id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
6655,Inclusion of <boost/signals2.hpp> causes valid code to fail to compile with Clang,Alexander Korsunsky <fat.lobyte9@…>,Frank Mori Hess,"The following well-formed program does not compile with Clang 3.1 (from trunk):

{{{
#include <memory>
#include <boost/signals2.hpp>

int main()
{
    std::shared_ptr<int> s;
}
}}}

The error message is:
{{{
signals2-bug.cpp:6:26: error: implicit instantiation of undefined template
      'std::shared_ptr<int>'
    std::shared_ptr<int> s;
                         ^
/home/alexander/usr/local/include/boost/signals2/detail/foreign_ptr.hpp:24:30: note: 
      template is declared here
  template<typename T> class shared_ptr;
                             ^
}}}

The header boost/signals2/detail/foreign_ptr.hpp declares two names in the std:: namespace:
{{{
#if !defined(BOOST_INTEL_STDCXX0X)
namespace std
{
  template<typename T> class shared_ptr;
  template<typename T> class weak_ptr;
}
#endif
}}}

which is undefined behaviour, according to the Standard 17.6.4.2.1:
  The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a
  namespace within namespace std unless otherwise specified. A program may add a template specialization
  for any standard library template to namespace std only if the declaration depends on a user-defined type
  and the specialization meets the standard library requirements for the original template and is not explicitly
  prohibited.

The compilation succeeds, if either the #include of <boost/signals2.hpp> is removed, or the offending lines are commented out.

Please remove the offending lines from the header.
",Bugs,closed,Boost 1.50.0,signals2,Boost 1.49.0,Problem,fixed,,