>> wrote:
>>>>  One view on this is simply: -Wsystem-headers means "don't give system
>>>>> headers special treatment when emitting diagnostics”.
>>>> This is it exactly. “Treat all headers like normal headers"
>>> We don't have a flag to treat all headers as normal headers at the
>>> moment. It'd be very simple to implement compared to -Wsystem-headers which
>>> somewhat intricate.
>> Could you elaborate, AFAICT '-Wsystem-headers' is treated specially, it's
>> outside the diagnostic group machinery, and acts essentially as a flag.
>> If you'd like to have something like '-fwarn-on-system-headers' or
>> something instead, that's another discussion, but as far as the PR is
>> concerned I don't see why we need to change what -Wsystem-headers is
>> currently doing.
> PR18327 reports a valid corner-case bug in the way a very small number
> (around 8 in total) diagnostics are upgraded from warnings/extensions to
> errors, and then were forgetting to downgrade them back to warnings as all
> the other diagnostics are seen. So it's just an implementation detail that
> was leaking and manifesting as errors in a context where it should have
> been impossible.

These diagnostics are set up as 'errors that we suppress in system headers'
(they're suppressed because they actually happen in some system headers).

If the viewpoint is that '-Wsystem-headers' means 'don't suppress
diagnostics in system headers', then issuing errors in these cases seems
correct to me. If the viewpoint is that '-Wsystem-headers' means 'warn on
problems in system headers', then issuing warnings, not errors, in these
cases seems correct. It depends on what the user of the flag intends it to

I think the former option makes a little more sense for system header
developers -- if their system headers contain ill-formed code, they
probably want to know about that with more urgency than if their system
headers merely contain dubious code. But we still don't know what the
person who filed the original PR was trying to do, and maybe there's a use
case where the latter view makes more sense.

Remember that this flag doesn't control the very many isInSystemHeader()
> and isInSystemMacro() checks that happen earlier than the diagnostic
> machinery. A -fno-system-headers flag to just disable the whole system
> header machinery would be separately useful though, agreed.

I agree. If we want this flag to mean 'there are no system headers', it
does not go far enough.

> Alp.
>>  Alp.
>>>  That would seem to make perfect sense to people developing system
>>>>> headers, and is our current behavior. What is the use case that leads to
>>>>> enabling -Wsystem-headers but not wanting that to lead to errors? PR18327
>>>>> doesn't make that obvious.
>>>> Not sure I’m following that report, if one doesn’t like that that
>>>> diagnostic is by default mapped to an error, maybe map it to a warning on
>>>> the command-line or discuss whether it should not be mapped to error by
>>>> default ?
>>>> I don’t see a need to complicate what -Wsystem-headers does.
>>>>  --
>>> http://www.nuanti.com
>>> the browser experts
> --
> http://www.nuanti.com
> the browser experts
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