Showcases some interesting functionality of a feature introduced in C++20 called Concepts. Note: at the time of coding, this had to be compiled with GCC 9 with the flag -fconcepts.
In its most simple form, Concepts are syntactic sugar for employing type traits. They offer an alternative to the traditional SFINAE (substitution failure is not an error) approach. Both SFINAE and Concepts result in drastically reduced compiler errors vs unconstrained template errors.
C++20 introduces the keyword requires as a way of imposing constraints on template parameters.