[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.

[Marshall Clow]

On Oct 1, 2014, at 10:17 AM, Bob Wilson <bob.wilson at apple.com> wrote:

>> 
>> On Oct 1, 2014, at 9:22 AM, Agustín K-ballo Bergé <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> 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
>>>> 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.


— Marshall

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