Concepts and constraints in C++20 work .. I have just redesigned the Gamma GOF patterns using them. Goodbye GOF, really.Introducing Concepts -- http://accu.org/index.php/journals/2157 -- Concepts in C++11 had many false starts. Andrew Sutton show why they are a big deal now they are with us. by Andrew SuttonAn Inline-variant-visitor with C++ Concepts -- http://accu.org/index.php/journals/2160 -- Concepts are abstract. Jonathan Coe and Andrew Sutton provide us with a concrete example of their use.