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

1template <typename T>
2concept Numeric = std::is_arithmetic_v<T>;
3
4template <Numeric T>
5T add(T a, T b) { return a + b; }
6
7// Constrained auto
8auto result = add(3, 4);    // OK
9// add("a", "b");           // Error: not Numeric

1template <typename T>
2concept Container = requires(T t) {
3    std::begin(t);
4    std::end(t);
5    t.size();
6};
7
8void print(Container auto const& c) {
9    for (const auto& elem : c) { /* ... */ }
10}