Add negative pointer cast tests.

test/pointer_cast_st_fail2.cpp

@@ -0,0 +1,18 @@
+//
+//  A negative test for unique_ptr static_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>
+
+int main()
+{
+    std::unique_ptr<int[]> p1( new int[ 1 ] );
+    std::unique_ptr<int> p2 = boost::static_pointer_cast<int>( std::move( p1 ) );
+}