The Java interfaces that Clojure relies on don't matter that much. On the other hand what is the bytecode representation of a protocol or of a multi-method?
Scala has an equivalent representation for everything it has. For example traits are just Java interfaces but with corresponding static methods. Another example is default parameters (a concept Java doesn't have) is done with method overloading. The Scala compiler has to infer Scala-specific stuff (like signatures using Scala features) from compiled bytecode. Functions that aren't methods (e.g. anonymous functions) get compiled either to static methods, or to classes that have to be instantiated (in case it's about closures closing over their context). Scala does not have the concept of static methods, but it has singleton objects, which can also implement interfaces. Of course, these get compiled to static methods, but there's also a singleton instance instantiated for those cases in which you want to use that singleton object as a real polymorphic instance. Etc, etc... So there's a protocol in place for how to encode this in the bytecode, there's a protocol for everything. And so even small changes in Scala's standard libraries can trigger big bytecode changes that end up being backwards incompatible.
Clojure doesn't have to do this, because Clojure dependencies get distributed as source-code, as I've said.
Scala has an equivalent representation for everything it has. For example traits are just Java interfaces but with corresponding static methods. Another example is default parameters (a concept Java doesn't have) is done with method overloading. The Scala compiler has to infer Scala-specific stuff (like signatures using Scala features) from compiled bytecode. Functions that aren't methods (e.g. anonymous functions) get compiled either to static methods, or to classes that have to be instantiated (in case it's about closures closing over their context). Scala does not have the concept of static methods, but it has singleton objects, which can also implement interfaces. Of course, these get compiled to static methods, but there's also a singleton instance instantiated for those cases in which you want to use that singleton object as a real polymorphic instance. Etc, etc... So there's a protocol in place for how to encode this in the bytecode, there's a protocol for everything. And so even small changes in Scala's standard libraries can trigger big bytecode changes that end up being backwards incompatible.
Clojure doesn't have to do this, because Clojure dependencies get distributed as source-code, as I've said.