[libcxx] r216909 - Fix PR#20834 - 'is_trivially_destructible yeilds wrong answer for arrays of unknown bound' Thanks to K-ballo for the bug report. Update a few of the other tests while we're here, and fix a typo in a test name.

[Argyrios Kyrtzidis]

> On Oct 1, 2014, at 12:03 PM, Marshall Clow <mclow.lists at gmail.com> wrote:
> 
> 
> On Oct 1, 2014, at 10:17 AM, Bob Wilson <bob.wilson at apple.com <mailto:bob.wilson at apple.com>> wrote:
> 
>>> 
>>> On Oct 1, 2014, at 9:22 AM, Agustín K-ballo Bergé <kaballo86 at hotmail.com <mailto:kaballo86 at hotmail.com>> wrote:
>>> 
>>> Hi Argyrios
>>> 
>>> On 30/09/2014 10:45 p.m., Argyrios Kyrtzidis wrote:
>>>> Hi Marshall,
>>>> 
>>>> This seems to have caused a regression with Objective-C++, see the following test case:
>>>> 
>>>> #include <type_traits>
>>>> 
>>>> class CXXForwardClass;
>>>> @class ObjCForwardClass;
>>>> 
>>>> static_assert(std::is_trivially_destructible<CXXForwardClass*>::value == true, "it is true"); // true
>>>> static_assert(std::is_trivially_destructible<ObjCForwardClass*>::value == true, "it is true"); // false ?
>>> 
>>> This sounds like a pre-existing issue to me. Does the following test case hold?
>>> 
>>> static_assert(std::is_destructible<CXXForwardClass*>::value == true, "it is true"); // true
>>> static_assert(std::is_destructible<ObjCForwardClass*>::value == true, "it is true"); // false ?
>> 
>> No, that doesn’t work either.
>> 
>>> 
>>>>> On Sep 2, 2014, at 9:19 AM, Marshall Clow <mclow.lists at gmail.com <mailto:mclow.lists at gmail.com>> wrote:
>>>>> 
>>>>> Author: marshall
>>>>> Date: Tue Sep  2 11:19:38 2014
>>>>> New Revision: 216909
>>>>> 
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=216909&view=rev <http://llvm.org/viewvc/llvm-project?rev=216909&view=rev>
>>>>> Log:
>>>>> Fix PR#20834 - 'is_trivially_destructible yeilds wrong answer for arrays of unknown bound' Thanks to K-ballo for the bug report. Update a few of the other tests while we're here, and fix a typo in a test name.
>>>>> 
>>>>> --- libcxx/trunk/include/type_traits (original)
>>>>> +++ libcxx/trunk/include/type_traits Tue Sep  2 11:19:38 2014
>>>>> @@ -2861,7 +2861,7 @@ template <class _Tp> struct _LIBCPP_TYPE
>>>>> #if __has_feature(has_trivial_destructor) || (_GNUC_VER >= 403)
>>>>> 
>>>>> template <class _Tp> struct _LIBCPP_TYPE_VIS_ONLY is_trivially_destructible
>>>>> -    : public integral_constant<bool, __has_trivial_destructor(_Tp)> {};
>>>>> +    : public integral_constant<bool, is_destructible<_Tp>::value && __has_trivial_destructor(_Tp)> {};
>>> 
>>> This is the relevant change ^. Something that is not destructible cannot be trivially destructible by definition.
>> 
>> Right, but the issue wasn’t exposed until r216909.
> 
> Interesting (but consistent) behavior here:
> 
> 	class CXXForwardClass;
> 	@class ObjCForwardClass;
> 
> 	template <class U>
> 	void destroy ( U& u ) { u.~U(); }
> 
> 	template <class U>
> 	void test () { U *p = nullptr; destroy<U*>(p); }
> 
> 	int main () {
> 		test<void>();				// compiles w/o error
> 		test<CXXForwardClass>();	// compiles w/o error
> 		test<ObjCForwardClass>();	// error.
> 		}
> 
> $ totclang11 -ObjC++ junk2.cpp
> junk2.cpp:39:26: error: member access into incomplete type 'ObjCForwardClass'
> void destroy ( U& u ) { u.~U(); }
>                          ^
> junk2.cpp:50:32: note: in instantiation of function template specialization
>       'destroy<ObjCForwardClass *>' requested here
> void test () { U *p = nullptr; destroy<U*>(p); }
>                                ^
> junk2.cpp:55:2: note: in instantiation of function template specialization
>       'test<ObjCForwardClass>' requested here
>         test<ObjCForwardClass>();
>         ^
> junk2.cpp:36:8: note: forward declaration of class here
> @class ObjCForwardClass;
>        ^
> 1 error generated.

Interesting but this behavior have existed in clang forever, is libcxx depending on this, recently, to do the type trait ?

> 
> 
> — Marshall
> 
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