[libcxx] Remove use of default function template parameters in type traits. Fixes DR20484

[EricWF]

Summary

This patch moves the SFINAE for __is_destructor_welformed out of the function template parameters. type_traits must compile in c++03 mode since it is included in c++03 headers.

[K-ballo]

include/type_traits

1554

Isn't the decltype still a problem if it has to compile in C++03 mode?

[EricWF]

decltype is #defined to __typeof__ when it is not available (ie. c++03 mode). The other uses of decltype in the type_traits header lead me to believe it should not be a problem. The tests for <cmath> (as en example since it includes <type_traits>) compile after this change in C++03 mode (for GCC tot) but not before.

[danalbert]

LGTM.

I can test this against the projects we had that were failing tomorrow if you'd like.

[EricWF]

@danalbert I would appreciate that. I wouldn't mind your verification since this needs to get merged into 3.5 on short notice.

[EricWF]

@mclow.lists I'm going to wait on your OK as well before I commit this.

[mclow.lists]

I tested this in C++03/11/14 modes. All tests passed in 11/14, but the is_destructible tests failed in C++03 mode (as they did before this change), due to the fact that __typeof__ is not really a replacement for declval.

This looks OK to me.

include/type_traits

1554

Yes. It will fail (cause a compilation failure) when the destructor is private or protected.

[danalbert]

Well, I still can't get a passing checkbuild (though I can build a system image) with ToT libc++, but it's a different set of issues now (partially due to bugs in our version of gcc). I think this patch is good to go.

include/type_traits

 //  Otherwise, return "std::declval<_Up&>().~_Up()" is well-formed
 //    where _Up is remove_all_extents<_Tp>::type
 
+template <class>
+struct __is_destructible_apply { typedef int type; };
+
 template <typename _Tp>
 struct __is_destructor_wellformed {
-  template <typename _Tp1, typename _Tp2 = decltype(_VSTD::declval<_Tp1&>().~_Tp1())>
-  static char  __test (int);
+  template <typename _Tp1>
+  static char  __test (
+        typename __is_destructible_apply<decltype(_VSTD::declval<_Tp1&>().~_Tp1())>::type
+    );
 
   template <typename _Tp1>
   static __two __test (...);
   
   static const bool value = sizeof(__test<_Tp>(12)) == sizeof(char);
-  };
 
 template <class _Tp, bool>
 struct __destructible_imp;