[cfe-dev] Possible bug with __has_cpp_attribute

[Aaron Ballman]

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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