Explain C++20 Concepts and how they improve templates.

Concepts are named constraints on template parameters, replacing SFINAE and enable_if. Syntax: template<typename T> concept Sortable = requires(T a) { a < a; }. Usage: template<Sortable T> or requires clause. Standard concepts: std::integral, std::floating_point, std::copyable, std::range. Benefits: clearer intent, better error messages, overload resolution. Requires expressions: test expressions, type requirements, compound requirements. Subsumption: more constrained overload preferred. Concepts enable: cleaner API documentation, constraint-based overloading, concept-based polymorphism. Major improvement for generic programming readability and usability.