One thing that concerns me about this proposal is ...

# language-proposals

[dmitry.petrov]

One thing that concerns me about this proposal is separate compilation.
Every time you design some (new) dispatch mechanism, you should take care that there are no surprises if it is used across compilation units (modules in Kotlin case).
E.g., if you need to see all implementations of the particular "multimethod" (or whatever it actually turns out in the end) to generate proper dispatching methods, that's bad.