[cfe-dev] [PATCH] New syntax and functionality for __has_attribute

[Sean Silva]

On Sun, Jan 12, 2014 at 6:53 PM, Aaron Ballman <aaron at aaronballman.com>wrote:

> > That's good news -- thanks for confirming.
> >
> > The feature detection macro itself will still need to have a different
> name
> > (or some other mechanism) so it can be used compatibly with existing
> clang
> > deployments, because _has_attribute() currently emits a parse error
> instead
> > of gracefully returning 0 when passed the new argument syntax:
> >
> > tmp/attr2.cpp:1:5: error: builtin feature check macro requires a
> > parenthesized identifier
> > #if __has_attribute(__attribute__((weakref)))
> >     ^
>
> Good catch. Unfortunately, __has_attribute is really the best
> identifier for the macro, so I am loathe to let it go.
>
> Due to the current design of __has_attribute, we can't get away with "
> magic" by expanding the non-function-like form into a value that could
> be tested. So we would really have to pick a new name if we are
> worried about this.
>
> Suggestions on the name are welcome.


Ok, I'll bite:

__has_attribute_written_as([[foo]])
__has_attribute_syntax([[foo]])
__has_attribute_spelling([[foo]])

-- Sean Silva


> Regardless of the name picked, if
> it's different from __has_attribute, we should deprecate the existing
> usage since the proposed implementation is backwards compatible and
> the existing __has_attribute is not forwards-compatible.
>
> > For forward compatibility your new version should warn, rather than
> emitting
> > an error, upon encountering invalid attribute syntaxes that may be valid
> in
> > future standards or other language dialects (e.g. single-braced C++/CLI
> []
> > attributes should return 0, indicating "unsupported" rather than
> triggering
> > a parse error).
>
> I agree, and will implement for the next round.
>
> ~Aaron
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