in a local partial community build run on JDK 12, I verified that several projects that failed before are green now: scalaz,scala-async,scala-partest

but let's do a full JDK 12 run on Jenkins, too, just in case: https://scala-ci.typesafe.com/view/scala-2.12.x/job/scala-2.12.x-jdk12-integrate-community-build/24/

I verified locally that scala/bug#11372 goes away with 2.12.9-bin-1220d3c-SNAPSHOT

let's do a full JDK 12 run on Jenkins, too, just in case

the results show that this PR raises the number of green projects in the community build from 98 to 139, and without introducing any additional failures

@adriaanm @retronym shall we merge?

(one additional piece of due diligence: https://asm.ow2.io/versions.html shows that 7.0 is still the most recent ASM version)

As far as I can tell, the compilation errors only happen when the backend optimizer is turned on, since that's what triggers ASM looking into classfiles from your dependencies, why not degrade gracefully and have the optimizer not look into classfiles it can't read ? Or if that's not possible, at least make the optimizer output an error message explaining to the user that he needs to downgrade his JVM, disable the optimizer, or upgrade his scalac.

As far as I can tell, the compilation errors only happen when the backend optimizer is turned on

yes, likely correct, I didn't get to the bottom of every individual failure, but every one I did look at it turned out to involved the optimizer

why not degrade gracefully

seems like a good suggestion, but it exceeds my own ambitions for this PR. perhaps someone else will take it up

I'm going to go ahead and merge this because I think we should have nightly builds that work on JDK 12. And for the community build.