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+[/
+Copyright 2018 Glen Joseph Fernandes
+<glenjofe -at- gmail.com>
+
+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).
+]
+
+[section:detected_or detected_or]
+
+ template<class Default, template<class...> class Op, class... Args>
+ using detected_or = __below;
+
+ template<class Default, template<class...> class Op, class... Args>
+ using detected_or_t = typename detected_or<Default, Op, Args...>::type;
+
+__alias An unspecified type with two public member type definitions:
+
+* `value_t` is __true_type if `Op<Args...>` is a valid template-id, otherwise
+ __false_type
+* `type` is `Op<Args...>` if it is a valid template-id, otherwise `Default`
+
+__std_paper [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf N4502]
+
+__compat Requires C++11 variadic templates and C++11 template aliases.
+
+__header `#include <boost/type_traits/detected_or.hpp>`
+
+__examples
+
+Suppose we wish to declare a type that represents the difference between two values of type T, it should be
+T::difference_type if such a type exists, or std::ptrdiff_t otherwise:
+
+ template<class T>
+ using difference_t = typename T::difference_type;
+
+ template<class T>
+ using difference_type = boost::detected_or_t<std::ptrdiff_t, difference_t, T>;
+
+Now the type `difference_type<T>` gives us what we need.
+
+See also: __is_detected, __is_detected_convertible, __is_detected_exact.
+
+[endsect]