Answer
A requires expression (requires(T a){ ... }) is a compile-time check that yields true if the enclosed operations/requirements are valid for the given types. It's used to define concepts, listing required expressions, their validity, and return-type constraints. It replaces verbose SFINAE with readable requirement lists.
template <class T>
concept Addable = requires(T a, T b) { { a + b } -> std::same_as<T>; };Real-world example A custom Addable concept is defined with a requires expression listing that a + b must be valid and return T.