Add use_default to Core

doc/core.qbk

@@ -58,3 +58,4 @@ criteria for inclusion is that the utility component be:
 [include swap.qbk]
 [include typeinfo.qbk]
 [include uncaught_exceptions.qbk]
+[include use_default.qbk]

doc/use_default.qbk

@@ -0,0 +1,47 @@
+[/
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+]
+
+[section:use_default use_default]
+
+[section Overview]
+
+The header <boost/core/use_default.hpp> provides the type `boost::use_default`
+which is used by other Boost libraries as a sentinel type in a templates to
+indicate defaults.
+
+[endsect]
+
+[section Example]
+
+```
+template<class Derived, class Base,
+   class Value = boost::use_default,
+   class CategoryOrTraversal = boost::use_default,
+   class Reference = boost::use_default,
+   class Difference = boost::use_default>
+class iterator_adaptor;
+
+template<class Value>
+class node_iterator
+    : public iterator_adaptor<node_iterator<Value>, Value*,
+        boost::use_default, boost::forward_traversal_tag>;
+```
+
+[endsect]
+
+[section Reference]
+
+```
+namespace boost {
+  struct use_default { };
+}
+```
+
+[endsect]
+
+[endsect]

include/boost/core/use_default.hpp

@@ -0,0 +1,17 @@
+/*
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#ifndef BOOST_CORE_USE_DEFAULT_HPP
+#define BOOST_CORE_USE_DEFAULT_HPP
+
+namespace boost {
+
+struct use_default { };
+
+} /* boost */
+
+#endif

test/Jamfile.v2

@@ -133,6 +133,8 @@ run empty_value_final_test.cpp ;
 run quick_exit_test.cpp ;
 run-fail quick_exit_fail.cpp ;
 
+compile use_default_test.cpp ;
+
 lib lib_typeid : lib_typeid.cpp : <link>shared:<define>LIB_TYPEID_DYN_LINK=1 ;
 
 run test_lib_typeid.cpp lib_typeid : : : <link>shared : test_lib_typeid_shared ;

test/use_default_test.cpp

@@ -0,0 +1,15 @@
+/*
+Copyright 2019 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#include <boost/core/use_default.hpp>
+
+template<class, class = boost::use_default>
+struct type { };
+
+template class type<int>;
+template class type<void, boost::use_default>;