Difference between requires keyword and requires expression

The requires keyword and requires expressions serve different purposes in C++20 concepts:

The requires keyword is used within a requires expression to assert that a specific expression must be valid and evaluate to true for a type to satisfy the concept. For example:

1template <typename T>
2concept Addable = requires(T a, T b) {
3  requires std::is_arithmetic_v<T>;
4  requires requires { a + b; };
5};

Here, the requires keyword is used to assert that T must be an arithmetic type and that the expression a + b must be valid.

A requires expression, on the other hand, is a way to group multiple requirements together. It consists of a parameter list and a body containing one or more requirements. A type satisfies the requires expression if all the requirements in its body are met. For example:

1template <typename T>
2concept Printable = requires(T a) {
3  std::cout << a;
4};

In this case, the requires expression states that for a type T to be Printable, the expression std::cout << a must be valid, where a is an instance of type T.

So, in summary, the requires keyword is used within a requires expression to assert individual requirements, while a requires expression is a way to group and define a set of requirements that a type must satisfy to meet a concept.