Currently the Class file parser uses ASM 4.1. This older version cannot read Java 8 / Java 9 class files (fails with Exception).
The upgrade to ASM 5.0.4 is very simple, just Maven dependency change. The code change is only to update the visitor version, so it gets new Java 8 features like lambdas reported, but this is not really required, but should be done for full support.
LUCENE-6729 we want to upgrade the Lucene Expressions module to ASM 5, too.
You can hot-swap ASM 4.1 with ASM 5.0.4 without recompilation (so we have no problem with Lucene using a newer version). Since ASM 4.x the updates are more easy (no visitor interfaces anymore, instead abstract classes), so it does not break if you just replace the JAR file. So just see this as a recommendatation, not urgent! Solr/Lucene will also work without this patch (it just replaces the shipped ASM by newer version in our packaging).