20 General utilities library [utilities]

20.2 Utility components [utility]

20.2.6 Function template declval [declval]

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
Mandates: This function is not odr-used ([basic.def.odr]).
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 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]