fixed obsolete use of class_requires

[SVN r8477]

bad_error_eg.cpp

@@ -1,11 +1,8 @@
 #include <list>
 #include <algorithm>
 
-struct foo {
-  bool operator<(const foo&) const { return false; }
-};
-int main(int, char*[]) {
-  std::list<foo> v;
+int main() {
+  std::list<int> v;
   std::stable_sort(v.begin(), v.end());
   return 0;
 }