18 Concepts library [concepts]

18.7 Callable concepts [concepts.callable]

18.7.3 Concept regular_invocable [concept.regularinvocable]

template<class F, class... Args> concept regular_invocable = invocable<F, Args...>;
The invoke function call expression shall be equality-preserving ([concepts.equality]) and shall not modify the function object or the arguments.
[Note 1: 
This requirement supersedes the annotation in the definition of invocable.
— end note]
[Example 1: 
A random number generator does not model regular_invocable.
— end example]
[Note 2: 
The distinction between invocable and regular_invocable is purely semantic.
— end note]