[cfe-dev] Does `#pragma GCC diagnostic warning` intentionally win over -Werror?

Nico Weber via cfe-dev cfe-dev at lists.llvm.org
Thu Jan 24 08:19:54 PST 2019

On Thu, Jan 24, 2019 at 11:12 AM James Y Knight <jyknight at google.com> wrote:

> I don't think we should have such a flag. The explicit intent of '#pragma
> gcc diagnostic warning' is to allow downgrading an error to a warning,
> while still allowing it to be emitted.
> Perhaps a new value for pragma diagnostic which emits an upgradeable
> warning would be reasonable.

The use case here is that we don't allow non-error diagnostics in chrome.
Some subproject used `#pragma GCC diagnostic warning` because they wanted
to configure warning flags in cc files instead of in build files for some
reason. They didn't expect this to trigger a warning instead of an error. A
new pragma would meet that use case, but I don't want source code to be
able to opt out of -Werror. In my experience, once it's known that such an
option exist, someone will use it. So we do need some kind of "_all_
warnings are errors" toggle.

> On Thu, Jan 24, 2019 at 9:56 AM Nico Weber via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>> Re "intentional?", it matches gcc which explicitly documents on
>> https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html that the
>> pragma wins over -Werror.
>> Does anyone know if there's a flag that says "I want all warnings to be
>> errors, not just most of them"?
>> If there isn't one yet, what would be a good UI for that?
>> On Wed, Jan 23, 2019 at 11:11 PM Nico Weber <thakis at chromium.org> wrote:
>>> Consider:
>>> $ cat test.cc
>>> #pragma GCC diagnostic warning "-Wsign-compare"
>>> bool f(int a, unsigned b) {
>>>   return a != b;
>>> }
>>> $ out/gn/bin/clang -c test.cc -Werror
>>> test.cc:4:12: warning: comparison of integers of different signs: 'int'
>>> and 'unsigned int' [-Wsign-compare]
>>>   return a != b;
>>>          ~ ^  ~
>>> 1 warning generated.
>>> I found it surprising that this is emitted as a warning, not as an
>>> error. If this is intentional, is there some other way to say "I want my
>>> compiler warnings to always be errors"?
>>> Thanks,
>>> Nico
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