Can anybody tell me if this would be true to any JVM targeted language, or is it a specific issue of the Scala compiler?
I'm asking because I just recently decided to abort learning Scala in favor of Clojure.
The 1.2 -> 1.3 transition has had a couple of bumps, but nothing compared to the pain I recall from my time across Scala versions years ago. FWIW, I have code I wrote years ago against pre-1.0 Clojure that still sits in jars in my Maven repository that is loaded and run in 1.4 alphas today. Backwards compatibility is mostly quite good.
What AOT really is, is when you first run a java program with certain flags, and some compilation starts, a special AOT version, which is usually a low optimization level which is saved to disk with some relocation markers.
This helps with startup time in subsequent runs as the compiler can just load its baseline pre-compiled classes straight into memory instead of having to do a bunch of grunt work during startup. Once things get going, the compiler can much more quickly work its way up to the higher optimization levels on the sections of code that need it.
THAT is what AOT means from a jitted language perspective.
Now, there have been some source-level incompatibilities, especially going from Clojure 1.2 to 1.3, and this seems to have been a bit of a headache within some parts of the Clojure community.
Neither Scala or Clojure's compilers generate byte code that is expected to work with that from another version of the respective compiler or with a different version of the language runtime.
In the case of Clojure this is mitigated by the fact that (since it is a dynamic language) that applications and libraries are generally delivered in source form and "ahead of time" compilation is seen as an optimization rather than a code delivery mechanism.
I think Scala is slowing down in this area, but have not stopped.