[libc++] Fix PR32979 - types with a private std::enable_shared_from_this base break shared_ptr

[EricWF]

Summary

This patch fixes bugs.llvm.org/PR32979.

[util.smartptr.shared.const] says:

In the constructor definitions below, enables shared_from_this with p, for a pointer p of type Y*, means
that if Y has an unambiguous and accessible base class that is a specialization of enable_shared_from_-
this.

This means that libc++ needs to respect the access specifier of the base class, and not attempt to construct
and enabled_shared_from_this base if it is private. However access specifiers don't affect overload resolution
so our current implementation will attempt to construct the private base.

This patch uses SFINAE to correctly detect if the shared_ptr input has an accessible enable_shared_from_this
base class.

[mclow.lists]

LGTM

include/memory

         allocate_shared(const _Alloc& __a, _A0& __a0, _A1& __a1, _A2& __a2);
 
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
 private:
 
     template <class _Yp, class _OrigPtr>
         _LIBCPP_INLINE_VISIBILITY
-        void
+        typename enable_if<is_convertible<_OrigPtr*,
+                                          const enable_shared_from_this<_Yp>*
+        >::value,
+            void>::type
         __enable_weak_this(const enable_shared_from_this<_Yp>* __e,
                            _OrigPtr* __ptr) _NOEXCEPT
         {
             typedef typename remove_cv<_Yp>::type _RawYp;
             if (__e && __e->__weak_this_.expired())
             {
                 __e->__weak_this_ = shared_ptr<_RawYp>(*this,
                     const_cast<_RawYp*>(static_cast<const _Yp*>(__ptr)));
             }
         }
 
     _LIBCPP_INLINE_VISIBILITY
     void __enable_weak_this(const volatile void*, const volatile void*) _NOEXCEPT {}
 
     template <class _Up> friend class _LIBCPP_TEMPLATE_VIS shared_ptr;
     template <class _Up> friend class _LIBCPP_TEMPLATE_VIS weak_ptr;
 };
 
+
 template<class _Tp>
 inline
 _LIBCPP_CONSTEXPR
 shared_ptr<_Tp>::shared_ptr() _NOEXCEPT
     : __ptr_(0),
       __cntrl_(0)
 {
 }

test/std/utilities/memory/util.smartptr/util.smartptr.enab/enable_shared_from_this.pass.cpp

struct Foo : virtual public std::enable_shared_from_this<Foo>
   virtual ~Foo() {}
 };
 
 struct Bar : public Foo {
     Bar(int) {}
 };
 
 
+struct PrivateBase : private std::enable_shared_from_this<PrivateBase> {
+    std::weak_ptr<PrivateBase> get_weak() { return weak_from_this(); }
+};
+
+
 int main()
 {
     {  // https://bugs.llvm.org/show_bug.cgi?id=18843
     std::shared_ptr<T const> t1(new T);
     std::shared_ptr<T const> t2(std::make_shared<T>());
     }
     { // https://bugs.llvm.org/show_bug.cgi?id=27115
     int x = 42;
int main()
     assert(!p.owner_before(q) && !q.owner_before(p)); // p and q share ownership
     }
     {
     std::shared_ptr<Y> p = std::make_shared<Z>();
     std::shared_ptr<T> q = p->shared_from_this();
     assert(p == q);
     assert(!p.owner_before(q) && !q.owner_before(p)); // p and q share ownership
     }
+  {
+    typedef std::shared_ptr<PrivateBase> APtr;
+    typedef std::weak_ptr<PrivateBase> WeakAPtr;
+    APtr a1 = std::make_shared<PrivateBase>();
+    assert(a1.use_count() == 1);
+  }
     // Test LWG issue 2529. Only reset '__weak_ptr_' when it's already expired.
     // http://cplusplus.github.io/LWG/lwg-active.html#2529.
     // Test two different ways:
     // * Using 'weak_from_this().expired()' in C++17.
     // * Using 'shared_from_this()' in all dialects.
     {
         assert(globalMemCounter.checkOutstandingNewEq(0));
         T* ptr = new T;