[PATCH] D59223: Objective-C++11: Support static_assert() in @interface/@implementation ivar lists and method declarations

Mon Mar 11 18:04:50 PDT 2019

thakis marked an inline comment as done.
thakis added inline comments.

================
Comment at: clang/test/Parser/objc-static-assert.mm:1
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-objc-root-class %s
+
----------------
aaron.ballman wrote:
> erik.pilkington wrote:
> > thakis wrote:
> > > aaron.ballman wrote:
> > > > thakis wrote:
> > > > > aaron.ballman wrote:
> > > > > > Can you try explicitly specifying C++98 as the underlying language standard mode? I feel like `_Static_assert()` will continue to work there (because we made it a language extension in all modes) but `static_assert()` may fail (because that's gated on C++11 support). If that turns out to be the case, then I think `objc_static_assert` should be more complex than expanding to `true`, or we should talk about supporting `static_assert()` in all C++ language modes like we do for `_Static_assert()`.
> > > > > Correct, with -std=c++98 static_assert isn't available but _Static_assert still is. If you want, I can add a test for this, but this is covered by regular c++ tests already.
> > > > > 
> > > > > I think the has_feature() should stay as-is though: Else you have no way to know if _Static_assert works in obj-c mode, and you can check if static_assert works by checkout has_feature && __cplusplus >= 201103L if you still care about c++98.
> > > > > 
> > > > > (And adding one feature each for static_assert and _Static_assert seems like overkill.)
> > > > > Correct, with -std=c++98 static_assert isn't available but _Static_assert still is. If you want, I can add a test for this, but this is covered by regular c++ tests already.
> > > > 
> > > > Please do (if we don't change the availability of `static_assert()`), because the test currently relies on the unwritten -std option in order to pass.
> > > > 
> > > > > I think the has_feature() should stay as-is though: Else you have no way to know if _Static_assert works in obj-c mode, and you can check if static_assert works by checkout has_feature && __cplusplus >= 201103L if you still care about c++98.
> > > > 
> > > > I don't think this is the correct approach. Testing for `static_assert()` support should not leave the user guessing at what the correct spelling is.
> > > > 
> > > > > (And adding one feature each for static_assert and _Static_assert seems like overkill.)
> > > > 
> > > > Definitely agreed there.
> > > > 
> > > > I think the correct way forward is to support `static_assert()` in all language modes like we already do for `_Static_assert()`, then `objc_static_assert` becomes useful as-is. I cannot think of a reason why we would want to allow `_Static_assert()` in C++ but not allow `static_assert()`.
> > > I updated the test.
> > > 
> > > Accepting `static_assert()` independent of language mode seems pretty unrelated to this patch here, so I don't want to do this.
> > > 
> > > If you don't like the current has_feature approach, I'm all ears for other approaches. The current approach allows you to detect if clang can handle static_assert in objc objects, and codebases that still care about c++98 will have a static_assert wrapping macro keyed off __cplusplus already, so that part will transparently just work as well. And codebases that are c++11 and newer are in a good position too. I think the current setup is pretty good. (But I'm happy to hear better suggestions.)
> > > Accepting static_assert() independent of language mode seems pretty unrelated to this patch here, so I don't want to do this.
> > 
> > Yeah, we shouldn't be treating `static_assert` as a keyword in C++98 or C, I think. It would break code.
> > 
> > > If you don't like the current has_feature approach, I'm all ears for other approaches. The current approach allows you to detect if clang can handle static_assert in objc objects, and codebases that still care about c++98 will have a static_assert wrapping macro keyed off __cplusplus already, so that part will transparently just work as well. And codebases that are c++11 and newer are in a good position too. I think the current setup is pretty good. (But I'm happy to hear better suggestions.)
> > 
> > This is pretty weird. This feature flag doesn't actually correspond to any feature, just the possibility of the existence of a feature (there isn't any way you could use this `__has_feature` portably without also including another `__has_feature` check). I think the most internally consistent way of doing this is to have two flags, as you mentioned above, `objc_c_static_assert` and `objc_cxx_static_assert`.
> > 
> > Just to keep the bikeshed going, maybe it should be spelt `objc_interface_c_static_assert` or something, to show that it doesn't control static_assert in ObjC, but static_assert in interfaces in ObjC.
> > Yeah, we shouldn't be treating static_assert as a keyword in C++98 or C, I think. It would break code.
> 
> That depends on how we implemented the feature (we could parse the token as an identifier and check the spelling in situations where static_assert() can grammatically appear, for instance). I do have a hunch that this should be possible to support, though it's nontrivial and I don't expect @thakis to do it as part of this feature.
> 
> > I think the most internally consistent way of doing this is to have two flags, as you mentioned above, objc_c_static_assert and objc_cxx_static_assert.
> 
> Yeah, as much as I didn't like the idea at first blush, I'm starting to think it's the best way forward. I don't want to ever explain why `__has_feature(objc_static_assert)` return true with `-std=c++98` and yet `static_assert(true, "");` fails to compile while `_Static_assert(true, "")` succeeds. That's inexplicable behavior, IMO. Having two feature test macros alleviates that concern.
> 
> > Just to keep the bikeshed going, maybe it should be spelt objc_interface_c_static_assert or something, to show that it doesn't control static_assert in ObjC, but static_assert in interfaces in ObjC.
> 
> That seems reasonable to me.
One last try to make a case for this patch as-is:

> This is pretty weird. This feature flag doesn't actually correspond to any feature, just the possibility of the existence of a feature (there isn't any way you could use this __has_feature portably without also including another __has_feature check). 

You can if you assume C++11, which most people probably do. And people who don't either use Objective-C (without ++) where _Static_assert always works, or use something like

```
#if __cplusplus >= 201103L
#define STATIC_ASSERT(x) static_assert(x)
#else
#define STATIC_ASSERT(x) static_assert(x)
#endif
```

For all three cases,

```
@interface Foo {
#if __has_feature(objc_static_assert)
  STATIC_ASSERT(...)
#endif
}
```

will do the right thing (with STATIC_ASSERT either being static_assert for folks who target C++11 and never, _Static_assert() (or static_assert + include assert.h) for Objective-C without ++, or literally STATIC_ASSERT with the macro above for people who target c++98 and c++11.

I don't think there's anything strange about this, and it allows having just a single feature check.

I don't feel super strongly about this, so if this still doesn't sway y'all I'll go with two features.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D59223/new/

https://reviews.llvm.org/D59223