Perfect forwarding error

#include <utility>
// using forward
#include <string>

namespace {
  struct Foo
  {
    std::string bar;

    template<class std_string>
    Foo(std_string&& bar):
      bar(std::forward(bar))
    {}
  };
}
int main()
{
  Foo baz(std::string(" "));
  return 0;
}

// clang error:
// forward.cpp:13:11: error: no matching function for call to 'forward'
//       bar(std::forward(bar))
//           ^~~~~~~~~~~~
// forward.cpp:21:7: note: in instantiation of function template specialization '<anonymous namespace>::Foo::Foo<std::basic_string<char> >' requested here
//   Foo baz(std::string(" "));
//       ^
// /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include/g++-v4/bits/move.h:62:5: note: candidate template ignored: couldn't infer template argument '_Tp'
//     forward(typename std::remove_reference<_Tp>::type& __t)
//     ^
// /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include/g++-v4/bits/move.h:67:5: note: candidate template ignored: couldn't infer template argument '_Tp'
//     forward(typename std::remove_reference<_Tp>::type&& __t)
//     ^
// 1 error generated.

// g++ error:
// forward.cpp: In constructor '{anonymous}::Foo::Foo(std_string&&) [with std_string = std::basic_string<char>]':
// forward.cpp:21:27:   instantiated from here
// forward.cpp:13:28: error: no matching function for call to 'forward(std::basic_string<char>&)'
// forward.cpp:13:28: note: candidates are:
// /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include/g++-v4/bits/move.h:62:5: note: template<class _Tp> _Tp&& std::forward(typename std::remove_reference<_Tp>::type&)
// /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include/g++-v4/bits/move.h:67:5: note: template<class _Tp> _Tp&& std::forward(typename std::remove_reference<_Tp>::type&&)