function_template.hpp:

- function partial specialization now allows assignment to zero
    (for clearing) and comparison against zero (for the empty check)
    (Brad King)

function_test.cpp:
  - Check comparison against zero
  - Check assignment to zero

function_test_fail1.cpp:
function_test_fail2.cpp:
  - Make them fail for the right reasons


[SVN r15803]

test/function_test_fail1.cpp

@@ -23,8 +23,8 @@ using namespace boost;
 int
 test_main(int, char*[])
 {
-  function<int> f1;
-  function<int> f2;
+  function0<int> f1;
+  function0<int> f2;
 
   if (f1 == f2) {
   }