Constraints and concepts

So far, we have covered quite a few important techniques for writing C++ metaprograms. You have seen how templates can generate concrete classes and functions for us with excellent support from the type traits library. Furthermore, you have seen how the use of constexpr, consteval, and if constexpr can help us move computations from runtime to compile time. In that way, we can detect programming errors at compile time and write programs with lower runtime costs. This is great, but there is still plenty of room for improvement when it comes to writing and consuming generic code in C++. Some of the issues that we haven't addressed yet include:

1. Interfaces are too generic. When using a template with some arbitrary type, it's hard to know what the requirements of that type are. This makes the templates hard to use if we only inspect the template interface. Instead, we have to rely on documentation or dig deep into the implementation of a template...