The Scala team has a large test suite and compiles against more than a million lines of code from published Scala code bases in the wild on regular intervals.

The Scala team has a large test suite and compiles against more than a million lines of code from published Scala code bases in the wild on regular intervals. They also guarantee that if something compiles against a major version without deprecation warnings it will also compile against the next major version. The only difference with Java is that Scala developers actually deal with deprecated things instead of letting them accumulate with every release and that binary artifacts are forward compatible.