Added BOOST_CONCEPT_USAGE for defining usage requirements: Using this

macro is better than simply defining a destructor because usage
patterns will be tested whenever associated types are accessed.


[SVN r35198]

usage_fail.cpp

@@ -0,0 +1,22 @@
+// Copyright David Abrahams 2006. Distributed under the Boost
+// Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/concept/usage.hpp>
+
+template <class T>
+struct StringInitializable
+{
+    typedef int associated;
+    BOOST_CONCEPT_USAGE(StringInitializable)
+    {
+        T x = "foo";
+    }
+};
+
+// Test that accessing an associated type will actually exercise usage
+// requirements
+typedef StringInitializable<int>::associated tee;
+
+
+