[libcxx] Fix LWG 2876: shared_ptr::shared_ptr(const weak_ptr<Y>&) constructor should be constrained.

[EricWF]

Summary

Fixes LWG 2876 and some issues with array types in weak_ptr.

[zoecarver]

I'm going to add a more comprehensive patch to fully support array types in weak_ptr later this week.

[zoecarver]

• Update 1z satus

[ldionne]

Can we add tests for these constructors with incompatible Yps too?

libcxx/include/memory

4533

I'm not seeing this change as part of LWG 2876.

Where does that come from? If we don't have proper support for array types in weak_ptr, we should add it separately (with tests too), not as part of fixing this simple LWG issue.

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/weak_ptr.pass.cpp

85

STD_TEST_VERSION doesn't exist either. Please see my comment in another patch.

[zoecarver]

If we don't have proper support for array types in weak_ptr, we should add it separately (with tests too), not as part of fixing this simple LWG issue.

I was hoping to put together a patch for this today but I didn't have time. I should be able to implement this tomorrow or sometime later this week. We should put this patch on hold until that's done.

[EricWF]

This was completed. Closing.

libcxx/include/memory

 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     _LIBCPP_INLINE_VISIBILITY
     shared_ptr(shared_ptr&& __r) _NOEXCEPT;
     template<class _Yp> _LIBCPP_INLINE_VISIBILITY  shared_ptr(shared_ptr<_Yp>&& __r,
                    typename enable_if<__compatible_with<_Yp, element_type>::value, __nat>::type = __nat())
                        _NOEXCEPT;
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
     template<class _Yp> explicit shared_ptr(const weak_ptr<_Yp>& __r,
-                   typename enable_if<is_convertible<_Yp*, element_type*>::value, __nat>::type= __nat());
+                   typename enable_if<__compatible_with<_Yp, _Tp>::value, __nat>::type= __nat());
 #if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR)
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     template<class _Yp>
         shared_ptr(auto_ptr<_Yp>&& __r,
                    typename enable_if<is_convertible<_Yp*, element_type*>::value, __nat>::type = __nat());
 #else
     template<class _Yp>
         shared_ptr(auto_ptr<_Yp> __r,

 }
 
 #endif  // _LIBCPP_NO_RTTI
 
 template<class _Tp>
 class _LIBCPP_TEMPLATE_VIS weak_ptr
 {
 public:
+#if _LIBCPP_STD_VER > 14
+    typedef remove_extent_t<_Tp> element_type;
+#else
     typedef _Tp element_type;
+#endif
+
 private:
     element_type*        __ptr_;
     __shared_weak_count* __cntrl_;
 
 public:
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR weak_ptr() _NOEXCEPT;
     template<class _Yp> _LIBCPP_INLINE_VISIBILITY weak_ptr(shared_ptr<_Yp> const& __r,

 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
     ~weak_ptr();
 
     _LIBCPP_INLINE_VISIBILITY
     weak_ptr& operator=(weak_ptr const& __r) _NOEXCEPT;
     template<class _Yp>
         typename enable_if
         <
-            is_convertible<_Yp*, element_type*>::value,
+            __compatible_with<_Yp, _Tp>::value,
             weak_ptr&
         >::type
         _LIBCPP_INLINE_VISIBILITY
         operator=(weak_ptr<_Yp> const& __r) _NOEXCEPT;
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
     _LIBCPP_INLINE_VISIBILITY
     weak_ptr& operator=(weak_ptr&& __r) _NOEXCEPT;
     template<class _Yp>
         typename enable_if
         <
-            is_convertible<_Yp*, element_type*>::value,
+            __compatible_with<_Yp, _Tp>::value,
             weak_ptr&
         >::type
         _LIBCPP_INLINE_VISIBILITY
         operator=(weak_ptr<_Yp>&& __r) _NOEXCEPT;
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
     template<class _Yp>
         typename enable_if
         <
-            is_convertible<_Yp*, element_type*>::value,
+            __compatible_with<_Yp, _Tp>::value,
             weak_ptr&
         >::type
         _LIBCPP_INLINE_VISIBILITY
         operator=(shared_ptr<_Yp> const& __r) _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     void swap(weak_ptr& __r) _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
weak_ptr<_Tp>::operator=(weak_ptr const& __r) _NOEXCEPT
     return *this;
 }
 
 template<class _Tp>
 template<class _Yp>
 inline
 typename enable_if
 <
-    is_convertible<_Yp*, _Tp*>::value,
+    __compatible_with<_Yp, _Tp>::value,
     weak_ptr<_Tp>&
 >::type
 weak_ptr<_Tp>::operator=(weak_ptr<_Yp> const& __r) _NOEXCEPT
 {
     weak_ptr(__r).swap(*this);
     return *this;
 }
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 template<class _Tp>
 inline
 weak_ptr<_Tp>&
 weak_ptr<_Tp>::operator=(weak_ptr&& __r) _NOEXCEPT
 {
     weak_ptr(_VSTD::move(__r)).swap(*this);
     return *this;
 }
 
