The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.

template<class T> add_rvalue_reference_t<T> declval() noexcept; // as unevaluated operand

Remarks: The template parameter T of declval may be an incomplete type.

[Example 1:

template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&); declares a function template convert which only participates in overload resolution if the type From can be explicitly converted to type To.

— end example]