18 Concepts library [concepts]

18.7 Callable concepts [concepts.callable]

18.7.2 Concept Invocable [concept.invocable]

The Invocable concept specifies a relationship between a callable type ([func.def]) F and a set of argument types Args... which can be evaluated by the library function invoke ([func.invoke]).
template<class F, class... Args> concept Invocable = requires(F&& f, Args&&... args) { invoke(std::forward<F>(f), std::forward<Args>(args)...); // not required to be equality-preserving };
A function that generates random numbers can satisfy Invocable, since the invoke function call expression is not required to be equality-preserving ([concepts.equality]).
end example