> quite a lot of changes are required to support Java 16+
Yes, for proper proxy and default method support there are a lot of changes. Not asking for a backport on that stuff. ASM 9.4 is required to parse the class files created by Java 19 target. FYI, I updated ASM from 9.3 to 9.4 for Groovy 2.5 and 3.0 yesterday.
