The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands (Clause [expr]).
template <class T>
typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand
Remarks: If this function is odr-used ([basic.def.odr]), the program is ill-formed.
Remarks: The template parameter T of declval may be an incomplete type.
[ Example:
template <class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);
declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To. For another example see class template common_type ([meta.trans.other]). — end example ]