Add negative pointer cast tests.

test/pointer_cast_dy_fail.cpp

@@ -0,0 +1,25 @@
+//
+//  A negative test for unique_ptr dynamic_cast
+//
+//  Copyright 2016 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/pointer_cast.hpp>
+#include <memory>
+
+struct B
+{
+    virtual ~B()
+    {
+    }
+};
+
+int main()
+{
+    std::unique_ptr<B> p1( new B );
+    std::unique_ptr<B[]> p2 = boost::dynamic_pointer_cast<B[]>( std::move( p1 ) );
+}