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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/concepts/invocable.html below:

std::invocable, std::regular_invocable - cppreference.com

template< class F, class... Args >

concept invocable =
    requires(F&& f, Args&&... args) {
        std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
            /* not required to be equality-preserving */

    }; (since C++20)

template< class F, class... Args >
concept regular_invocable = std::invocable<F, Args...>;

(since C++20)

The invocable concept specifies that a callable type F can be called with a set of arguments Args... using the function template std::invoke.

The regular_invocable concept adds to the invocable concept by requiring the invoke expression to be equality-preserving and not modify either the function object or the arguments.

[edit] Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

[edit] Notes

The distinction between invocable and regular_invocable is purely semantic.

A random number generator may satisfy invocable but cannot satisfy regular_invocable (comical ones excluded).

[edit] References
[edit] See also [edit] External links A joke example of a random number generator that satisfies both invocable and regular_invocable.

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