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

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.

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33033

llvm-svn: 302709

1 files changed, 11 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.enab/enable_shared_from_this.pass.cpp b/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.enab/enable_shared_from_this.pass.cpp
index 5a0d9259c11..5124e025253 100644
--- a/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.enab/enable_shared_from_this.pass.cpp
+++ b/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.enab/enable_shared_from_this.pass.cpp
@@ -49,6 +49,11 @@ struct Bar : public Foo {
 };
 
 
+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
@@ -74,6 +79,12 @@ int main()
     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: