Sorry for the late reply but yes you can write some Groovy that will not work on Java (I'm trying to find when and where it happened to me... that could have changed).
The issue is that there are things you can do in JVM bytecode that you just can't do in Java and as the JVM develops along with these languages I imagine further interoperability.
You are right about the languages not making promises but there are plenty of library writers who disregard this. A fine example would be Akka and Finagle. Those libraries could have been written in Java thus minimizing dependencies and maybe even overall code size since they have to have a wrapper for Java anyway (typesafe did choose Java for typesafe config). But then again maybe Scala/Clojure buys enough that the wrappers are trivial. I'm just not sure how trivial interoperable wrappers for WebAssembly will be as I imagine WebAssembly will be a superset of what Javascript can do (or maybe not?).
The issue is that there are things you can do in JVM bytecode that you just can't do in Java and as the JVM develops along with these languages I imagine further interoperability.
You are right about the languages not making promises but there are plenty of library writers who disregard this. A fine example would be Akka and Finagle. Those libraries could have been written in Java thus minimizing dependencies and maybe even overall code size since they have to have a wrapper for Java anyway (typesafe did choose Java for typesafe config). But then again maybe Scala/Clojure buys enough that the wrappers are trivial. I'm just not sure how trivial interoperable wrappers for WebAssembly will be as I imagine WebAssembly will be a superset of what Javascript can do (or maybe not?).