How do you define a custom concept?

Define a concept with the concept keyword equal to a compile-time boolean expression, often a requires expression listing required operations and their constraints. The concept can then constrain templates. Compose concepts with && and || and refine existing ones to build precise, reusable requirements.

template <class T>
concept Container = requires(T c) {
    c.begin(); c.end(); typename T::value_type;
};