 template<class _Tp>
 template<class _Yp>
 inline
 typename enable_if
 <
-    is_convertible<_Yp*, _Tp*>::value,
+    __compatible_with<_Yp, _Tp>::value,
     weak_ptr<_Tp>&
 >::type
 weak_ptr<_Tp>::operator=(weak_ptr<_Yp>&& __r) _NOEXCEPT
 {
     weak_ptr(_VSTD::move(__r)).swap(*this);
     return *this;
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 template<class _Tp>
 template<class _Yp>
 inline
 typename enable_if
 <
-    is_convertible<_Yp*, _Tp*>::value,
+    __compatible_with<_Yp, _Tp>::value,
     weak_ptr<_Tp>&
 >::type
 weak_ptr<_Tp>::operator=(shared_ptr<_Yp> const& __r) _NOEXCEPT
 {
     weak_ptr(__r).swap(*this);
     return *this;
 }
 

 weak_ptr<_Tp>::reset() _NOEXCEPT
 {
     weak_ptr().swap(*this);
 }
 
 template<class _Tp>
 template<class _Yp>
 shared_ptr<_Tp>::shared_ptr(const weak_ptr<_Yp>& __r,
-                            typename enable_if<is_convertible<_Yp*, element_type*>::value, __nat>::type)
+                            typename enable_if<__compatible_with<_Yp, _Tp>::value, __nat>::type)
     : __ptr_(__r.__ptr_),
       __cntrl_(__r.__cntrl_ ? __r.__cntrl_->lock() : __r.__cntrl_)
 {
     if (__cntrl_ == 0)
         __throw_bad_weak_ptr();
 }
 
 template<class _Tp>

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/weak_ptr.pass.cpp

#ifndef TEST_HAS_NO_EXCEPTIONS
         }
         catch (std::bad_weak_ptr&)
         {
         }
     }
     assert(A::count == 0);
 #endif
 
+#if STD_TEST_VERSION > 14
+    {
+        std::shared_ptr<A[4]> sp0(new A[4]);
+        std::weak_ptr<A[4]> wp(sp0);
+        std::shared_ptr<A[4]> sp(wp);
+        assert(sp.use_count() == 2);
+        assert(sp.get() == sp0.get());
+        assert(A::count == 4);
+    }
+    assert(A::count == 0);
+
+    {
+        std::shared_ptr<A[4]> sp0(new A[4]);
+        std::weak_ptr<A[4]> wp(sp0);
+        std::shared_ptr<B[4]> sp(wp);
+        assert(sp.use_count() == 2);
+        assert(sp.get() == sp0.get());
+        assert(A::count == 4);
+    }
+    assert(A::count == 0);
+
+    {
+        std::shared_ptr<A[4]> sp0(new A[4]);
+        std::weak_ptr<A[4]> wp(sp0);
+        std::shared_ptr<A> sp(wp);
+        assert(sp.use_count() == 2);
+        assert(sp.get() == sp0.get());
+        assert(A::count == 4);
+    }
+    assert(A::count == 0);
+
+    {
+        std::shared_ptr<A[4]> sp0(new A[4]);
+        std::weak_ptr<A[4]> wp(sp0);
+        std::shared_ptr<B> sp(wp);
+        assert(sp.use_count() == 2);
+        assert(sp.get() == sp0.get());
+        assert(A::count == 4);
+    }
+    assert(A::count == 0);
+#endif // STD_TEST_VERSION > 14
+
   return 0;
 }

libcxx/www/cxx1z_status.html

<!--   <I>Note: "NAD" means that the issue was deemed "Not a defect"</I> -->
   <tr><td><a href="https://wg21.link/LWG2857">2857</a></td><td>{variant,optional,any}::emplace should return the constructed value</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2861">2861</a></td><td>basic_string should require that charT match traits::char_type</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2866">2866</a></td><td>Incorrect derived classes constraints</td><td>Kona</td><td><I>Nothing to do</I></td></tr>
   <tr><td><a href="https://wg21.link/LWG2868">2868</a></td><td>Missing specification of bad_any_cast::what()</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2872">2872</a></td><td>Add definition for direct-non-list-initialization</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2873">2873</a></td><td>Add noexcept to several shared_ptr related functions</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2874">2874</a></td><td>Constructor shared_ptr::shared_ptr(Y*) should be constrained</td><td>Kona</td><td></td></tr>
   <tr><td><a href="https://wg21.link/LWG2875">2875</a></td><td>shared_ptr::shared_ptr(Y*, D, [&hellip;]) constructors should be constrained</td><td>Kona</td><td></td></tr>
-  <tr><td><a href="https://wg21.link/LWG2876">2876</a></td><td>shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;) constructor should be constrained</td><td>Kona</td><td></td></tr>
+  <tr><td><a href="https://wg21.link/LWG2876">2876</a></td><td>shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;) constructor should be constrained</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2878">2878</a></td><td>Missing DefaultConstructible requirement for istream_iterator default constructor</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2890">2890</a></td><td>The definition of 'object state' applies only to class types</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2900">2900</a></td><td>The copy and move constructors of optional are not constexpr</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2903">2903</a></td><td>The form of initialization for the emplace-constructors is not specified</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2904">2904</a></td><td>Make variant move-assignment more exception safe</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2905">2905</a></td><td>is_constructible_v&lt;unique_ptr&lt;P, D&gt;, P, D const &amp;&gt; should be false when D is not copy constructible</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2908">2908</a></td><td>The less-than operator for shared pointers could do more</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="https://wg21.link/LWG2911">2911</a></td><td>An is_aggregate type trait is needed</td><td>Kona</td><td>Complete</td></tr>