Groovy 3.x snapshots use ASM7_EXPERIMENTAL. For Groovy 2.5.x, the current plan (subject to change) is that we were planning on waiting for the ASM 7 release and doing a new release after that.
in that case, perhaps Groovy 2.5.x should reject classfiles V11 and V12 with an error message instead of having ASM throwing an UnsupportedOperationException later, at least until ASM 7 is integrated
But, I must admit that I haven't really had a chance to look at the full implications of using ASM7_EXPERIMENTAL. If we added that already in 2.5.x but don't use any of the new features, is there any backward compatibility issues when we then switched to ASM7?
Currently no, just replacing ASM7_EXPERIMENTAL by ASM7 is enough and given that Java 11 is already in release candidate phase, we do not expect any compatibility issues.
Also, there is no bytecode changes planned for 12 so the latest release of ASM 6 (6.2.1) also allows Java 12 bytecode.
Forget to ling to the related bug:
----- Mail original -----
> De: "Remi Forax" <email@example.com>
> À: "dev" <firstname.lastname@example.org>
> Envoyé: Dimanche 2 Septembre 2018 18:04:12
> Objet: Support of Java 11 using ASM 6.2.1 and ASM 7
> Hi all,
> Java 11 introduces several new forward incompatible features* in the class file.
> Currently javac only uses one of them, nestmates , which allows to declare
> that several classes are part of the same nest thus allow access to private
> members in between them, obviously changing the semantics of the private access
> for invokevirtual (see  if you want more info) if not forward compatible.
> Even if NestMates are declared as class attributes, we have decided in ASM to
> not ignore them if there are present in the bytecode (a class compiled with
> javac 11) because this change for the VM is far from innocuous, if you scrap
> those attributes, you get IllegalAccessError laters, so if you use a
> ClassVisitor configured with the ASM6 API and ASM see a nestmate related
> attributes (NestHost or NestMembers) it will fail with an
> Moreover, even if the support of Java 11 will came with ASM7, we have decided to
> introduce a new experimental API version (currently ASM7_EXPERIMENTAL) which
> let you parse Java 11 using ASM6 (6.2+), so if you want your favorite language
> to support Java 11, all visitors need to be upgraded to the api version
> You can also decide as before to wait until we release ASM7 (the first week end
> after the release of Java as usual, so at the end of September) to use the ASM7
> * see also Constant Dynamic (http://openjdk.java.net/jeps/309) and Preview
> Feature (http://openjdk.java.net/jeps/12)
>  http://openjdk.java.net/jeps/181
>  https://youtu.be/-k_IicifbxQ?list=PLX8CzqL3ArzVnxC6PYxMlngEMv3W1pIkn