C++: What are C++20 concepts?

1#include <concepts>
2
3// Define a concept
4template<typename T>
5concept Numeric = std::is_arithmetic_v<T>;
6
7// Use concept as constraint
8template<Numeric T>
9T add(T a, T b) {
10    return a + b;
11}
12
13// Shorthand syntax
14auto multiply(Numeric auto a, Numeric auto b) {
15    return a * b;
16}
17
18add(1, 2);       // OK
19add(1.5, 2.5);   // OK
20// add("a", "b"); // Error — constraint not satisfied
21
22// Requires expression
23template<typename T>
24concept Streamable = requires(T t) {
25    { std::cout << t } -> std::same_as<std::ostream&>;
26};