It's honestly a seriously hard problem. Yes, it's definitely nice to be able to typecheck generic code before instantiation. But supporting that ends up adding

It's honestly a seriously hard problem. Yes, it's definitely nice to be able to typecheck generic code before instantiation. But supporting that ends up adding a lot of complexity to the typesystem. C++-style templates are sort of like "compile-time dynamic types" where the type system is much simpler because you can just write templates that try to do stuff and if the instantiation works, it works. C++ templates are more powerful than generics in most other languages, while not having to deal with covariance/contravariance, bounded quantification, F-bounded quantification, traits, and all sorts of other complex machinery that Java, C#, etc. have. I still generally prefer languages that do the type-checking before instantiation, but I think C++ picks a really interesting point in the design space.