[cfe-dev] Possible bug with __has_cpp_attribute

[Aaron Ballman]

I've fixed this in r236996; the built-in macro is now only added when
compiling with C++ enabled.

~Aaron

On Wed, May 6, 2015 at 9:31 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Wed, May 6, 2015 at 8:17 AM, Kal <b17c0de at gmail.com> wrote:
>> Am 06.05.15 um 14:21 schrieb Aaron Ballman:
>>
>> On Wed, May 6, 2015 at 4:28 AM, Kal <b17c0de at gmail.com> wrote:
>>
>> Hi,
>> The following seems like a clang bug.
>>
>> You can't use __has_cpp_attribute with namespaces in a header that is
>> included from C. For example:
>> #if __has_cpp_attribute(clang::fallthrough)
>>    ...
>> #endif
>>
>> Yields:
>> error: builtin feature check macro requires a parenthesized identifier.
>>
>> Shouldn't __has_cpp_attribute() evaluate to 0 for C programs?
>>
>> That's debatable. I would say it should be an error for C programs to
>> refer to __has_cpp_attribute because it should not be a predefined
>> macro there. I think the bug is that __has_cpp_attribute isn't flag as
>> being available only for C++. Then you could use the canonical
>> #if/#elif block for C++ attribute feature testing, without needing the
>> __cplusplus bit in yours.
>>
>> ~Aaron
>>
>>
>> From claim documentation for __has_cpp_attribute: " If the attribute is not
>> supported by the current compliation target, this macro evaluates to 0." No
>> C++ attributes are supported when compiling for C, so shouldn't they just
>> all return 0? If __has_cpp_attribute wasn't defined at all for C programs
>> that would also be good. Either way, what clang does now seems broken.
>
> Agreed that what Clang is doing now is not correct. I've filed a bug for it:
>
> https://llvm.org/bugs/show_bug.cgi?id=23435
>
> Thank you!
>
> ~Aaron
>
>>
>>
>> I worked around the issue with something like:
>>
>> #ifndef __has_cpp_attribute
>> #define __has_cpp_attribute(x) 0
>> #elif defined(__clang__) && !defined(__cplusplus)
>> #pragma clang diagnostic push
>> #pragma clang diagnostic ignored "-Wbuiltin-macro-redefined"
>> #define __has_cpp_attribute(x) 0
>> #pragma clang diagnostic pop
>> #endif
>>
>> Is there a better way?
>>
>> -Kal
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>>