(1) note that “statically-typed language with generics” excludes a lot of statically typed languages, including C and Go (at least pre generics).

(2) this misses the meat of the question which is how to express that (eg) a GET request doesn’t come with a body and a POST request does. I suppose that you’re suggesting that one registers a url handler with a method type and that forces the handler to accept responses of a certain type. Or perhaps you are implicitly allowing for sun types (which aren’t a thing in many static type systems.)

(3) even in C++, isn’t this suggestion hard to work with. That is, isn’t it annoying to write a program which works for any request whether or not it has a body because the type of the body must be a template parameter that adds templates to the type of every method which is generic to it. But maybe that is ok or I just don’t understand C++.