[cfe-dev] Objective-C++11: concerns about adding static_assert support to @interface / @implementation?

[Arthur O'Dwyer via cfe-dev]

On Fri, Mar 8, 2019 at 11:11 AM Nico Weber via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> I'm trying to enable -Wextra-semi in Chromium. I'm almost there; the last
> remaining issue is that we have a macro THREAD_CHECKER(name) that expands
> to nothing in release builds but to `ThreadChecker name` in debug builds.
> It's supposed to be used like
>
> class C {
>   THREAD_CHECKER(checker_);
> };
>
> [...] Since this is a declaration, this triggers -Wextra-semi. I figured
> the way to spell the usual `do {} while (0)` you tag on the end of a
> statement macro to silence this warning could be `static_assert(1, "")`, so
> I changed THREAD_CHECKER() to expand to that in release builds instead of
> nothing.
>

For what little it's worth, my opinion is that supporting
static_assert/_Static_assert in Objective-C++/Objective-C sounds like a
great idea.

However, if you need something for Chromium that works right now and in
previous versions of Clang too, have you considered

    #define THREAD_CHECKER(name) friend void __dummy()

or

    #define THREAD_CHECKER(name) static constexpr int __dummy = 0

The latter should work in most places (as long as you only need one per
scope, or you could do `__COUNTER__` tricks if you need several in the same
scope). The former should work as long as you only ever need it in class
bodies.
(Bikeshedding the appropriate spelling of `__dummy` is left as an exercise
for Chromium's maintainers.)

–Arthur
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