[rectang]

I believe that both Go and Rust use two-word fat pointers for interface types. I was actually thinking of Rust traits first and foremost; in Rust, traits can supply default method implementations which invoke other methods defined in the same trait.

While I take your point about introducing assumptions, I'm reluctant to give up the convenience of default implementations; they seem to present fewer problems than classically inherited methods because shallow hierarchies are more common with interfaces than with classical inheritance, and because default interface methods cannot directly access member variables because they do not know about object layout — unlike methods inherited from a parent class under classical inheritance.