Make atomic_shared_ptr's default constructor constexpr

doc/smart_ptr/atomic_shared_ptr.adoc

@@ -35,7 +35,7 @@ namespace boost {
 
   public:
 
-    atomic_shared_ptr() noexcept = default;
+    constexpr atomic_shared_ptr() noexcept;
     atomic_shared_ptr( shared_ptr<T> p ) noexcept;
 
     atomic_shared_ptr& operator=( shared_ptr<T> r ) noexcept;
@@ -64,6 +64,14 @@ namespace boost {
 
 ## Members
 
+```
+constexpr atomic_shared_ptr() noexcept;
+```
+[none]
+* {blank}
++
+Effects:: Default-initializes `p_`.
+
 ```
 atomic_shared_ptr( shared_ptr<T> p ) noexcept;
 ```

include/boost/smart_ptr/atomic_shared_ptr.hpp

@@ -57,12 +57,22 @@ private:
 
 public:
 
+#if !defined( BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX ) && !defined( BOOST_NO_CXX11_CONSTEXPR )
+
+    constexpr atomic_shared_ptr() BOOST_SP_NOEXCEPT: l_ BOOST_DETAIL_SPINLOCK_INIT
+    {
+    }
+
+#else
+
     atomic_shared_ptr() BOOST_SP_NOEXCEPT
     {
         boost::detail::spinlock init = BOOST_DETAIL_SPINLOCK_INIT;
         std::memcpy( &l_, &init, sizeof( init ) );
     }
 
+#endif
+
     atomic_shared_ptr( shared_ptr<T> p ) BOOST_SP_NOEXCEPT
 #if !defined( BOOST_NO_CXX11_RVALUE_REFERENCES )
         : p_( std::move( p ) )

test/Jamfile.v2

@@ -216,6 +216,8 @@ import testing ;
           [ run sp_constexpr_test.cpp ]
           [ run sp_constexpr_test2.cpp ]
 
+          [ run atomic_sp_constexpr_test.cpp ]
+
           [ run local_sp_test.cpp ]
           [ run lsp_array_test.cpp ]
           [ run lsp_array_n_test.cpp ]

test/atomic_sp_constexpr_test.cpp

@@ -0,0 +1,67 @@
+//
+// atomic_sp_constexpr_test.cpp
+//
+// Copyright 2017 Peter Dimov
+//
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+//
+
+#include <boost/config.hpp>
+#include <boost/detail/workaround.hpp>
+
+#define HAVE_CONSTEXPR_INIT
+
+#if defined( BOOST_NO_CXX11_CONSTEXPR )
+# undef HAVE_CONSTEXPR_INIT
+#endif
+
+#if BOOST_WORKAROUND( BOOST_MSVC, <= 1910 )
+# undef HAVE_CONSTEXPR_INIT
+#endif
+
+#if defined(__clang__) && defined( BOOST_NO_CXX14_CONSTEXPR )
+# undef HAVE_CONSTEXPR_INIT
+#endif
+
+#if !defined( HAVE_CONSTEXPR_INIT ) || !defined( BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX )
+
+int main()
+{
+}
+
+#else
+
+#include <boost/atomic_shared_ptr.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct X
+{
+};
+
+struct Z
+{
+    Z();
+};
+
+static Z z;
+
+static boost::atomic_shared_ptr<X> p1;
+
+Z::Z()
+{
+    p1 = boost::shared_ptr<X>( new X );
+}
+
+int main()
+{
+    bost::shared_ptr<X> p2 = p1;
+
+    BOOST_TEST( p2.get() != 0 );
+    BOOST_TEST_EQ( p2.use_count(), 1 );
+
+    return boost::report_errors();
+}
+
+#endif // #if defined( BOOST_NO_CXX11_CONSEXPR )