deduction guides for
Defined in header
template<class R, class... ArgTypes>
function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>;
function(F) -> function</*see below*/>;
2) If decltype(&F::operator()) is of the form R(G::*)(A...) (optionally cv-qualified, optionally noexcept, optionally lvalue reference qualified) for some class type G, then the deduced type is std::function<R(A...)>. This overload only participates in overload resolution if &F::operator() is well-formed when treated as an unevaluated operand.
The type deduced by these deduction guides may change in a later standard revision (in particular, this might happen if noexcept support is added to std::function in a later standard).