I would think a preferable (in the sense of separation of concerns, modularity, what-have-you) solution would be an addition to Boost.Functional (or whatever) that basically wraps a callable object and forces its result_type to be some specified template parameter. E.g.,

template< class F, class R >
struct result_binder
{
  F f;
  typedef R result_type;
  template< class... T >
  result_type operator()(T&&... x) const
  { return f(std::forward<T>(x)...); }
};

template< class R, class F >
result_binder<F,R>
bind_result(F&& f)
{ return result_binder<F,R>{std::forward<F>(f)}; }

[or something like that]

- Jeff