Excellent point! I think it's fair to say that substitutability can only be satisfied (in general) for a fixed set of operations. This is why I believe Haskell'

Excellent point! I think it's fair to say that substitutability can only be satisfied (in general) for a fixed set of operations. This is why I believe Haskell's (and by extension Rust's) approach to bounded ad-hoc polymorphism is the best that I've seen in a production language. Going beyond Haskell, Rust also allows you to choose static dispatch, making bounded polymorphism zero cost. Dynamic dispatch is still available, when desired or needed (but it's almost never needed).