[boost] [move] forward with lvalue ref template param

Subject: [boost] [move] forward with lvalue ref template param
From: Andrew Ho (helloworld922_at_[hidden])
Date: 2013-08-08 18:26:14

Hi during my implementation of unique_ptr I noticed that boost::forward 
doesn't play nicely with lvalue ref types.

int a;
int &b = a;
int &c = std::forward<int&>(b); // compiles fine
int &d = boost::forward<int&>(b); // compiler error

I did a little playing around with the declarations of forward and I managed 
to get this to work:

// if T is not a lvalue ref and movable
   template <class T>
   inline typename ::boost::move_detail::enable_if_c
      < enable_move_utility_emulation<T>::value && 
::boost::move_detail::is_rv<T>::value &&
      !::boost::move_detail::is_lvalue_reference<T>::value, const T &>::type
         forward(typename ::boost::move_detail::identity<T>::type &x) 
BOOST_NOEXCEPT
   {
      return const_cast<T&>(x);
   }

   // if T is not a lvalue ref and not movable
   template <class T>
   inline typename ::boost::move_detail::enable_if_c
      < enable_move_utility_emulation<T>::value && 
!::boost::move_detail::is_rv<T>::value &&
        !::boost::move_detail::is_lvalue_reference<T>::value, const T 
&>::type
      forward(const typename ::boost::move_detail::identity<T>::type &x) 
BOOST_NOEXCEPT
   {
      return x;
   }

   // if T is a lvalue ref
   template <class T>
   inline typename ::boost::move_detail::enable_if_c
      < ::boost::move_detail::is_lvalue_reference<T>::value , T>::type
          forward(T x) BOOST_NOEXCEPT
   {
      return x;
   }

Basically, this forces all calls to forward<A&> to simply return the lvalue 
ref (const-ness is maintained, I think), otherwise it does whatever 
boost::forward did before.

As far as I can tell, this doesn't break boost::forward, and it enables it 
to properly handle taking lvalue refs. Am I right in my assumptions? Are 
there any cases where this would fail to produce standard-compliant/desired 
results?