made void_ a complete type

[SVN r17182]

include/boost/mpl/void.hpp

@@ -23,7 +23,11 @@
 namespace boost {
 namespace mpl {
 
-struct void_;
+//  [JDG Feb-4-2003] made void_ a complete type to allow it to be
+//  instantiated so that it can be passed in as an object that can be
+//  used to select an overloaded function. Possible use includes signaling
+//  a zero arity functor evaluation call.
+struct void_ {};
 
 template< typename T >
 struct is_void_