When I write two functions in the same project I get a compile error if I change the signature of one without changing the call site. This is analogous to a monorepo, in which I have projects in separate directories but with a unified build.

If you really want to take this analogy to its logical conclusion for separate repos, you're gonna have to do something more like compile every function as a dynamically loaded library. And then hope you notice a function signature change because the dynamic linker won't tell you.