I'm interested in the question https://wg21.link/p2279r0 illustrated. Is there ongoing work trying to solve the customization problem with a solution base on the C++0x Concepts or anything related to the external polymorphism (such as Haskell typeclasses, Rust traits, and Swift protocol)? Or a plan of standardizing the approach of Circle(see https://github.com/seanbaxter/circle/blob/master/new-circle/README.md#interface)?