[/ 
  Copyright 2015 Peter Dimov.
  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:declval declval]

   template <class T>
   typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand

__std_ref C++11 20.2.4 [declval].

__header ` #include <boost/type_traits/declval.hpp>` or ` #include <boost/type_traits.hpp>`

The function template `declval` is used when a value of a certain type is required in
a type computation context. For example, the type of the result of adding an `int` and
a `float` can be obtained with the expression `decltype( declval<int>() + declval<float>() )`.

[endsect]