Calls to static interface methods compile on Java 8 but not on Java 9 because of a new bytecode restriction. As the usage of static interface methods increases, this problem will manifest more. I ran into it while using the AWS SDK (2.0 preview) which makes extensive usage of these methods.
JVMS Section 22.214.171.124 and 126.96.36.199 describe method resolution
The v2 patches have a full description in each commit. The basic approach is to link the method description correctly (a one-line change) but in order to do that we must emit JDK8 bytecode, not the current JDK5 bytecode. If this approach is decent I recommend getting in the change early in order to enjoy extended testing.
The original ticket had a repro here: https://github.com/ztellman/java9-failure