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

Alp Toker alp at nuanti.com
Tue Jan 7 20:33:59 PST 2014

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.

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.


>> 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.
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