[func.wrap.func.cap]
22 General utilities library [utilities]
22.10 Function objects [function.objects]
22.10.17 Polymorphic function wrappers [func.wrap]
22.10.17.3 Class template function [func.wrap.func]
22.10.17.3.4 Capacity [func.wrap.func.cap]
explicit operator bool() const noexcept;