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### declval

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

C++ Standard Reference: 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>() )`.