libstdc++: Fix assigning nullptr to std::atomic<shared_ptr<T>> (LWG 3893)

LWG voted this to Tentatively Ready recently.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr_atomic.h (atomic::operator=(nullptr_t)):
	Add overload, as per LWG 3893.
	* testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc:
	Check assignment from nullptr.

2 files changed, 15 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/shared_ptr_atomic.h b/libstdc++-v3/include/bits/shared_ptr_atomic.h
index d142616..2295b48 100644
--- a/libstdc++-v3/include/bits/shared_ptr_atomic.h
+++ b/libstdc++-v3/include/bits/shared_ptr_atomic.h
@@ -650,6 +650,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator=(shared_ptr<_Tp> __desired) noexcept
       { _M_impl.swap(__desired, memory_order_seq_cst); }
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3893. LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;
+      void
+      operator=(nullptr_t) noexcept
+      { store(nullptr); }
+
       shared_ptr<_Tp>
       exchange(shared_ptr<_Tp> __desired,
 	       memory_order __o = memory_order_seq_cst) noexcept
diff --git a/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc b/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
index a190274..54cf262 100644
--- a/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
+++ b/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
@@ -145,6 +145,14 @@ test_counting()
   VERIFY( counter == 2 );
 }
 
+void
+test_lwg3893()
+{
+  // LWG 3893. LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;
+  std::atomic<std::shared_ptr<int>> a;
+  a = nullptr;
+}
+
 int
 main()
 {
@@ -152,4 +160,5 @@ main()
   test_atomic_shared_ptr();
   test_wait_notify();
   test_counting();
+  test_lwg3893();
 }